Mixed-source cflags (#41049)

Allow flags from different sources (compilers, `require:`, command-line
specs, and `depends_on`) to be merged together, and enforce a consistent
order among them.

The order is based on the sources, e.g. flags on specs from the command
line always come last. Some flag order consistency issues are fixed:

1. Flags from `compilers.yaml` and the command line were always intra- and
   inter-source order consistent.
2. Flags from dependents and packages.yaml (introduced via `require:`)
   were not: for `-a -b` from one source and `-c` from another, the final
   result might rearrange `-a -b`, and would also be inconsistent in terms
   of whether `-c` came before or after.

(1) is/was handled by going back to the original source, i.e., flags are
retrieved directly from the command line spec rather than the solver.

(2) is addressed by:

* Keeping track of grouped flags in the solver
* Keeping track of flag sources in the solver on a per-flag basis

The latter info is used in this PR to enforce DAG ordering on flags
applied from multiple dependents to the same package, e.g., for this
graph:

```
   a
  /|\
 b | c
  \|/
   d
```

If `a`, `b`, and `c` impose flags on `d`, the combined flags on `d` will
contain the flags of `a`, `b`, and `c` -- in that order. 

Conflicting flags are allowed (e.g. -O2 and -O3). `Spec.satisifes()` has
been updated such that X satisfies Y as long as X has *at least* all of
the flags that Y has. This is also true in the solver constraints.
`.satisfies` does not account for how order can change behavior (so
`-O2 -O3` can satisfy `-O3 -O2`); it is expected that this can be
addressed later (e.g. by prohibiting flag conflicts).

`Spec.constrain` and `.intersects` have been updated to be consistent
with this new definition of `.satisfies`.

lib/spack/spack/solver/asp.py

@@ -1022,6 +1022,102 @@ def __iter__(self):
 ConditionSpecCache = Dict[str, Dict[ConditionSpecKey, ConditionIdFunctionPair]]
 
 
+class ConstraintOrigin(enum.Enum):
+    """Generates identifiers that can be pased into the solver attached
+    to constraints, and then later retrieved to determine the origin of
+    those constraints when ``SpecBuilder`` creates Specs from the solve
+    result.
+    """
+
+    DEPENDS_ON = 1
+    REQUIRE = 2
+
+    @staticmethod
+    def _SUFFIXES() -> Dict["ConstraintOrigin", str]:
+        return {ConstraintOrigin.DEPENDS_ON: "_dep", ConstraintOrigin.REQUIRE: "_req"}
+
+    @staticmethod
+    def append_type_suffix(pkg_id: str, kind: "ConstraintOrigin") -> str:
+        """Given a package identifier and a constraint kind, generate a string ID."""
+        suffix = ConstraintOrigin._SUFFIXES()[kind]
+        return f"{pkg_id}{suffix}"
+
+    @staticmethod
+    def strip_type_suffix(source: str) -> Tuple[int, Optional[str]]:
+        """Take a combined package/type ID generated by
+        ``append_type_suffix``, and extract the package ID and
+        an associated weight.
+        """
+        if not source:
+            return -1, None
+        for kind, suffix in ConstraintOrigin._SUFFIXES().items():
+            if source.endswith(suffix):
+                return kind.value, source[: -len(suffix)]
+        return -1, source
+
+
+class SourceContext:
+    """Tracks context in which a Spec's clause-set is generated (i.e.
+    with ``SpackSolverSetup.spec_clauses``).
+
+    Facts generated for the spec may include this context.
+    """
+
+    def __init__(self):
+        # This can be "literal" for constraints that come from a user
+        # spec (e.g. from the command line); it can be the output of
+        # `ConstraintOrigin.append_type_suffix`; the default is "none"
+        # (which means it isn't important to keep track of the source
+        # in that case).
+        self.source = "none"
+
+
+class ConditionIdContext(SourceContext):
+    """Derived from a ``ConditionContext``: for clause-sets generated by
+    imposed/required specs, stores an associated transform.
+
+    This is primarily used for tracking whether we are generating clauses
+    in the context of a required spec, or for an imposed spec.
+
+    Is not a subclass of ``ConditionContext`` because it exists in a
+    lower-level context with less information.
+    """
+
+    def __init__(self):
+        super().__init__()
+        self.transform = None
+
+
+class ConditionContext(SourceContext):
+    """Tracks context in which a condition (i.e. ``SpackSolverSetup.condition``)
+    is generated (e.g. for a `depends_on`).
+
+    This may modify the required/imposed specs generated as relevant
+    for the context.
+    """
+
+    def __init__(self):
+        super().__init__()
+        # transformation applied to facts from the required spec. Defaults
+        # to leave facts as they are.
+        self.transform_required = None
+        # transformation applied to facts from the imposed spec. Defaults
+        # to removing "node" and "virtual_node" facts.
+        self.transform_imposed = None
+
+    def requirement_context(self) -> ConditionIdContext:
+        ctxt = ConditionIdContext()
+        ctxt.source = self.source
+        ctxt.transform = self.transform_required
+        return ctxt
+
+    def impose_context(self) -> ConditionIdContext:
+        ctxt = ConditionIdContext()
+        ctxt.source = self.source
+        ctxt.transform = self.transform_imposed
+        return ctxt
+
+
 class SpackSolverSetup:
     """Class to set up and run a Spack concretization solve."""
 
@@ -1197,8 +1293,9 @@ def compiler_facts(self):
             if compiler.compiler_obj is not None:
                 c = compiler.compiler_obj
                 for flag_type, flags in c.flags.items():
+                    flag_group = " ".join(flags)
                     for flag in flags:
-                        self.gen.fact(fn.compiler_flag(compiler_id, flag_type, flag))
+                        self.gen.fact(fn.compiler_flag(compiler_id, flag_type, flag, flag_group))
 
             if compiler.available:
                 self.gen.fact(fn.compiler_available(compiler_id))
@@ -1375,7 +1472,7 @@ def _get_condition_id(
         named_cond: spack.spec.Spec,
         cache: ConditionSpecCache,
         body: bool,
-        transform: Optional[TransformFunction] = None,
+        context: ConditionIdContext,
     ) -> int:
         """Get the id for one half of a condition (either a trigger or an imposed constraint).
 
@@ -1389,15 +1486,15 @@ def _get_condition_id(
         """
         pkg_cache = cache[named_cond.name]
 
-        named_cond_key = (str(named_cond), transform)
+        named_cond_key = (str(named_cond), context.transform)
         result = pkg_cache.get(named_cond_key)
         if result:
             return result[0]
 
         cond_id = next(self._id_counter)
-        requirements = self.spec_clauses(named_cond, body=body)
-        if transform:
-            requirements = transform(named_cond, requirements)
+        requirements = self.spec_clauses(named_cond, body=body, context=context)
+        if context.transform:
+            requirements = context.transform(named_cond, requirements)
         pkg_cache[named_cond_key] = (cond_id, requirements)
 
         return cond_id
@@ -1408,8 +1505,7 @@ def condition(
         imposed_spec: Optional[spack.spec.Spec] = None,
         name: Optional[str] = None,
         msg: Optional[str] = None,
-        transform_required: Optional[TransformFunction] = None,
-        transform_imposed: Optional[TransformFunction] = remove_node,
+        context: Optional[ConditionContext] = None,
     ):
         """Generate facts for a dependency or virtual provider condition.
 
