Add -fno-verbose-asm to clang

[verhovsky]

This removes useless comments (namely repeated function/label names, <n>-byte Folded Spill and others) from the generated assembly.

Fixes #5964

[partouf]

This is maybe an Ok change, but calling in @OfekShilon and @mattgodbolt

It might be appropriate to only hide these when comments are filtered out, but not sure

[OfekShilon]

Great find! I wasn't aware of this switch. I looked in the llvm source and indeed it controls only comments, but I'm not sure all are useless. One random example (first in the list): https://github.com/llvm/llvm-project/blob/31774b6a8a88b435ce79f9ba048ef8bb00e2817e/llvm/lib/CodeGen/AsmPrinter/ARMException.cpp#L96-L110

  bool VerboseAsm = Asm->OutStreamer->isVerboseAsm();

  int Entry = 0;
  // Emit the Catch TypeInfos.
  if (VerboseAsm && !TypeInfos.empty()) {
    Asm->OutStreamer->AddComment(">> Catch TypeInfos <<");
    Asm->OutStreamer->addBlankLine();
    Entry = TypeInfos.size();
  }

  for (const GlobalValue *GV : reverse(TypeInfos)) {
    if (VerboseAsm)
      Asm->OutStreamer->AddComment("TypeInfo " + Twine(Entry--));
    Asm->emitTTypeReference(GV, TTypeEncoding);
  }

Can certainly be useful to anyone debugging ARM exceptions.

So I'd say this switch would be activated only when the comments-filter is checked, but that might require a server roundtrip where (I think?) today it isn't required. Do you think that's doable?

[mattgodbolt]

Yeah doing it when comments are filtered out? I dunno. We've had folks asking before for -fverbose-asm to be the default before now, #4130, #297, #1004

I think I'm ok to merge this, but

• perhaps a more sane/unified approach to verbosity
• there's an argument we've used before that "we do whatever the compiler does by default"

[OfekShilon]

Suggestion: maybe add a checkbox specific to this switch, defaulting to checked? Like the fno-discard-value-names in llvm ir :

That one also triggers a re-compilation so I guess it's doable.

[mattgodbolt]

That sounds like a winner, Ofek; and if we do it right fixes a number of those linked issues too!

[partouf]

Interestingly #4130, #297, #1004 are about GCC, and there it's off by default and if on - most comments are removed by comment filter

I don't know, I'm conflicted. The extra checkbox might be the only correct way to go.

[OfekShilon]

Interestingly #4130, #297, #1004 are about GCC, and there it's off by default and if on - most comments are removed by comment filter

I don't know, I'm conflicted. The extra checkbox might be the only correct way to go.

Actually I take it back - after looking at the old issues I think it would be best to indeed link -fverbose-asm/-fno-verbose-asm to the existing 'comments' checkbox.
i.e.: if it's checked - add -fno-verbose-asm to the build switches (for supporting compilers) AND apply the regular comment-cleanup-logic. If it is unchecked - add -fverbose-asm to the build switches (for supporting compilers) and don't apply comment-cleanup-logic.