[ELF] Emit Verbose Asm when using --lto-emit-asm

[Pierre-vh]

Summary

D138560 was abandonned as the use case can already be covered by -Xoffload-linker --lto-emit-asm.
However the output from --lto-emit-asm doesn't have
comments like the Clang -S output.

This patch adds verbose assembly output to LLD ELF LTO
so that the resulting assembly file more closely matches Clang's.

Having comments is especially important on targets such as AMDGPU because
they contain additional information about the kernel(s) being compiled.

[MaskRay]

[ELF] tag is more common for lld/ELF changes. lto-emit-asm => --lto-emit-asm (drop quotes in the subject).

lld/test/ELF/lto/emit-asm.ll

14

16

You can drop -DAG for all CHECK lines. I don't think they make tests more maintainable.

[Pierre-vh]

Rebase, update test docs

lld/test/ELF/lto/emit-asm.ll

16

There's cat %t2.s %t2.s1 | FileCheck %s above f2 comes before f1 for that one (f2 is in %t2.s), so if I remove -DAG the test fails.
I'm not sure how lto-partitions=2 works. Is f2 always in %t2.s and f1 always in %t2.s1? If yes I think we can swap the arguments to cat & remove -DAG

[MaskRay]

lld/test/ELF/lto/emit-asm.ll

14

[Pierre-vh]

Address comment

lld/ELF/LTO.cpp

if (config->emitLLVM) {
     c.PostInternalizeModuleHook = [](size_t task, const Module &m) {
       if (std::unique_ptr<raw_fd_ostream> os =
               openLTOOutputFile(config->outputFile))
         WriteBitcodeToFile(m, *os, false);
       return false;
     };
   }
 
-  if (config->ltoEmitAsm)
+  if (config->ltoEmitAsm) {
     c.CGFileType = CGFT_AssemblyFile;
+    c.Options.MCOptions.AsmVerbose = true;
+  }
 
   if (!config->saveTempsArgs.empty())
     checkError(c.addSaveTemps(config->outputFile.str() + ".",
                               /*UseInputModulePath*/ true,
                               config->saveTempsArgs));
   return c;
 }
 

lld/test/ELF/lto/emit-asm.ll

 ; REQUIRES: x86
 ; RUN: llvm-as %s -o %t.o
 ; RUN: ld.lld --lto-emit-asm -shared %t.o -o - | FileCheck %s
 ; RUN: ld.lld --plugin-opt=emit-asm --plugin-opt=lto-partitions=2 -shared %t.o -o %t2.s
 ; RUN: cat %t2.s %t2.s1 | FileCheck %s
 
 ; RUN: ld.lld --lto-emit-asm --save-temps -shared %t.o -o %t3.s
 ; RUN: FileCheck --input-file %t3.s %s
 ; RUN: llvm-dis %t3.s.0.4.opt.bc -o - | FileCheck --check-prefix=OPT %s
 
 target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64-unknown-linux-gnu"
 
+;; Note: we also check for the presence of comments; --lto-emit-asm output should be verbose.
+
+; CHECK-DAG: # -- Begin function f1
 ; CHECK-DAG: f1:
-; OPT-DAG: define void @f1()
+; OPT: define void @f1()
 define void @f1() {
   ret void
 }
 
+; CHECK-DAG: # -- Begin function f2
 ; CHECK-DAG: f2:
-; OPT-DAG: define void @f2()
+; OPT: define void @f2()
 define void @f2() {
   ret void
 }