@@ -1418,10 +1514,8 @@ def condition(
             imposed_spec: the constraints that are imposed when this condition is triggered
             name: name for `required_spec` (required if required_spec is anonymous, ignored if not)
             msg: description of the condition
-            transform_required: transformation applied to facts from the required spec. Defaults
-                to leave facts as they are.
-            transform_imposed: transformation applied to facts from the imposed spec. Defaults
-                to removing "node" and "virtual_node" facts.
+            context: if provided, indicates how to modify the clause-sets for the required/imposed
+                specs based on the type of constraint they are generated for (e.g. `depends_on`)
         Returns:
             int: id of the condition created by this function
         """
@@ -1429,14 +1523,19 @@ def condition(
         if not name:
             raise ValueError(f"Must provide a name for anonymous condition: '{required_spec}'")
 
+        if not context:
+            context = ConditionContext()
+            context.transform_imposed = remove_node
+
         with spec_with_name(required_spec, name):
             # Check if we can emit the requirements before updating the condition ID counter.
             # In this way, if a condition can't be emitted but the exception is handled in the
             # caller, we won't emit partial facts.
 
             condition_id = next(self._id_counter)
+            requirement_context = context.requirement_context()
             trigger_id = self._get_condition_id(
-                required_spec, cache=self._trigger_cache, body=True, transform=transform_required
+                required_spec, cache=self._trigger_cache, body=True, context=requirement_context
             )
             self.gen.fact(fn.pkg_fact(required_spec.name, fn.condition(condition_id)))
             self.gen.fact(fn.condition_reason(condition_id, msg))
@@ -1446,8 +1545,9 @@ def condition(
             if not imposed_spec:
                 return condition_id
 
+            impose_context = context.impose_context()
             effect_id = self._get_condition_id(
-                imposed_spec, cache=self._effect_cache, body=False, transform=transform_imposed
+                imposed_spec, cache=self._effect_cache, body=False, context=impose_context
             )
             self.gen.fact(
                 fn.pkg_fact(required_spec.name, fn.condition_effect(condition_id, effect_id))
@@ -1455,8 +1555,8 @@ def condition(
 
             return condition_id
 
-    def impose(self, condition_id, imposed_spec, node=True, name=None, body=False):
-        imposed_constraints = self.spec_clauses(imposed_spec, body=body, required_from=name)
+    def impose(self, condition_id, imposed_spec, node=True, body=False):
+        imposed_constraints = self.spec_clauses(imposed_spec, body=body)
         for pred in imposed_constraints:
             # imposed "node"-like conditions are no-ops
             if not node and pred.args[0] in ("node", "virtual_node"):
@@ -1528,14 +1628,14 @@ def dependency_holds(input_spec, requirements):
                         if t & depflag
                     ]
 
-                self.condition(
-                    cond,
-                    dep.spec,
-                    name=pkg.name,
-                    msg=msg,
-                    transform_required=track_dependencies,
-                    transform_imposed=dependency_holds,
+                context = ConditionContext()
+                context.source = ConstraintOrigin.append_type_suffix(
+                    pkg.name, ConstraintOrigin.DEPENDS_ON
                 )
+                context.transform_required = track_dependencies
+                context.transform_imposed = dependency_holds
+
+                self.condition(cond, dep.spec, name=pkg.name, msg=msg, context=context)
 
                 self.gen.newline()
 
@@ -1613,17 +1713,21 @@ def emit_facts_from_requirement_rules(self, rules: List[RequirementRule]):
                     when_spec = spack.spec.Spec(pkg_name)
 
                 try:
-                    # With virtual we want to emit "node" and "virtual_node" in imposed specs
-                    transform: Optional[TransformFunction] = remove_node
-                    if virtual:
-                        transform = None
+                    context = ConditionContext()
+                    context.source = ConstraintOrigin.append_type_suffix(
+                        pkg_name, ConstraintOrigin.REQUIRE
+                    )
+                    if not virtual:
+                        context.transform_imposed = remove_node
+                    # else: for virtuals we want to emit "node" and
+                    # "virtual_node" in imposed specs
 
                     member_id = self.condition(
                         required_spec=when_spec,
                         imposed_spec=spec,
                         name=pkg_name,
-                        transform_imposed=transform,
                         msg=f"{input_spec} is a requirement for package {pkg_name}",
+                        context=context,
                     )
                 except Exception as e:
                     # Do not raise if the rule comes from the 'all' subsection, since usability
@@ -1722,7 +1826,9 @@ def external_imposition(input_spec, requirements):
                     ]
 
                 try:
-                    self.condition(spec, spec, msg=msg, transform_imposed=external_imposition)
+                    context = ConditionContext()
+                    context.transform_imposed = external_imposition
+                    self.condition(spec, spec, msg=msg, context=context)
                 except (spack.error.SpecError, RuntimeError) as e:
                     warnings.warn(f"while setting up external spec {spec}: {e}")
                     continue
@@ -1797,6 +1903,7 @@ def spec_clauses(
         expand_hashes: bool = False,
         concrete_build_deps=False,
         required_from: Optional[str] = None,
+        context: Optional[SourceContext] = None,
     ) -> List[AspFunction]:
         """Wrap a call to `_spec_clauses()` into a try/except block with better error handling.
 
@@ -1812,6 +1919,7 @@ def spec_clauses(
                 transitive=transitive,
                 expand_hashes=expand_hashes,
                 concrete_build_deps=concrete_build_deps,
+                context=context,
             )
         except RuntimeError as exc:
             msg = str(exc)
@@ -1828,6 +1936,7 @@ def _spec_clauses(
         transitive: bool = True,
         expand_hashes: bool = False,
         concrete_build_deps: bool = False,
+        context: Optional[SourceContext] = None,
     ) -> List[AspFunction]:
         """Return a list of clauses for a spec mandates are true.
 
@@ -1839,6 +1948,8 @@ def _spec_clauses(
             expand_hashes: if True, descend into hashes of concrete specs (default False)
             concrete_build_deps: if False, do not include pure build deps of concrete specs
                 (as they have no effect on runtime constraints)
+            context: tracks what constraint this clause set is generated for (e.g. a
+                `depends_on` constraint in a package.py file)
 
         Normally, if called with ``transitive=True``, ``spec_clauses()`` just generates
         hashes for the dependency requirements of concrete specs. If ``expand_hashes``
@@ -1925,13 +2036,19 @@ def _spec_clauses(
                 self.compiler_version_constraints.add(spec.compiler)
 
         # compiler flags
+        source = context.source if context else "none"
         for flag_type, flags in spec.compiler_flags.items():
+            flag_group = " ".join(flags)
             for flag in flags:
-                clauses.append(f.node_flag(spec.name, flag_type, flag))
+                clauses.append(
+                    f.node_flag(spec.name, fn.node_flag(flag_type, flag, flag_group, source))
+                )
                 if not spec.concrete and flag.propagate is True:
                     clauses.append(
                         f.propagate(
-                            spec.name, fn.node_flag(flag_type, flag), fn.edge_types("link", "run")
+                            spec.name,
+                            fn.node_flag(flag_type, flag, flag_group, source),
+                            fn.edge_types("link", "run"),
                         )
                     )
 
