Add support for the CompCert C Compiler

* Add preliminary support for the CompCert C Compiler

The intention is to use this with the picolibc, so some GCC flags are
automatically filtered. Since CompCert uses GCC is for linking, those
GCC-linker flags which are used by picolibc, are automatically prefixed
with '-WUl', so that they're passed to GCC.

Squashed commit of the following:

commit 4e0ad66dca9de301d2e41e74aea4142afbd1da7d
Author: Sebastian Meyer <meyer@absint.com>
Date:   Mon Aug 31 14:20:39 2020 +0200

    remove '-fall' from default arguments, also filter -ftls-model=.*

commit 41afa3ccc62ae72824eb319cb8b34b7e6693cb67
Author: Sebastian Meyer <meyer@absint.com>
Date:   Mon Aug 31 14:13:55 2020 +0200

    use regex for filtering ccomp args

commit d68d242d0ad22f8bf53923ce849da9b86b696a75
Author: Sebastian Meyer <meyer@absint.com>
Date:   Mon Aug 31 13:54:36 2020 +0200

    filter some gcc arguments

commit 982a01756266bddbbd211c54e8dbfa2f43dec38f
Author: Sebastian Meyer <meyer@absint.com>
Date:   Fri Aug 28 15:03:14 2020 +0200

    fix ccomp meson configuration

commit dce0bea00b1caa094b1ed0c6c77cf6c12f0f58d9
Author: Sebastian Meyer <meyer@absint.com>
Date:   Thu Aug 27 13:02:19 2020 +0200

    add CompCert to meson (does not fully work, yet)

* remove unused import and s/cls/self/

fixes the two obvious LGTM warnings

* CompCert: Do not ignore unsupported GCC flags

Some are safe to ignore, however, as per
https://github.com/mesonbuild/meson/pull/7674, they should not be
ignored by meson itself. Instead the meson.build should take care to
select only those which are actually supported by the compiler.

* remove unused variable

* Only add arguments once.

* Apply suggestions from code review

Co-authored-by: Dylan Baker <dylan@pnwbakers.com>

* Remove erroneous ' ' from '-o {}'.format()

As noticed by @dcbaker

* added release note snippet for compcert

* properly split parameters

As suggested by @dcbaker, these parameters should be properly split into multiple strings.

Co-authored-by: Dylan Baker <dylan@pnwbakers.com>

* Update add_compcert_compiler.md

Added a sentence about the state of the implementation (experimental); use proper markdown

* properly separate arguments

Co-authored-by: Dylan Baker <dylan@pnwbakers.com>

3 files changed, 160 insertions, 0 deletions

diff --git a/mesonbuild/compilers/__init__.py b/mesonbuild/compilers/__init__.py
index efae0ef..ec5d30f 100644
--- a/mesonbuild/compilers/__init__.py
+++ b/mesonbuild/compilers/__init__.py
@@ -97,6 +97,7 @@ __all__ = [
     'CcrxCCompiler',
     'CcrxCPPCompiler',
     'Xc16CCompiler',
+    'CompCertCCompiler',
     'C2000CCompiler',
     'C2000CPPCompiler',
     'SunFortranCompiler',
@@ -145,6 +146,7 @@ from .c import (
     PGICCompiler,
     CcrxCCompiler,
     Xc16CCompiler,
+    CompCertCCompiler,
     C2000CCompiler,
     VisualStudioCCompiler,
 )
diff --git a/mesonbuild/compilers/c.py b/mesonbuild/compilers/c.py
index 13ca1d4..5cfa06c 100644
--- a/mesonbuild/compilers/c.py
+++ b/mesonbuild/compilers/c.py
@@ -22,6 +22,7 @@ from .c_function_attributes import C_FUNC_ATTRIBUTES
 from .mixins.clike import CLikeCompiler
 from .mixins.ccrx import CcrxCompiler
 from .mixins.xc16 import Xc16Compiler
+from .mixins.compcert import CompCertCompiler
 from .mixins.c2000 import C2000Compiler
 from .mixins.arm import ArmCompiler, ArmclangCompiler
 from .mixins.visualstudio import MSVCCompiler, ClangClCompiler
@@ -554,6 +555,36 @@ class Xc16CCompiler(Xc16Compiler, CCompiler):
             path = '.'
         return ['-I' + path]
 
+class CompCertCCompiler(CompCertCompiler, CCompiler):
+    def __init__(self, exelist, version, for_machine: MachineChoice,
+                 is_cross, info: 'MachineInfo', exe_wrapper=None, **kwargs):
+        CCompiler.__init__(self, exelist, version, for_machine, is_cross,
+                           info, exe_wrapper, **kwargs)
+        CompCertCompiler.__init__(self)
+
+    def get_options(self):
+        opts = CCompiler.get_options(self)
+        opts.update({'c_std': coredata.UserComboOption('C language standard to use',
+                                                       ['none', 'c89', 'c99'],
+                                                       'none')})
+        return opts
+
+    def get_option_compile_args(self, options):
+        return []
+
+    def get_no_optimization_args(self):
+        return ['-O0']
+
+    def get_output_args(self, target):
+        return ['-o{}'.format(target)]
+
+    def get_werror_args(self):
+        return ['-Werror']
+
+    def get_include_args(self, path, is_system):
+        if path == '':
+            path = '.'
+        return ['-I' + path]
 
 class C2000CCompiler(C2000Compiler, CCompiler):
     def __init__(self, exelist, version, for_machine: MachineChoice,
diff --git a/mesonbuild/compilers/mixins/compcert.py b/mesonbuild/compilers/mixins/compcert.py
new file mode 100644
index 0000000..ebcccf5
--- /dev/null
+++ b/mesonbuild/compilers/mixins/compcert.py
@@ -0,0 +1,127 @@
+# Copyright 2012-2019 The Meson development team
+
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+
+#     http://www.apache.org/licenses/LICENSE-2.0
+
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Representations specific to the CompCert C compiler family."""
+
+import os
+import re
+import typing as T
+
+if T.TYPE_CHECKING:
+    from ...environment import Environment
+
+ccomp_buildtype_args = {
+    'plain': [''],
+    'debug': ['-O0', '-g'],
+    'debugoptimized': ['-O0', '-g'],
+    'release': ['-03'],
+    'minsize': ['-Os'],
+    'custom': ['-Obranchless'],
+}  # type: T.Dict[str, T.List[str]]
+
+ccomp_optimization_args = {
+    '0': ['-O0'],
+    'g': ['-O0'],
+    '1': ['-O1'],
+    '2': ['-O2'],
+    '3': ['-O3'],
+    's': ['-Os']
+}  # type: T.Dict[str, T.List[str]]
+
+ccomp_debug_args = {
+    False: [],
+    True: ['-g']
+}  # type: T.Dict[bool, T.List[str]]
+
+# As of CompCert 20.04, these arguments should be passed to the underlying gcc linker (via -WUl,<arg>)
+# There are probably (many) more, but these are those used by picolibc
+ccomp_args_to_wul = [
+        r"^-ffreestanding$",
+        r"^-r$"
+] # type: T.List[str]
+
+class CompCertCompiler:
+    def __init__(self) -> None:
+        self.id = 'ccomp'
+        # Assembly
+        self.can_compile_suffixes.add('s')
+        default_warn_args = []  # type: T.List[str]
+        self.warn_args = {'0': [],
+                          '1': default_warn_args,
+                          '2': default_warn_args + [],
+                          '3': default_warn_args + []}
+
+    def get_always_args(self) -> T.List[str]:
+        return []
+
+    def get_pic_args(self) -> T.List[str]:
+        # As of now, CompCert does not support PIC
+        return []
+
+    def get_buildtype_args(self, buildtype: str) -> T.List[str]:
+        return ccomp_buildtype_args[buildtype]
+
+    def get_pch_suffix(self) -> str:
+        return 'pch'
+
+    def get_pch_use_args(self, pch_dir: str, header: str) -> T.List[str]:
+        return []
+
+    def unix_args_to_native(self, args):
+        "Always returns a copy that can be independently mutated"
+        patched_args = []
+        for arg in args:
+            added = 0
+            for ptrn in ccomp_args_to_wul:
+                if re.match(ptrn, arg):
+                    patched_args.append('-WUl,' + arg)
+                    added = 1
+            if not added:
+                patched_args.append(arg)
+        return patched_args
+
+    # Override CCompiler.get_dependency_gen_args
+    def get_dependency_gen_args(self, outtarget: str, outfile: str) -> T.List[str]:
+        return []
+
+    def thread_flags(self, env: 'Environment') -> T.List[str]:
+        return []
+
+    def get_preprocess_only_args(self) -> T.List[str]:
+        return ['-E']
+
+    def get_compile_only_args(self) -> T.List[str]:
+        return ['-c']
+
+    def get_coverage_args(self) -> T.List[str]:
+        return []
+
+    def get_no_stdinc_args(self) -> T.List[str]:
+        return ['-nostdinc']
+
+    def get_no_stdlib_link_args(self) -> T.List[str]:
+        return ['-nostdlib']
+
+    def get_optimization_args(self, optimization_level: str) -> T.List[str]:
+        return ccomp_optimization_args[optimization_level]
+
+    def get_debug_args(self, is_debug: bool) -> T.List[str]:
+        return ccomp_debug_args[is_debug]
+
+    def compute_parameters_with_absolute_paths(self, parameter_list: T.List[str], build_dir: str) -> T.List[str]:
+        for idx, i in enumerate(parameter_list):
+            if i[:9] == '-I':
+                parameter_list[idx] = i[:9] + os.path.normpath(os.path.join(build_dir, i[9:]))
+
+        return parameter_list