Specify GCC prefix in LLVM-based compilers (#33146)

* spack.compiler.Compiler: introduce prefix property

We currently don't really have something that gives the GCC install
path, which is used by many LLVM-based compilers (llvm, llvm-amdgpu,
nvhpc, ...) to fix the GCC toolchain once and for all.

This `prefix` property is dynamic in the sense that it queries the
compiler itself. This is necessary because it's not easy to deduce the
install path from the `cc` property (might be a symlink, might be a
filename like `gcc` which works by having the compiler load a module
that sets the PATH variable, might be a generic compiler wrapper based
on environment variables like on cray...).

With this property introduced, we can clean up some recipes that have
the logic repeated for GCC.

* intel-oneapi-compilers: set --gcc-sysroot to %gcc prefix

lib/spack/spack/compiler.py

@@ -537,6 +537,14 @@ def get_real_version(self):
             )
             return self.extract_version_from_output(output)
 
+    @property
+    def prefix(self):
+        """Query the compiler for its install prefix. This is the install
+        path as reported by the compiler. Note that paths for cc, cxx, etc
+        are not enough to find the install prefix of the compiler, since
+        the can be symlinks, wrappers, or filenames instead of absolute paths."""
+        raise NotImplementedError("prefix is not implemented for this compiler")
+
     #
     # Compiler classes have methods for querying the version of
     # specific compiler executables.  This is used when discovering compilers.

lib/spack/spack/compilers/gcc.py

@@ -6,8 +6,11 @@
 import os
 import re
 
+from llnl.util.filesystem import ancestor
+
 import spack.compiler
 import spack.compilers.apple_clang as apple_clang
+import spack.util.executable
 from spack.version import ver
 
 
@@ -196,3 +199,21 @@ def f77_version(cls, f77):
     @property
     def stdcxx_libs(self):
         return ("-lstdc++",)
+
+    @property
+    def prefix(self):
+        # GCC reports its install prefix when running ``-print-search-dirs``
+        # on the first line ``install: <prefix>``.
+        cc = spack.util.executable.Executable(self.cc)
+        with self.compiler_environment():
+            gcc_output = cc("-print-search-dirs", output=str, error=str)
+
+            for line in gcc_output.splitlines():
+                if line.startswith("install:"):
+                    gcc_prefix = line.split(":")[1].strip()
+                    # Go from <prefix>/lib/gcc/<triplet>/<version>/ to <prefix>
+                    return ancestor(gcc_prefix, 4)
+
+            raise RuntimeError(
+                "could not find install prefix of GCC from output:\n\t{}".format(gcc_output)
+            )