@@ -2013,6 +2130,7 @@ def _spec_clauses(
                             body=body,
                             expand_hashes=expand_hashes,
                             concrete_build_deps=concrete_build_deps,
+                            context=context,
                         )
                     )
 
@@ -2648,7 +2766,9 @@ def literal_specs(self, specs):
                 effect_id, requirements = cache[imposed_spec_key]
             else:
                 effect_id = next(self._id_counter)
-                requirements = self.spec_clauses(spec)
+                context = SourceContext()
+                context.source = "literal"
+                requirements = self.spec_clauses(spec, context=context)
             root_name = spec.name
             for clause in requirements:
                 clause_name = clause.args[0]
@@ -3405,11 +3525,10 @@ def node_compiler_version(self, node, compiler, version):
         self._specs[node].compiler = spack.spec.CompilerSpec(compiler)
         self._specs[node].compiler.versions = vn.VersionList([vn.Version(version)])
 
-    def node_flag(self, node, flag_type, flag):
-        self._specs[node].compiler_flags.add_flag(flag_type, flag, False)
-
-    def node_flag_source(self, node, flag_type, source):
-        self._flag_sources[(node, flag_type)].add(source)
+    def node_flag(self, node, node_flag):
+        self._specs[node].compiler_flags.add_flag(
+            node_flag.flag_type, node_flag.flag, False, node_flag.flag_group, node_flag.source
+        )
 
     def external_spec_selected(self, node, idx):
         """This means that the external spec and index idx has been selected for this package."""
@@ -3450,15 +3569,23 @@ def virtual_on_edge(self, parent_node, provider_node, virtual):
         dependencies[0].update_virtuals((virtual,))
 
     def reorder_flags(self):
-        """Order compiler flags on specs in predefined order.
-
-        We order flags so that any node's flags will take priority over
-        those of its dependents.  That is, the deepest node in the DAG's
-        flags will appear last on the compile line, in the order they
-        were specified.
+        """For each spec, determine the order of compiler flags applied to it.
 
         The solver determines which flags are on nodes; this routine
-        imposes order afterwards.
+        imposes order afterwards. The order is:
+
+        1. Flags applied in compiler definitions should come first
+        2. Flags applied by dependents are ordered topologically (with a
+           dependency on `traverse` to resolve the partial order into a
+           stable total order)
+        3. Flags from requirements are then applied (requirements always
+           come from the package and never a parent)
+        4. Command-line flags should come last
+
+        Additionally, for each source (requirements, compiler, command line, and
+        dependents), flags from that source should retain their order and grouping:
+        e.g. for `y cflags="-z -a"` "-z" and "-a" should never have any intervening
+        flags inserted, and should always appear in that order.
         """
         # reverse compilers so we get highest priority compilers that share a spec
         compilers = dict(
@@ -3473,40 +3600,78 @@ def reorder_flags(self):
                 flagmap_from_compiler = compilers[spec.compiler].flags
 
             for flag_type in spec.compiler_flags.valid_compiler_flags():
-                from_compiler = flagmap_from_compiler.get(flag_type, [])
-                from_sources = []
-
-                # order is determined by the  DAG. A spec's flags come after any of its ancestors
-                # on the compile line
                 node = SpecBuilder.make_node(pkg=spec.name)
-                source_key = (node, flag_type)
-                if source_key in self._flag_sources:
-                    order = [
-                        SpecBuilder.make_node(pkg=s.name)
-                        for s in spec.traverse(order="post", direction="parents")
-                    ]
-                    sorted_sources = sorted(
-                        self._flag_sources[source_key], key=lambda s: order.index(s)
+
+                ordered_flags = []
+
+                # 1. Put compiler flags first
+                from_compiler = tuple(flagmap_from_compiler.get(flag_type, []))
+                extend_flag_list(ordered_flags, from_compiler)
+
+                # 2. Add all sources (the compiler is one of them, so skip any
+                # flag group that matches it exactly)
+                flag_groups = set()
+                for flag in self._specs[node].compiler_flags.get(flag_type, []):
+                    flag_groups.add(
+                        spack.spec.CompilerFlag(
+                            flag.flag_group,
+                            propagate=flag.propagate,
+                            flag_group=flag.flag_group,
+                            source=flag.source,
+                        )
                     )
 
-                    # add flags from each source, lowest to highest precedence
-                    for node in sorted_sources:
-                        all_src_flags = list()
-                        per_pkg_sources = [self._specs[node]]
-                        if node.pkg in cmd_specs:
-                            per_pkg_sources.append(cmd_specs[node.pkg])
-                        for source in per_pkg_sources:
-                            all_src_flags.extend(source.compiler_flags.get(flag_type, []))
-                        extend_flag_list(from_sources, all_src_flags)
+                # For flags that are applied by dependents, put flags from parents
+                # before children; we depend on the stability of traverse() to
+                # achieve a stable flag order for flags introduced in this manner.
+                topo_order = list(s.name for s in spec.traverse(order="post", direction="parents"))
+                lex_order = list(sorted(flag_groups))
+
+                def _order_index(flag_group):
+                    source = flag_group.source
+                    # Note: if 'require: ^dependency cflags=...' is ever possible,
+                    # this will topologically sort for require as well
+                    type_index, pkg_source = ConstraintOrigin.strip_type_suffix(source)
+                    if pkg_source in topo_order:
+                        major_index = topo_order.index(pkg_source)
+                        # If for x->y, x has multiple depends_on declarations that
+                        # are activated, and each adds cflags to y, we fall back on
+                        # alphabetical ordering to maintain a total order
+                        minor_index = lex_order.index(flag_group)
+                    else:
+                        major_index = len(topo_order) + lex_order.index(flag_group)
+                        minor_index = 0
+                    return (type_index, major_index, minor_index)
+
+                prioritized_groups = sorted(flag_groups, key=lambda x: _order_index(x))
+
+                for grp in prioritized_groups:
+                    grp_flags = tuple(
+                        x for (x, y) in spack.compiler.tokenize_flags(grp.flag_group)
+                    )
+                    if grp_flags == from_compiler:
+                        continue
+                    as_compiler_flags = list(
+                        spack.spec.CompilerFlag(
+                            x,
+                            propagate=grp.propagate,
+                            flag_group=grp.flag_group,
+                            source=grp.source,
+                        )
+                        for x in grp_flags
+                    )
+                    extend_flag_list(ordered_flags, as_compiler_flags)
+
+                # 3. Now put cmd-line flags last
+                if node.pkg in cmd_specs:
+                    cmd_flags = cmd_specs[node.pkg].compiler_flags.get(flag_type, [])
+                    extend_flag_list(ordered_flags, cmd_flags)
 
-                # compiler flags from compilers config are lowest precedence
-                ordered_compiler_flags = list(llnl.util.lang.dedupe(from_compiler + from_sources))
                 compiler_flags = spec.compiler_flags.get(flag_type, [])
+                msg = "%s does not equal %s" % (set(compiler_flags), set(ordered_flags))
+                assert set(compiler_flags) == set(ordered_flags), msg
 
