gcc: add build_type and profiled variants (#30660)

Add a `build_type` variant, which allows building optimized compilers,
as well as target libraries (libstdc++ and friends).

The default is `build_type=RelWithDebInfo`, which corresponds to GCC's
default of -O2 -g.

When building with `+bootstrap %gcc`, also add Spack's arch specific
flags using the common denominator between host and new GCC.

It is done by creating a config/spack.mk file in def patch, that looks
as follows:
```
BOOT_CFLAGS := $(filter-out -O% -g%, $(BOOT_CFLAGS)) -O2 -g -march=znver2 -mtune=znver2
CFLAGS_FOR_TARGET := $(filter-out -O% -g%, $(CFLAGS_FOR_TARGET)) -O2 -g -march=znver2 -mtune=znver2
CXXFLAGS_FOR_TARGET := $(filter-out -O% -g%, $(CXXFLAGS_FOR_TARGET)) -O2 -g -march=znver2 -mtune=znver2
```

var/spack/repos/builtin/packages/gcc/package.py

@@ -8,6 +8,8 @@
 import re
 import sys
 
+from archspec.cpu import UnsupportedMicroarchitecture
+
 import llnl.util.tty as tty
 
 import spack.platforms
@@ -111,6 +113,13 @@ class Gcc(AutotoolsPackage, GNUMirrorPackage):
     variant('graphite',
             default=False,
             description='Enable Graphite loop optimizations (requires ISL)')
+    variant('build_type', default='RelWithDebInfo',
+            values=('Debug', 'Release', 'RelWithDebInfo', 'MinSizeRel'),
+            description='CMake-like build type. '
+                        'Debug: -O0 -g; Release: -O3; '
+                        'RelWithDebInfo: -O2 -g; MinSizeRel: -Os')
+    variant('profiled', default=False, description='Use Profile Guided Optimization',
+            when='+bootstrap %gcc')
 
     depends_on('flex', type='build', when='@master')
 
@@ -508,6 +517,57 @@ def patch(self):
                         'Enum(ptx_isa) Var(ptx_isa_option) Init(PTX_ISA_SM35)',
                         'gcc/config/nvptx/nvptx.opt',
                         string=True)
+        self.build_optimization_config()
+
+    def get_common_target_flags(self, spec):
+        """Get the right (but pessimistic) architecture specific flags supported by
+        both host gcc and to-be-built gcc. For example: gcc@7 %gcc@12 target=znver3
+        should pick -march=znver1, since that's what gcc@7 supports."""
+        archs = [spec.target] + spec.target.ancestors
+        for arch in archs:
+            try:
+                return arch.optimization_flags('gcc', spec.version)
+            except UnsupportedMicroarchitecture:
+                pass
+        # no arch specific flags in common, unlikely to happen.
+        return ''
+
+    def build_optimization_config(self):
+        """Write a config/spack.mk file with sensible optimization flags, taking into
+        account bootstrapping subtleties."""
+        build_type_flags = {
+            'Debug': '-O0 -g',
+            'Release': '-O3',
+            'RelWithDebInfo': '-O2 -g',
+            'MinSizeRel': '-Os'
+        }
+
+        # Generic optimization flags.
+        flags = build_type_flags[self.spec.variants['build_type'].value]
+
+        # Pessimistic target specific flags. For example, when building
+        # gcc@11 %gcc@7 on znver3, Spack will fix the target to znver1 during
+        # concretization, so we'll stick to that. The other way around however can
+        # result in compilation errors, when gcc@7 is built with gcc@11, and znver3
+        # is taken as a the target, which gcc@7 doesn't support.
+        if '+bootstrap %gcc' in self.spec:
+            flags += ' ' + self.get_common_target_flags(self.spec)
+
+        if '+bootstrap' in self.spec:
+            variables = ['BOOT_CFLAGS', 'CFLAGS_FOR_TARGET', 'CXXFLAGS_FOR_TARGET']
+        else:
+            variables = ['CFLAGS', 'CXXFLAGS']
+
+        # Redefine a few variables without losing other defaults:
+        # BOOT_CFLAGS = $(filter-out -O% -g%, $(BOOT_CFLAGS)) -O3
+        # This makes sure that build_type=Release is really -O3, not -O3 -g.
+        fmt_string = '{} := $(filter-out -O% -g%, $({})) {}\n'
+        with open('config/spack.mk', 'w') as f:
+            for var in variables:
+                f.write(fmt_string.format(var, var, flags))
+            # Improve the build time for stage 2 a bit by enabling -O1 in stage 1.
+            # Note: this is ignored under ~bootstrap.
+            f.write('STAGE1_CFLAGS += -O1\n')
 
     # https://gcc.gnu.org/install/configure.html
     def configure_args(self):
@@ -606,6 +666,7 @@ def configure_args(self):
         boot_ldflags = stage1_ldflags + ' -static-libstdc++ -static-libgcc'
         options.append('--with-stage1-ldflags=' + stage1_ldflags)
         options.append('--with-boot-ldflags=' + boot_ldflags)
+        options.append('--with-build-config=spack')
 
         return options
 
@@ -668,6 +729,12 @@ def nvptx_install(self):
             make()
             make('install')
 
+    @property
+    def build_targets(self):
+        if '+profiled' in self.spec:
+            return ['profiledbootstrap']
+        return []
+
     @property
     def install_targets(self):
         if '+strip' in self.spec: