[clang][driver] Warn when '-mno-outline-atomics' is used with a non-AArch64 triple

[nathanchance]

Summary

The Linux kernel has a make macro called cc-option that invokes the
compiler with an option in isolation to see if it is supported before
adding it to CFLAGS. The exit code of the compiler is used to determine
if the flag is supported and should be added to the compiler invocation.

A call to cc-option with '-mno-outline-atomics' was added to prevent
linking errors with newer GCC versions but this call succeeds with a
non-AArch64 target because there is no warning from clang with
'-mno-outline-atomics', just '-moutline-atomics'. Because the call
succeeds and adds '-mno-outline-atomics' to the compiler invocation,
there is a warning from LLVM because the 'outline-atomics target
feature is only supported by the AArch64 backend.

$ echo | clang -target x86_64 -moutline-atomics -Werror -x c -c -o /dev/null -
clang-14: error: The 'x86_64' architecture does not support -moutline-atomics; flag ignored [-Werror,-Woption-ignored]

$ echo $?
1

$ echo | clang -target x86_64 -mno-outline-atomics -Werror -x c -c -o /dev/null -
'-outline-atomics' is not a recognized feature for this target (ignoring feature)

$ echo $?
0

This does not match GCC's behavior, which errors when the flag is added
to a non-AArch64 target.

$ echo | gcc -moutline-atomics -x c -c -o /dev/null -
gcc: error: unrecognized command-line option ‘-moutline-atomics’; did you mean ‘-finline-atomics’?

$ echo | gcc -mno-outline-atomics -x c -c -o /dev/null -
gcc: error: unrecognized command-line option ‘-mno-outline-atomics’; did you mean ‘-fno-inline-atomics’?

$ echo | aarch64-linux-gnu-gcc -moutline-atomics -x c -c -o /dev/null -

$ echo | aarch64-linux-gnu-gcc -mno-outline-atomics -x c -c -o /dev/null -

To get closer to GCC's behavior, issue a warning when
'-mno-outline-atomics' is used without an AArch64 triple and do not add
'{-,+}outline-atomic" to the list of target features in these cases.

Link: https://github.com/ClangBuiltLinux/linux/issues/1552

[nathanchance]

If there are more qualified people to review and accept this, please add them, I was unsure of who else to add.

[nickdesaulniers]

clang/test/Driver/x86-outline-atomics.c

1–7

If clang/test/Driver/aarch64-features.c is only testing outline atomics, I wonder if this test should actually go in there? @t.p.northover , thoughts? Otherwise LGTM and thanks for the patch!

[nathanchance]

clang/test/Driver/x86-outline-atomics.c

1–7

I did consider that initially but:

1. It does appear to test a few other AArch64 specific things.
2. It seemed weird to add an x86_64 test to an AArch64 file.

No strong opinions though, I'll go along with whatever works and makes the majority happy.

[melver]

clang/test/Driver/x86-outline-atomics.c

1–7

There is a similar precedent in clang/test/Driver/aarch64-outliner.c, although maybe that should have been in aarch64-features.c

However, the fact it's called x86-outline-atomics.c makes me think that x86 supports outline atomics.

Other similar tests appear in clang/test/Driver/unsupported-*.

My suggestion is to call it unsupported-outline-atomics.c.

Furthermore, this shouldn't just warn on x86, so one more arch (say riscv or ppc?) would be useful to check as well.

[nathanchance]

• Move test from x86-outline-atomics.c to unsupported-outline-atomics.c (Marco).

• Add a test for riscv64 to ensure it works with multiple architectures (Marco).

clang/include/clang/Basic/DiagnosticDriverKinds.td

def warn_drv_global_isel_incomplete_opt : Warning<
   "-fglobal-isel support is incomplete for this architecture at the current optimization level">,
   InGroup<GlobalISel>;
 
 def warn_drv_moutline_unsupported_opt : Warning<
   "'%0' does not support '-moutline'; flag ignored">,
   InGroup<OptionIgnored>;
 
 def warn_drv_moutline_atomics_unsupported_opt : Warning<
-  "'%0' does not support '-moutline-atomics'; flag ignored">,
+  "'%0' does not support '-%1'; flag ignored">,
   InGroup<OptionIgnored>;
 
 def warn_drv_darwin_sdk_invalid_settings : Warning<
   "SDK settings were ignored as 'SDKSettings.json' could not be parsed">,
   InGroup<DiagGroup<"darwin-sdk-settings">>;
 
 def err_drv_trivial_auto_var_init_zero_disabled : Error<
   "'-ftrivial-auto-var-init=zero' hasn't been enabled; enable it at your own "

clang/lib/Driver/ToolChains/Clang.cpp

 //===-- Clang.cpp - Clang+LLVM ToolChain Implementations --------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 

   if (!Args.hasFlag(options::OPT_fcxx_static_destructors,
                     options::OPT_fno_cxx_static_destructors, true))
     CmdArgs.push_back("-fno-c++-static-destructors");
 
   addMachineOutlinerArgs(D, Args, CmdArgs, Triple, /*IsLTO=*/false);
 
   if (Arg *A = Args.getLastArg(options::OPT_moutline_atomics,
                                options::OPT_mno_outline_atomics)) {
-    if (A->getOption().matches(options::OPT_moutline_atomics)) {
-      // Option -moutline-atomics supported for AArch64 target only.
-      if (!Triple.isAArch64()) {
-        D.Diag(diag::warn_drv_moutline_atomics_unsupported_opt)
-            << Triple.getArchName();
-      } else {
+          << Triple.getArchName() << A->getOption().getName();
+      if (A->getOption().matches(options::OPT_moutline_atomics)) {
         CmdArgs.push_back("-target-feature");
         CmdArgs.push_back("+outline-atomics");
-      }
       } else {
         CmdArgs.push_back("-target-feature");
         CmdArgs.push_back("-outline-atomics");
       }
+    }
   } else if (Triple.isAArch64() &&
              getToolChain().IsAArch64OutlineAtomicsDefault(Args)) {
     CmdArgs.push_back("-target-feature");
     CmdArgs.push_back("+outline-atomics");
   }
 
   if (Args.hasFlag(options::OPT_faddrsig, options::OPT_fno_addrsig,
                    (TC.getTriple().isOSBinFormatELF() ||

clang/test/Driver/x86-outline-atomics.c

+// RUN: %clang -target x86_64 -moutline-atomics -S %s -### 2>&1 | FileCheck %s -check-prefix=CHECK-OUTLINE-ATOMICS
+// CHECK-OUTLINE-ATOMICS: warning: 'x86_64' does not support '-moutline-atomics'; flag ignored [-Woption-ignored]
+// CHECK-OUTLINE-ATOMICS-NOT: "-target-feature" "+outline-atomics"
+
+// RUN: %clang -target x86_64 -mno-outline-atomics -S %s -### 2>&1 | FileCheck %s -check-prefix=CHECK-NO-OUTLINE-ATOMICS
+// CHECK-NO-OUTLINE-ATOMICS: warning: 'x86_64' does not support '-mno-outline-atomics'; flag ignored [-Woption-ignored]
+// CHECK-NO-OUTLINE-ATOMICS-NOT: "-target-feature" "-outline-atomics"