-                msg = f"{set(compiler_flags)} does not equal {set(ordered_compiler_flags)}"
-                assert set(compiler_flags) == set(ordered_compiler_flags), msg
-
-                spec.compiler_flags.update({flag_type: ordered_compiler_flags})
+                spec.compiler_flags.update({flag_type: ordered_flags})
 
     def deprecated(self, node: NodeArgument, version: str) -> None:
         tty.warn(f'using "{node.pkg}@{version}" which is a deprecated version')
@@ -3570,10 +3735,9 @@ def build_specs(self, function_tuples):
                     continue
 
                 # if we've already gotten a concrete spec for this pkg,
-                # do not bother calling actions on it except for node_flag_source,
-                # since node_flag_source is tracking information not in the spec itself
+                # do not bother calling actions on it
                 spec = self._specs.get(args[0])
-                if spec and spec.concrete and name != "node_flag_source":
+                if spec and spec.concrete:
                     continue
 
             action(*args)
@@ -3633,7 +3797,8 @@ def _develop_specs_from_env(spec, env):
         assert spec.variants["dev_path"].value == path, error_msg
     else:
         spec.variants.setdefault("dev_path", spack.variant.SingleValuedVariant("dev_path", path))
-    spec.constrain(dev_info["spec"])
+
+    assert spec.satisfies(dev_info["spec"])
 
 
 def _is_reusable(spec: spack.spec.Spec, packages, local: bool) -> bool:

lib/spack/spack/solver/concretize.lp

@@ -43,7 +43,7 @@
    internal_error("Only nodes can have node_compiler_version").
 :- attr("variant_value", PackageNode, _, _), not attr("node", PackageNode),
    internal_error("variant_value true for a non-node").
-:- attr("node_flag", PackageNode, _, _), not attr("node", PackageNode),
+:- attr("node_flag", PackageNode, _), not attr("node", PackageNode),
    internal_error("node_flag assigned for non-node").
 :- attr("external_spec_selected", PackageNode, _), not attr("node", PackageNode),
    internal_error("external_spec_selected for non-node").
@@ -51,10 +51,6 @@
    internal_error("non-node depends on something").
 :- attr("depends_on", _, ChildNode, _), not attr("node", ChildNode),
    internal_error("something depends_on a non-node").
-:- attr("node_flag_source", Node, _, _), not attr("node", Node),
-   internal_error("node_flag_source assigned for a non-node").
-:- attr("node_flag_source", _, _, SourceNode), not attr("node", SourceNode),
-   internal_error("node_flag_source assigned with a non-node source").
 :- attr("virtual_node", VirtualNode), not provider(_, VirtualNode),
    internal_error("virtual node with no provider").
 :- provider(_, VirtualNode), not attr("virtual_node", VirtualNode),
@@ -152,7 +148,6 @@ unification_set(SetID, VirtualNode)
 % TODO: literals, at the moment, can only influence the "root" unification set. This needs to be extended later.
 
 % Node attributes that have multiple node arguments (usually, only the first argument is a node)
-multiple_nodes_attribute("node_flag_source").
 multiple_nodes_attribute("depends_on").
 multiple_nodes_attribute("virtual_on_edge").
 multiple_nodes_attribute("provider_set").
@@ -390,7 +385,6 @@ trigger_condition_holds(ID, RequestorNode) :-
   attr(Name, node(X, A1), A2, A3)     : condition_requirement(ID, Name, A1, A2, A3),     condition_nodes(ID, PackageNode, node(X, A1)), not multiple_nodes_attribute(Name);
   attr(Name, node(X, A1), A2, A3, A4) : condition_requirement(ID, Name, A1, A2, A3, A4), condition_nodes(ID, PackageNode, node(X, A1));
   % Special cases
-  attr("node_flag_source", node(X, A1), A2, node(Y, A3)) : condition_requirement(ID, "node_flag_source", A1, A2, A3), condition_nodes(ID, PackageNode, node(X, A1)), condition_nodes(ID, PackageNode, node(Y, A3));
   not cannot_hold(ID, PackageNode).
 
 condition_holds(ConditionID, node(X, Package))
@@ -438,13 +432,6 @@ attr(Name, node(X, A1), A2)         :- impose(ID, PackageNode), imposed_constrai
 attr(Name, node(X, A1), A2, A3)     :- impose(ID, PackageNode), imposed_constraint(ID, Name, A1, A2, A3),     imposed_nodes(ID, PackageNode, node(X, A1)), not multiple_nodes_attribute(Name).
 attr(Name, node(X, A1), A2, A3, A4) :- impose(ID, PackageNode), imposed_constraint(ID, Name, A1, A2, A3, A4), imposed_nodes(ID, PackageNode, node(X, A1)).
 
-% For node flag sources we need to look at the condition_set of the source, since it is the dependent
-% of the package on which I want to impose the constraint
-attr("node_flag_source", node(X, A1), A2, node(Y, A3))
-  :- impose(ID, node(X, A1)),
-     imposed_constraint(ID, "node_flag_source", A1, A2, A3),
-     condition_set(node(Y, A3), node(X, A1)).
-
 % Provider set is relevant only for literals, since it's the only place where `^[virtuals=foo] bar`
 % might appear in the HEAD of a rule
 attr("provider_set", node(min_dupe_id, Provider), node(min_dupe_id, Virtual))
@@ -485,8 +472,8 @@ virtual_condition_holds(node(Y, A2), Virtual)
 % we cannot have additional flag values when we are working with concrete specs
 :- attr("node", node(ID, Package)),
    attr("hash", node(ID, Package), Hash),
-   attr("node_flag", node(ID, Package), FlagType, Flag),
-   not imposed_constraint(Hash, "node_flag", Package, FlagType, Flag),
+   attr("node_flag", node(ID, Package), node_flag(FlagType, Flag, _, _)),
+   not imposed_constraint(Hash, "node_flag", Package, node_flag(FlagType, Flag, _, _)),
    internal_error("imposed hash without imposing all flag values").
 
 #defined condition/2.
@@ -787,22 +774,15 @@ required_provider(Provider, Virtual)
 
 :- provider(node(Y, Package), node(X, Virtual)), required_provider(Provider, Virtual), Package != Provider.
 
-% TODO: the following two choice rules allow the solver to add compiler
+% TODO: the following choice rule allows the solver to add compiler
 % flags if their only source is from a requirement. This is overly-specific
 % and should use a more-generic approach like in https://github.com/spack/spack/pull/37180
 
-{ attr("node_flag", node(ID, Package), FlagType, FlagValue) } :-
+{ attr("node_flag", node(ID, Package), NodeFlag) } :-
   requirement_group_member(ConditionID, Package, RequirementID),
   activate_requirement(node(ID, Package), RequirementID),
   pkg_fact(Package, condition_effect(ConditionID, EffectID)),
-  imposed_constraint(EffectID, "node_flag_set", Package, FlagType, FlagValue).
-
-{ attr("node_flag_source", node(NodeID1, Package1), FlagType, node(NodeID2, Package2)) } :-
-  requirement_group_member(ConditionID, Package1, RequirementID),
-  activate_requirement(node(NodeID1, Package1), RequirementID),
-  pkg_fact(Package1, condition_effect(ConditionID, EffectID)),
-  imposed_constraint(EffectID, "node_flag_source", Package1, FlagType, Package2),
-  imposed_nodes(EffectID, node(NodeID2, Package2), node(NodeID1, Package1)).
+  imposed_constraint(EffectID, "node_flag_set", Package, NodeFlag).
 
 requirement_weight(node(ID, Package), Group, W) :-
   W = #min {
@@ -1048,23 +1028,22 @@ variant_is_propagated(PackageNode, Variant) :-
 % 1. The same flag type is not set on this node
 % 2. This node has the same compiler as the propagation source
 
-propagated_flag(node(PackageID, Package), node_flag(FlagType, Flag), SourceNode) :-
-  propagate(node(PackageID, Package), node_flag(FlagType, Flag), _),
-  not attr("node_flag_set", node(PackageID, Package), FlagType, _),
+propagated_flag(node(PackageID, Package), node_flag(FlagType, Flag, FlagGroup, Source), SourceNode) :-
+  propagate(node(PackageID, Package), node_flag(FlagType, Flag, FlagGroup, Source), _),
+  not attr("node_flag_set", node(PackageID, Package), node_flag(FlagType, _, _, "literal")),
   % Same compiler as propagation source
   node_compiler(node(PackageID, Package), CompilerID),
   node_compiler(SourceNode, CompilerID),
-  attr("propagate", SourceNode, node_flag(FlagType, Flag), _),
+  attr("propagate", SourceNode, node_flag(FlagType, Flag, FlagGroup, Source), _),
   node(PackageID, Package) != SourceNode,
   not runtime(Package).
 
-attr("node_flag", PackageNode, FlagType, Flag) :- propagated_flag(PackageNode, node_flag(FlagType, Flag), _).
-attr("node_flag_source", PackageNode, FlagType, SourceNode) :- propagated_flag(PackageNode, node_flag(FlagType, _), SourceNode).
+attr("node_flag", PackageNode, NodeFlag) :- propagated_flag(PackageNode, NodeFlag, _).
 
 % Cannot propagate the same flag from two distinct sources
 error(100, "{0} and {1} cannot both propagate compiler flags '{2}' to {3}", Source1, Source2, Package, FlagType) :-
-  propagated_flag(node(ID, Package), node_flag(FlagType, _), node(_, Source1)),
-  propagated_flag(node(ID, Package), node_flag(FlagType, _), node(_, Source2)),
+  propagated_flag(node(ID, Package), node_flag(FlagType, _, _, _), node(_, Source1)),
+  propagated_flag(node(ID, Package), node_flag(FlagType, _, _, _), node(_, Source2)),
   Source1 < Source2.
 
 %----
@@ -1351,23 +1330,18 @@ error(100, "Compiler {1}@{2} requested for {0} cannot be found. Set install_miss
 % Compiler flags
 %-----------------------------------------------------------------------------
 
-% remember where flags came from
-attr("node_flag_source", PackageNode, FlagType, PackageNode) :- attr("node_flag_set", PackageNode, FlagType, _).
-attr("node_flag_source", PackageNode, FlagType, PackageNode) :- attr("node_flag", PackageNode, FlagType, _), attr("hash", PackageNode, _).
-
 % compiler flags from compilers.yaml are put on nodes if compiler matches
-attr("node_flag", PackageNode, FlagType, Flag)
-  :- compiler_flag(CompilerID, FlagType, Flag),
+attr("node_flag", PackageNode, node_flag(FlagType, Flag, FlagGroup, CompilerID))
+  :- compiler_flag(CompilerID, FlagType, Flag, FlagGroup),
      node_compiler(PackageNode, CompilerID),
      flag_type(FlagType),
      compiler_id(CompilerID),
      compiler_name(CompilerID, CompilerName),
      compiler_version(CompilerID, Version).
 
-% Flag set to something
-attr("node_flag", PackageNode, FlagType, Flag) :- attr("node_flag_set", PackageNode, FlagType, Flag).
+attr("node_flag", PackageNode, NodeFlag) :- attr("node_flag_set", PackageNode, NodeFlag).
 
-#defined compiler_flag/3.
+#defined compiler_flag/4.
 
 
 %-----------------------------------------------------------------------------

lib/spack/spack/solver/core.py

@@ -230,6 +230,13 @@ class NodeArgument(NamedTuple):
     pkg: str
 
 
+class NodeFlag(NamedTuple):
+    flag_type: str
+    flag: str
+    flag_group: str
+    source: str
+
+
 def intermediate_repr(sym):
     """Returns an intermediate representation of clingo models for Spack's spec builder.
 
@@ -248,6 +255,13 @@ def intermediate_repr(sym):
             return NodeArgument(
                 id=intermediate_repr(sym.arguments[0]), pkg=intermediate_repr(sym.arguments[1])
             )
+        elif sym.name == "node_flag":
+            return NodeFlag(
+                flag_type=intermediate_repr(sym.arguments[0]),
+                flag=intermediate_repr(sym.arguments[1]),
+                flag_group=intermediate_repr(sym.arguments[2]),
+                source=intermediate_repr(sym.arguments[3]),
+            )
     except RuntimeError:
         # This happens when using clingo w/ CFFI and trying to access ".name" for symbols
         # that are not functions

lib/spack/spack/solver/display.lp

@@ -13,6 +13,7 @@
 #show attr/2.
 #show attr/3.
 #show attr/4.
+#show attr/5.
 
 % names of optimization criteria
 #show opt_criterion/2.

lib/spack/spack/spec.py

@@ -781,17 +781,49 @@ class CompilerFlag(str):
         propagate (bool): if ``True`` the flag value will
             be passed to the package's dependencies. If
             ``False`` it will not
+        flag_group (str): if this flag was introduced along
+            with several flags via a single source, then
+            this will store all such flags
+        source (str): identifies the type of constraint that
+            introduced this flag (e.g. if a package has
+            ``depends_on(... cflags=-g)``, then the ``source``
+            for "-g" would indicate ``depends_on``.
     """
 
     def __new__(cls, value, **kwargs):
         obj = str.__new__(cls, value)
         obj.propagate = kwargs.pop("propagate", False)
+        obj.flag_group = kwargs.pop("flag_group", value)
+        obj.source = kwargs.pop("source", None)
         return obj
 
 
 _valid_compiler_flags = ["cflags", "cxxflags", "fflags", "ldflags", "ldlibs", "cppflags"]
 
 
+def _shared_subset_pair_iterate(container1, container2):
+    """
+    [0, a, c, d, f]
+    [a, d, e, f]
+
+    yields [(a, a), (d, d), (f, f)]
+
+    no repeated elements
+    """
+    a_idx, b_idx = 0, 0
+    max_a, max_b = len(container1), len(container2)
+    while a_idx < max_a and b_idx < max_b:
+        if container1[a_idx] == container2[b_idx]:
+            yield (container1[a_idx], container2[b_idx])
+            a_idx += 1
+            b_idx += 1
+        else:
+            while container1[a_idx] < container2[b_idx]:
+                a_idx += 1
+            while container1[a_idx] > container2[b_idx]:
+                b_idx += 1
+
+
 class FlagMap(lang.HashableMap):
     __slots__ = ("spec",)
 
