[MC] Make UseAssemblerInfoForParsing mostly true

Commit 6c0665e22174d474050e85ca367424f6e02476be
(https://reviews.llvm.org/D45164) enabled certain constant expression
evaluation for `MCObjectStreamer` at parse time (e.g. `.if` directives,
see llvm/test/MC/AsmParser/assembler-expressions.s).

`getUseAssemblerInfoForParsing` was added to make `clang -c` handling
inline assembly similar to `MCAsmStreamer` (e.g. `llvm-mc -filetype=asm`),
where such expression folding (related to
`AttemptToFoldSymbolOffsetDifference`) is unavailable.

I believe this is overly conservative. We can make some parse-time
expression folding work for `clang -c` even if `clang -S` would still
report an error, a MCAsmStreamer issue (we cannot print `.if`
directives) that should not restrict the functionality of
MCObjectStreamer.

```
% cat b.cc
asm(R"(
.pushsection .text,"ax"
.globl _start; _start: ret
.if . -_start == 1
  ret
.endif
.popsection
)");
% gcc -S b.cc && gcc -c b.cc
% clang -S -fno-integrated-as b.cc     # succeeded

% clang -c b.cc     # succeeded with this patch
% clang -S b.cc     # still failed
<inline asm>:4:5: error: expected absolute expression
    4 | .if . -_start == 1
      |     ^
1 error generated.
```

However, removing `getUseAssemblerInfoForParsing` would make
MCDwarfFrameEmitter::Emit (for .eh_frame FDE) slow (~4% compile time
regression for sqlite3.c amalgamation) due to expensive
`AttemptToFoldSymbolOffsetDifference`. For now, make
`UseAssemblerInfoForParsing` false in MCDwarfFrameEmitter::Emit.

Close #62520
Link: https://discourse.llvm.org/t/rfc-clang-assembly-object-equivalence-for-files-with-inline-assembly/78841

Pull Request: https://github.com/llvm/llvm-project/pull/91082

1 files changed, 6 insertions, 0 deletions

diff --git a/llvm/lib/MC/MCDwarf.cpp b/llvm/lib/MC/MCDwarf.cpp
index 2ee0c3e..aba4071 100644
--- a/llvm/lib/MC/MCDwarf.cpp
+++ b/llvm/lib/MC/MCDwarf.cpp
@@ -1910,6 +1910,11 @@ void MCDwarfFrameEmitter::Emit(MCObjectStreamer &Streamer, MCAsmBackend *MAB,
                     [](const MCDwarfFrameInfo &X, const MCDwarfFrameInfo &Y) {
                       return CIEKey(X) < CIEKey(Y);
                     });
+  // Disable AttemptToFoldSymbolOffsetDifference folding of fdeStart-cieStart
+  // for EmitFDE due to the the performance issue. The label differences will be
+  // evaluate at write time.
+  assert(Streamer.getUseAssemblerInfoForParsing());
+  Streamer.setUseAssemblerInfoForParsing(false);
   for (auto I = FrameArrayX.begin(), E = FrameArrayX.end(); I != E;) {
     const MCDwarfFrameInfo &Frame = *I;
     ++I;
@@ -1930,6 +1935,7 @@ void MCDwarfFrameEmitter::Emit(MCObjectStreamer &Streamer, MCAsmBackend *MAB,
 
     Emitter.EmitFDE(*CIEStart, Frame, I == E, *SectionStart);
   }
+  Streamer.setUseAssemblerInfoForParsing(true);
 }
 
 void MCDwarfFrameEmitter::encodeAdvanceLoc(MCContext &Context,