build: Add LLVM_WINSYSROOT to make setting /winsysroot easy on Win

[thakis]

Summary

Also add a script for sysroot management. For now, it can only create
fake sysroots that just symlink to local folders. This is useful for
testing.

[thakis]

tiny tweak: rebase_path() sysroot in gn

[hans]

llvm/utils/sysroot.py

4

Can you expand on what "manage" could mean in the future? The current filename is pretty general, so if the main purpose is to set up sysroots (fake or not) maybe a better name would be make_sysroot.py?

[thakis]

llvm/utils/sysroot.py

4

I was thinking (at least on linux): creating, uploading a created sysroot, downloading a pre-uploaded sysroot. Hence the general name :)

[hans]

llvm/utils/sysroot.py

4

Ah, yes that sounds pretty general. Maybe put some of that in the comment here?

[thakis]

llvm/utils/sysroot.py

4

I thought I'd update the comment once more stuff works if that's ok.

llvm/cmake/modules/HandleLLVMOptions.cmake

add_definitions(
     )
 
   # Tell MSVC to use the Unicode version of the Win32 APIs instead of ANSI.
   add_definitions(
     -DUNICODE
     -D_UNICODE
   )
 
+  # Allow setting clang-cl's /winsysroot flag.
+  set(LLVM_WINSYSROOT "" CACHE STRING
+    "If set, argument to clang-cl's /winsysroot")
+  if (LLVM_WINSYSROOT)
+    if (NOT CLANG_CL)
+      message(ERROR "LLVM_WINSYSROOT requires clang-cl")
+    endif()
+    append("/winsysroot${LLVM_WINSYSROOT}" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
+  endif()
+
   if (LLVM_ENABLE_WERROR)
     append("/WX" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
   endif (LLVM_ENABLE_WERROR)
 
   append("/Zc:inline" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
 
   # Some projects use the __cplusplus preprocessor macro to check support for
   # a particular version of the C++ standard. When this option is not specified

llvm/utils/gn/build/BUILD.gn

declare_args() {
   # most debuggers harder to use. See "Getting to local determinism" and
   # "Getting debuggers to work well with locally deterministic builds" in
   # http://blog.llvm.org/2019/11/deterministic-builds-with-clang-and-lld.html
   # for more information.
   use_relative_paths_in_debug_info = false
 
   # The version of host gcc. Ignored if is_clang is true.
   gcc_version = 9
+
+  # Path of sysroot to use.
+  sysroot = ""
 }
 
 assert(!llvm_build_instrumented_coverage || is_clang,
        "llvm_build_instrumented_coverage requires clang as host compiler")
 
 config("compiler_defaults") {
   defines = []
 
if (is_clang) {
     }
     if (use_relative_paths_in_debug_info) {
       cflags += [
         "-fdebug-compilation-dir",
         ".",
       ]
     }
   }
+  if (sysroot != "") {
+    assert(current_os == "win", "FIXME: Make sysroot work on non-win")
+    assert(is_clang, "sysroot only works with clang-cl as host compiler")
+    cflags += [ "/winsysroot" + rebase_path(sysroot, root_build_dir) ]
+  }
 
   if (use_ubsan) {
     assert(is_clang && current_os == "linux",
            "ubsan only supported on Linux/Clang")
     cflags += [ "-fsanitize=undefined" ]
     ldflags += [ "-fsanitize=undefined" ]
   }
 

llvm/utils/sysroot.py

+#!/usr/bin/env python3
+
+"""Helps manage sysroots."""
+
+import argparse
+import os
+import subprocess
+import sys
+
+
+def make_fake_sysroot(out_dir):
+    def cmdout(cmd):
+        return subprocess.check_output(cmd).decode(sys.stdout.encoding).strip()
+
+    if sys.platform == 'win32':
+        p = os.getenv('ProgramFiles(x86)', 'C:\\Program Files (x86)')
+
+        winsdk = os.getenv('WindowsSdkDir')
+        if not winsdk:
+            winsdk = os.path.join(p, 'Windows Kits', '10')
+            print('%WindowsSdkDir% not set. You might want to run this from')
+            print('a Visual Studio cmd prompt. Defaulting to', winsdk)
+
+        vswhere = os.path.join(
+                p, 'Microsoft Visual Studio', 'Installer', 'vswhere')
+        vcid = 'Microsoft.VisualStudio.Component.VC.Tools.x86.x64'
+        vsinstalldir = cmdout(
+                [vswhere, '-latest', '-products', '*', '-requires', vcid,
+                    '-property', 'installationPath'])
+
+        def mkjunction(dst, src):
+            subprocess.check_call(['mklink', '/j', dst, src], shell=True)
+        os.mkdir(out_dir)
+        mkjunction(os.path.join(out_dir, 'VC'),
+                   os.path.join(vsinstalldir, 'VC'))
+        os.mkdir(os.path.join(out_dir, 'Windows Kits')) 
+        mkjunction(os.path.join(out_dir, 'Windows Kits', '10'), winsdk)
+    else:
+        assert False, "FIXME: Implement on non-win"
+
+    print('Done.')
+    if sys.platform == 'win32':
+        # CMake doesn't like backslashes in commandline args.
+        abs_out_dir = os.path.abspath(out_dir).replace(os.path.sep, '/')
+        print('Pass -DLLVM_WINSYSROOT=' + abs_out_dir + ' to cmake.')
+    else:
+        print('Pass -DCMAKE_SYSROOT=' + abs_out_dir + ' to cmake.')
+
+
+def main():
+    parser = argparse.ArgumentParser(description=__doc__)
+
+    subparsers = parser.add_subparsers(dest='command', required=True)
+
+    makefake = subparsers.add_parser('make-fake',
+            help='Create a sysroot that symlinks to local directories.')
+    makefake.add_argument('--out-dir', required=True)
+
+    args = parser.parse_args()
+
+    assert args.command == 'make-fake'
+    make_fake_sysroot(args.out_dir)
+
+
+if __name__ == '__main__':
+    main()