@@ -800,23 +832,9 @@ def __init__(self, spec):
         self.spec = spec
 
     def satisfies(self, other):
-        return all(f in self and self[f] == other[f] for f in other)
+        return all(f in self and set(self[f]) >= set(other[f]) for f in other)
 
     def intersects(self, other):
-        common_types = set(self) & set(other)
-        for flag_type in common_types:
-            if not self[flag_type] or not other[flag_type]:
-                # At least one of the two is empty
-                continue
-
-            if self[flag_type] != other[flag_type]:
-                return False
-
-            if not all(
-                f1.propagate == f2.propagate for f1, f2 in zip(self[flag_type], other[flag_type])
-            ):
-                # At least one propagation flag didn't match
-                return False
         return True
 
     def constrain(self, other):
@@ -824,28 +842,28 @@ def constrain(self, other):
 
         Return whether the spec changed.
         """
-        if other.spec and other.spec._concrete:
-            for k in self:
-                if k not in other:
-                    raise UnsatisfiableCompilerFlagSpecError(self[k], "<absent>")
-
         changed = False
-        for k in other:
-            if k in self and not set(self[k]) <= set(other[k]):
-                raise UnsatisfiableCompilerFlagSpecError(
-                    " ".join(f for f in self[k]), " ".join(f for f in other[k])
-                )
-            elif k not in self:
-                self[k] = other[k]
+        for flag_type in other:
+            if flag_type not in self:
+                self[flag_type] = other[flag_type]
                 changed = True
+            else:
+                extra_other = set(other[flag_type]) - set(self[flag_type])
+                if extra_other:
+                    self[flag_type] = list(self[flag_type]) + list(
+                        x for x in other[flag_type] if x in extra_other
+                    )
+                    changed = True
+
+                # Next, if any flags in other propagate, we force them to propagate in our case
+                shared = list(sorted(set(other[flag_type]) - extra_other))
+                for x, y in _shared_subset_pair_iterate(shared, sorted(self[flag_type])):
+                    if x.propagate:
+                        y.propagate = True
+
+        # TODO: what happens if flag groups with a partial (but not complete)
+        # intersection specify different behaviors for flag propagation?
 
-            # Check that the propagation values match
-            if self[k] == other[k]:
-                for i in range(len(other[k])):
-                    if self[k][i].propagate != other[k][i].propagate:
-                        raise UnsatisfiableCompilerFlagSpecError(
-                            self[k][i].propagate, other[k][i].propagate
-                        )
         return changed
 
     @staticmethod
@@ -858,7 +876,7 @@ def copy(self):
             clone[name] = compiler_flag
         return clone
 
-    def add_flag(self, flag_type, value, propagation):
+    def add_flag(self, flag_type, value, propagation, flag_group=None, source=None):
         """Stores the flag's value in CompilerFlag and adds it
         to the FlagMap
 
@@ -869,7 +887,8 @@ def add_flag(self, flag_type, value, propagation):
             propagation (bool): if ``True`` the flag value will be passed to
                 the packages' dependencies. If``False`` it will not be passed
         """
-        flag = CompilerFlag(value, propagate=propagation)
+        flag_group = flag_group or value
+        flag = CompilerFlag(value, propagate=propagation, flag_group=flag_group, source=source)
 
         if flag_type not in self:
             self[flag_type] = [flag]
@@ -1665,8 +1684,9 @@ def _add_flag(self, name, value, propagate):
         elif name in valid_flags:
             assert self.compiler_flags is not None
             flags_and_propagation = spack.compiler.tokenize_flags(value, propagate)
+            flag_group = " ".join(x for (x, y) in flags_and_propagation)
             for flag, propagation in flags_and_propagation:
-                self.compiler_flags.add_flag(name, flag, propagation)
+                self.compiler_flags.add_flag(name, flag, propagation, flag_group)
         else:
             # FIXME:
             # All other flags represent variants. 'foo=true' and 'foo=false'

lib/spack/spack/test/concretize.py

@@ -401,14 +401,6 @@ def test_spec_flags_maintain_order(self, mutable_config, gcc11_with_flags):
                 s.compiler_flags[x] == ["-O0", "-g"] for x in ("cflags", "cxxflags", "fflags")
             )
 
-    @pytest.mark.xfail(reason="Broken, needs to be fixed")
-    def test_compiler_flags_from_compiler_and_dependent(self):
-        client = Spec("cmake-client %clang@12.2.0 platform=test os=fe target=fe cflags==-g")
-        client.concretize()
-        cmake = client["cmake"]
-        for spec in [client, cmake]:
-            assert spec.compiler_flags["cflags"] == ["-O3", "-g"]
-
     def test_compiler_flags_differ_identical_compilers(self, mutable_config, clang12_with_flags):
         mutable_config.set("compilers", [clang12_with_flags])
         # Correct arch to use test compiler that has flags
@@ -442,6 +434,13 @@ def test_compiler_flags_differ_identical_compilers(self, mutable_config, clang12
                 ["hypre cflags='-g'", "^openblas cflags='-O3'"],
                 ["^openblas cflags='-g'"],
             ),
+            # Setting propagation on parent and dependency -> the
+            # dependency propagation flags override
+            (
+                "hypre cflags=='-g' ^openblas cflags=='-O3'",
+                ["hypre cflags='-g'", "^openblas cflags='-O3'"],
+                ["^openblas cflags='-g'"],
+            ),
             # Propagation doesn't go across build dependencies
             (
                 "cmake-client cflags=='-O2 -g'",

