gn build: Build clang with -fno-strict-aliasing, make building with gcc much quieter

[thakis]

Summary

• gcc doesn't understand -Wstring-conversion, so pass that only to clang
• disable a few gcc warnings that are noisy and also disabled in the cmake build
• -Wstrict-aliasing pointed out that the cmake build builds clang with -fno-strict-aliasing, so do that too

[thakis]

I'm getting a bunch of

[1593/1698] CXX stage2_unix/obj/compiler-rt/lib/sanitizer_common/sources.sanitizer_platform_limits_solaris.o
warning: unknown warning option '-Wno-maybe-uninitialized'; did you mean '-Wno-uninitialized'? [-Wunknown-warning-option]
1 warning generated.

in my local build with this patch applied. Does the stage2_unix toolchain set is_clang to true? If not, it probably should?

[thakis]

In D57191#1370152, @thakis wrote:

I'm getting a bunch of

[1593/1698] CXX stage2_unix/obj/compiler-rt/lib/sanitizer_common/sources.sanitizer_platform_limits_solaris.o
warning: unknown warning option '-Wno-maybe-uninitialized'; did you mean '-Wno-uninitialized'? [-Wunknown-warning-option]
1 warning generated.

in my local build with this patch applied. Does the stage2_unix toolchain set is_clang to true? If not, it probably should?

(Still, this is a big net noise reduction, even as-is.)

[pcc]

LGTM

In D57191#1370152, @thakis wrote:

I'm getting a bunch of

[1593/1698] CXX stage2_unix/obj/compiler-rt/lib/sanitizer_common/sources.sanitizer_platform_limits_solaris.o
warning: unknown warning option '-Wno-maybe-uninitialized'; did you mean '-Wno-uninitialized'? [-Wunknown-warning-option]
1 warning generated.

in my local build with this patch applied. Does the stage2_unix toolchain set is_clang to true? If not, it probably should?

Agreed, sent D57202.

llvm/utils/gn/build/BUILD.gn

153 ↗

(On Diff #183393)

current_os?

[thakis]

Thanks!

llvm/utils/gn/build/BUILD.gn

153 ↗

(On Diff #183393)

The file uses host_os for the other win checks too. I agree it's wrong, but I want to keep the file self-consistent until I get around to setting up a win/cross build.

llvm/trunk/utils/gn/build/BUILD.gn

if (host_os == "win" && !is_clang) {
     if (host_os == "win") {
       cflags += [ "/W4" ]
     } else {
       cflags += [
         "-Wall",
         "-Wextra",
       ]
     }
+    cflags += [ "-Wno-unused-parameter" ]
+    if (is_clang) {
       cflags += [
-      "-Wno-unused-parameter",
+        "-Wdelete-non-virtual-dtor",
         "-Wstring-conversion",
       ]
-    if (is_clang) {
-      cflags += [ "-Wdelete-non-virtual-dtor" ]
+    } else {
+      cflags += [
+        # GCC's -Wcomment complains about // comments ending with '\' if the
+        # next line is also a // comment.
+        "-Wno-comment",
+
+        # Disable gcc's potentially uninitialized use analysis as it presents
+        # lots of false positives.
+        "-Wno-maybe-uninitialized",
+      ]
+      cflags_cc += [
+        # The LLVM libraries have no stable C++ API, so -Wnoexcept-type is not
+        # useful.
+        "-Wno-noexcept-type",
+      ]
     }
     if (is_clang && use_goma) {
       # goma converts all paths to lowercase on the server, breaking this
       # warning.
       cflags += [ "-Wno-nonportable-include-path" ]
     }
   }
 

 config("lld_code") {
   include_dirs = [
     "//lld/include",
     "$root_gen_dir/lld/include",
   ]
 }
 
 config("clang_code") {
+  if (host_os != "win") {
+    cflags = [ "-fno-strict-aliasing" ]
+  }
   include_dirs = [
     "//clang/include",
     "$root_gen_dir/clang/include",
   ]
 }
 
 config("crt_code") {
   include_dirs = [ "//compiler-rt/lib" ]