lib/spack/spack/test/flag_mixing.py

@@ -0,0 +1,333 @@
+# Copyright 2013-2024 Lawrence Livermore National Security, LLC and other
+# Spack Project Developers. See the top-level COPYRIGHT file for details.
+#
+# SPDX-License-Identifier: (Apache-2.0 OR MIT)
+import pytest
+
+import spack.build_systems.generic
+import spack.config
+import spack.environment as ev
+import spack.error
+import spack.package_base
+import spack.repo
+import spack.util.spack_yaml as syaml
+import spack.version
+from spack.spec import Spec
+from spack.test.conftest import create_test_repo
+
+"""
+These tests include the following package DAGs:
+
+Firstly, w, x, y where w and x apply cflags to y.
+
+w
+|\
+x |
+|/
+y
+
+Secondly, v, y which where v does not apply cflags to y - this is for testing
+mixing with compiler flag propagation in the absence of compiler flags applied
+by dependents.
+
+v
+|
+y
+
+Finally, a diamond dag to check that the topological order is resolved into
+a total order:
+
+t
+|\
+u x
+|/
+y
+"""
+
+_pkgx = (
+    "x",
+    """\
+class X(Package):
+    version("1.1")
+    version("1.0")
+
+    variant("activatemultiflag", default=False)
+    depends_on('y cflags="-d1"', when="~activatemultiflag")
+    depends_on('y cflags="-d1 -d2"', when="+activatemultiflag")
+""",
+)
+
+
+_pkgy = (
+    "y",
+    """\
+class Y(Package):
+    version("2.1")
+    version("2.0")
+""",
+)
+
+
+_pkgw = (
+    "w",
+    """\
+class W(Package):
+    version("3.1")
+    version("3.0")
+
+    variant("moveflaglater", default=False)
+
+    depends_on('x +activatemultiflag')
+    depends_on('y cflags="-d0"', when="~moveflaglater")
+    depends_on('y cflags="-d3"', when="+moveflaglater")
+""",
+)
+
+
+_pkgv = (
+    "v",
+    """\
+class V(Package):
+    version("4.1")
+    version("4.0")
+
+    depends_on("y")
+""",
+)
+
+
+_pkgt = (
+    "t",
+    """\
+class T(Package):
+    version("5.0")
+
+    depends_on("u")
+    depends_on("x+activatemultiflag")
+    depends_on("y cflags='-c1 -c2'")
+""",
+)
+
+
+_pkgu = (
+    "u",
+    """\
+class U(Package):
+    version("6.0")
+
+    depends_on("y cflags='-e1 -e2'")
+""",
+)
+
+
+@pytest.fixture
+def _create_test_repo(tmpdir, mutable_config):
+    yield create_test_repo(tmpdir, [_pkgt, _pkgu, _pkgv, _pkgw, _pkgx, _pkgy])
+
+
+@pytest.fixture
+def test_repo(_create_test_repo, monkeypatch, mock_stage):
+    with spack.repo.use_repositories(_create_test_repo) as mock_repo_path:
+        yield mock_repo_path
+
+
+def update_concretize_scope(conf_str, section):
+    conf = syaml.load_config(conf_str)
+    spack.config.set(section, conf[section], scope="concretize")
+
+
+def test_mix_spec_and_requirements(concretize_scope, test_repo):
+    conf_str = """\
+packages:
+  y:
+    require: cflags="-c"
+"""
+    update_concretize_scope(conf_str, "packages")
+
+    s1 = Spec('y cflags="-a"').concretized()
+    assert s1.satisfies('cflags="-a -c"')
+
+
+def test_mix_spec_and_dependent(concretize_scope, test_repo):
+    s1 = Spec('x ^y cflags="-a"').concretized()
+    assert s1["y"].satisfies('cflags="-a -d1"')
+
+
+def _compiler_cfg_one_entry_with_cflags(cflags):
+    return f"""\
+compilers::
+- compiler:
+    spec: gcc@12.100.100
+    paths:
+      cc: /usr/bin/fake-gcc
+      cxx: /usr/bin/fake-g++
+      f77: null
+      fc: null
+    flags:
+      cflags: {cflags}
+    operating_system: debian6
+    modules: []
+"""
+
+
+def test_mix_spec_and_compiler_cfg(concretize_scope, test_repo):
+    conf_str = _compiler_cfg_one_entry_with_cflags("-Wall")
+    update_concretize_scope(conf_str, "compilers")
+
+    s1 = Spec('y %gcc@12.100.100 cflags="-O2"').concretized()
+    assert s1.satisfies('cflags="-Wall -O2"')
+
+
+@pytest.mark.parametrize(
+    "cmd_flags,req_flags,cmp_flags,dflags,expected_order",
+    [
+        ("-a -b", "-c", None, False, "-c -a -b"),
+        ("-x7 -x4", "-x5 -x6", None, False, "-x5 -x6 -x7 -x4"),
+        ("-x7 -x4", "-x5 -x6", "-x3 -x8", False, "-x3 -x8 -x5 -x6 -x7 -x4"),
+        ("-x7 -x4", "-x5 -x6", "-x3 -x8", True, "-x3 -x8 -d1 -d2 -x5 -x6 -x7 -x4"),
+        ("-x7 -x4", None, "-x3 -x8", False, "-x3 -x8 -x7 -x4"),
+        ("-x7 -x4", None, "-x3 -x8", True, "-x3 -x8 -d1 -d2 -x7 -x4"),
+        # The remaining test cover cases of intersection
+        ("-a -b", "-a -c", None, False, "-c -a -b"),
+        ("-a -b", None, "-a -c", False, "-c -a -b"),
+        ("-a -b", "-a -c", "-a -d", False, "-d -c -a -b"),
+        ("-a -d2 -d1", "-d2 -c", "-d1 -b", True, "-b -c -a -d2 -d1"),
+        ("-a", "-d0 -d2 -c", "-d1 -b", True, "-b -d1 -d0 -d2 -c -a"),
+    ],
+)
+def test_flag_order_and_grouping(
+    concretize_scope, test_repo, cmd_flags, req_flags, cmp_flags, dflags, expected_order
+):
+    """Check consistent flag ordering and grouping on a package "y"
+    with flags introduced from a variety of sources.
+
+    The ordering rules are explained in ``asp.SpecBuilder.reorder_flags``.
+    """
+    if req_flags:
+        conf_str = f"""\
+packages:
+  y:
+    require: cflags="{req_flags}"
+"""
+        update_concretize_scope(conf_str, "packages")
+
+    if cmp_flags:
+        conf_str = _compiler_cfg_one_entry_with_cflags(cmp_flags)
+        update_concretize_scope(conf_str, "compilers")
+
+    compiler_spec = ""
+    if cmp_flags:
+        compiler_spec = "%gcc@12.100.100"
+
+    if dflags:
+        spec_str = f"x+activatemultiflag {compiler_spec} ^y"
+        expected_dflags = "-d1 -d2"
+    else:
+        spec_str = f"y {compiler_spec}"
+        expected_dflags = None
+
+    if cmd_flags:
+        spec_str += f' cflags="{cmd_flags}"'
+
+    root_spec = Spec(spec_str).concretized()
+    spec = root_spec["y"]
+    satisfy_flags = " ".join(x for x in [cmd_flags, req_flags, cmp_flags, expected_dflags] if x)
+    assert spec.satisfies(f'cflags="{satisfy_flags}"')
+    assert spec.compiler_flags["cflags"] == expected_order.split()
+
+
+def test_two_dependents_flag_mixing(concretize_scope, test_repo):
+    root_spec1 = Spec("w~moveflaglater").concretized()
+    spec1 = root_spec1["y"]
+    assert spec1.compiler_flags["cflags"] == "-d0 -d1 -d2".split()
+
+    root_spec2 = Spec("w+moveflaglater").concretized()
+    spec2 = root_spec2["y"]
+    assert spec2.compiler_flags["cflags"] == "-d3 -d1 -d2".split()
+
+
+def test_propagate_and_compiler_cfg(concretize_scope, test_repo):
+    conf_str = _compiler_cfg_one_entry_with_cflags("-f2")
+    update_concretize_scope(conf_str, "compilers")
+
+    root_spec = Spec("v %gcc@12.100.100 cflags=='-f1'").concretized()
+    assert root_spec["y"].satisfies("cflags='-f1 -f2'")
+
+
+# Note: setting flags on a dependency overrides propagation, which
+# is tested in test/concretize.py:test_compiler_flag_propagation
+
+
+def test_propagate_and_pkg_dep(concretize_scope, test_repo):
+    root_spec1 = Spec("x ~activatemultiflag cflags=='-f1'").concretized()
+    assert root_spec1["y"].satisfies("cflags='-f1 -d1'")
+
+
+def test_propagate_and_require(concretize_scope, test_repo):
+    conf_str = """\
+packages:
+  y:
+    require: cflags="-f2"
+"""
+    update_concretize_scope(conf_str, "packages")
+
+    root_spec1 = Spec("v cflags=='-f1'").concretized()
+    assert root_spec1["y"].satisfies("cflags='-f1 -f2'")
+
+    # Next, check that a requirement does not "undo" a request for
+    # propagation from the command-line spec
+    conf_str = """\
+packages:
+  v:
+    require: cflags="-f1"
+"""
+    update_concretize_scope(conf_str, "packages")
+
+    root_spec2 = Spec("v cflags=='-f1'").concretized()
+    assert root_spec2["y"].satisfies("cflags='-f1'")
+
+    # Note: requirements cannot enforce propagation: any attempt to do
+    # so will generate a concretization error; this likely relates to
+    # the note about #37180 in concretize.lp
+
+
+def test_dev_mix_flags(tmp_path, concretize_scope, mutable_mock_env_path, test_repo):
+    src_dir = tmp_path / "x-src"
+
+    env_content = f"""\
+spack:
+  specs:
+  - y %gcc@12.100.100 cflags=='-fsanitize=address'
+  develop:
+    y:
+      spec: y cflags=='-fsanitize=address'
+      path: {src_dir}
+"""
+
+    conf_str = _compiler_cfg_one_entry_with_cflags("-f1")
+    update_concretize_scope(conf_str, "compilers")
+
+    manifest_file = tmp_path / ev.manifest_name
+    manifest_file.write_text(env_content)
+    e = ev.create("test", manifest_file)
+    with e:
+        e.concretize()
+    e.write()
+
+    (result,) = list(j for i, j in e.concretized_specs() if j.name == "y")
+
+    assert result["y"].satisfies("cflags='-fsanitize=address -f1'")
+
+
+def test_diamond_dep_flag_mixing(concretize_scope, test_repo):
+    """A diamond where each dependent applies flags to the bottom
+    dependency. The goal is to ensure that the flag ordering is
+    (a) topological and (b) repeatable for elements not subject to
+    this partial ordering (i.e. the flags for the left and right
+    nodes of the diamond always appear in the same order).
+    `Spec.traverse` is responsible for handling both of these needs.
+    """
+    root_spec1 = Spec("t").concretized()
+    spec1 = root_spec1["y"]
+    assert spec1.satisfies('cflags="-c1 -c2 -d1 -d2 -e1 -e2"')
+    assert spec1.compiler_flags["cflags"] == "-c1 -c2 -e1 -e2 -d1 -d2".split()

lib/spack/spack/test/spec_semantics.py

@@ -245,6 +245,65 @@ def test_abstract_specs_can_constrain_each_other(self, lhs, rhs, expected):
         assert c1 == c2
         assert c1 == expected
 
+    @pytest.mark.parametrize(
+        "lhs,rhs,expected_lhs,expected_rhs,propagated_lhs,propagated_rhs",
+        [
+            (
+                'mpich cppflags="-O3"',
+                'mpich cppflags="-O2"',
+                'mpich cppflags="-O3 -O2"',
+                'mpich cppflags="-O2 -O3"',
+                [],
+                [],
+            ),
+            (
+                'mpich cflags="-O3 -g"',
+                'mpich cflags=="-O3"',
+                'mpich cflags="-O3 -g"',
+                'mpich cflags=="-O3 -g"',
+                [("cflags", "-O3")],
+                [("cflags", "-O3")],
+            ),
+        ],
+    )
+    def test_constrain_compiler_flags(
+        self, lhs, rhs, expected_lhs, expected_rhs, propagated_lhs, propagated_rhs
+    ):
+        """Constraining is asymmetric for compiler flags. Also note that
+        Spec equality does not account for flag propagation, so the checks
+        here are manual.
+        """
+        lhs, rhs, expected_lhs, expected_rhs = (
+            Spec(lhs),
+            Spec(rhs),
+            Spec(expected_lhs),
+            Spec(expected_rhs),
+        )
+
+        assert lhs.intersects(rhs)
+        assert rhs.intersects(lhs)
+
+        c1, c2 = lhs.copy(), rhs.copy()
+        c1.constrain(rhs)
+        c2.constrain(lhs)
+
+        assert c1 == expected_lhs
+        assert c2 == expected_rhs
+        for x in [c1, c2]:
+            assert x.satisfies(lhs)
+            assert x.satisfies(rhs)
+
+        def _propagated_flags(_spec):
+            result = set()
+            for flagtype in _spec.compiler_flags:
+                for flag in _spec.compiler_flags[flagtype]:
+                    if flag.propagate:
+                        result.add((flagtype, flag))
+            return result
+
+        assert set(propagated_lhs) <= _propagated_flags(c1)
+        assert set(propagated_rhs) <= _propagated_flags(c2)
+
     def test_constrain_specs_by_hash(self, default_mock_concretization, database):
         """Test that Specs specified only by their hashes can constrain eachother."""
         mpich_dag_hash = "/" + database.query_one("mpich").dag_hash()
@@ -311,14 +370,11 @@ def test_concrete_specs_which_satisfies_abstract(self, lhs, rhs, default_mock_co
             ("mpich~~foo", "mpich++foo"),
             ("mpich++foo", "mpich~~foo"),
             ("mpich foo==True", "mpich foo==False"),
-            ('mpich cppflags="-O3"', 'mpich cppflags="-O2"'),
-            ('mpich cppflags="-O3"', 'mpich cppflags=="-O3"'),
             ("libelf@0:2.0", "libelf@2.1:3"),
             ("libelf@0:2.5%gcc@4.8:4.9", "libelf@2.1:3%gcc@4.5:4.7"),
             ("libelf+debug", "libelf~debug"),
             ("libelf+debug~foo", "libelf+debug+foo"),
             ("libelf debug=True", "libelf debug=False"),
-            ('libelf cppflags="-O3"', 'libelf cppflags="-O2"'),
             ("libelf platform=test target=be os=be", "libelf target=fe os=fe"),
             ("namespace=builtin.mock", "namespace=builtin"),
         ],
@@ -347,10 +403,6 @@ def test_constraining_abstract_specs_with_empty_intersection(self, lhs, rhs):
             ("mpich", "mpich++foo"),
             ("mpich", "mpich~~foo"),
             ("mpich", "mpich foo==1"),
-            # Flags semantics is currently different from other variant
-            ("mpich", 'mpich cflags="-O3"'),
-            ("mpich cflags=-O3", 'mpich cflags="-O3 -Ofast"'),
-            ("mpich cflags=-O2", 'mpich cflags="-O3"'),
             ("multivalue-variant foo=bar", "multivalue-variant +foo"),
             ("multivalue-variant foo=bar", "multivalue-variant ~foo"),
             ("multivalue-variant fee=bar", "multivalue-variant fee=baz"),
@@ -1451,8 +1503,8 @@ def test_abstract_contains_semantic(lhs, rhs, expected, mock_packages):
         (CompilerSpec, "gcc@5", "gcc@5-tag", (True, False, True)),
         # Flags (flags are a map, so for convenience we initialize a full Spec)
         # Note: the semantic is that of sv variants, not mv variants
-        (Spec, "cppflags=-foo", "cppflags=-bar", (False, False, False)),
-        (Spec, "cppflags='-bar -foo'", "cppflags=-bar", (False, False, False)),
+        (Spec, "cppflags=-foo", "cppflags=-bar", (True, False, False)),
+        (Spec, "cppflags='-bar -foo'", "cppflags=-bar", (True, True, False)),
         (Spec, "cppflags=-foo", "cppflags=-foo", (True, True, True)),
         (Spec, "cppflags=-foo", "cflags=-foo", (True, False, False)),
         # Versions