[MC] Make UseAssemblerInfoForParsing mostly true

Commit 6c0665e
(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 llvm#62520
Link: https://discourse.llvm.org/t/rfc-clang-assembly-object-equivalence-for-files-with-inline-assembly/78841

Pull Request: llvm#91082

Author: MaskRay

clang/tools/driver/cc1as_main.cpp

@@ -576,9 +576,6 @@ static bool ExecuteAssemblerImpl(AssemblerInvocation &Opts,
     Str.get()->emitZeros(1);
   }
 
-  // Assembly to object compilation should leverage assembly info.
-  Str->setUseAssemblerInfoForParsing(true);
-
   bool Failed = false;
 
   std::unique_ptr<MCAsmParser> Parser(

llvm/include/llvm/MC/MCStreamer.h

@@ -245,7 +245,7 @@ class MCStreamer {
   /// requires.
   unsigned NextWinCFIID = 0;
 
-  bool UseAssemblerInfoForParsing;
+  bool UseAssemblerInfoForParsing = true;
 
   /// Is the assembler allowed to insert padding automatically?  For
   /// correctness reasons, we sometimes need to ensure instructions aren't
@@ -296,6 +296,8 @@ class MCStreamer {
 
   MCContext &getContext() const { return Context; }
 
+  // MCObjectStreamer has an MCAssembler and allows more expression folding at
+  // parse time.
   virtual MCAssembler *getAssemblerPtr() { return nullptr; }
 
   void setUseAssemblerInfoForParsing(bool v) { UseAssemblerInfoForParsing = v; }

llvm/lib/CodeGen/AsmPrinter/AsmPrinterInlineAsm.cpp

@@ -102,9 +102,6 @@ void AsmPrinter::emitInlineAsm(StringRef Str, const MCSubtargetInfo &STI,
   std::unique_ptr<MCAsmParser> Parser(
       createMCAsmParser(SrcMgr, OutContext, *OutStreamer, *MAI, BufNum));
 
-  // Do not use assembler-level information for parsing inline assembly.
-  OutStreamer->setUseAssemblerInfoForParsing(false);
-
   // We create a new MCInstrInfo here since we might be at the module level
   // and not have a MachineFunction to initialize the TargetInstrInfo from and
   // we only need MCInstrInfo for asm parsing. We create one unconditionally

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,

llvm/lib/MC/MCObjectStreamer.cpp

@@ -40,9 +40,6 @@ MCObjectStreamer::MCObjectStreamer(MCContext &Context,
 
 MCObjectStreamer::~MCObjectStreamer() = default;
 
-// AssemblerPtr is used for evaluation of expressions and causes
-// difference between asm and object outputs. Return nullptr to in
-// inline asm mode to limit divergence to assembly inputs.
 MCAssembler *MCObjectStreamer::getAssemblerPtr() {
   if (getUseAssemblerInfoForParsing())
     return Assembler.get();

llvm/lib/MC/MCStreamer.cpp

@@ -93,7 +93,7 @@ void MCTargetStreamer::emitAssignment(MCSymbol *Symbol, const MCExpr *Value) {}
 
 MCStreamer::MCStreamer(MCContext &Ctx)
     : Context(Ctx), CurrentWinFrameInfo(nullptr),
-      CurrentProcWinFrameInfoStartIndex(0), UseAssemblerInfoForParsing(false) {
+      CurrentProcWinFrameInfoStartIndex(0) {
   SectionStack.push_back(std::pair<MCSectionSubPair, MCSectionSubPair>());
 }
 

llvm/lib/Target/AMDGPU/AMDGPUAsmPrinter.cpp

@@ -517,12 +517,9 @@ bool AMDGPUAsmPrinter::runOnMachineFunction(MachineFunction &MF) {
 
   DumpCodeInstEmitter = nullptr;
   if (STM.dumpCode()) {
-    // For -dumpcode, get the assembler out of the streamer, even if it does
-    // not really want to let us have it. This only works with -filetype=obj.
-    bool SaveFlag = OutStreamer->getUseAssemblerInfoForParsing();
-    OutStreamer->setUseAssemblerInfoForParsing(true);
+    // For -dumpcode, get the assembler out of the streamer. This only works
+    // with -filetype=obj.
     MCAssembler *Assembler = OutStreamer->getAssemblerPtr();
-    OutStreamer->setUseAssemblerInfoForParsing(SaveFlag);
     if (Assembler)
       DumpCodeInstEmitter = Assembler->getEmitterPtr();
   }

llvm/lib/Target/SPIRV/SPIRVAsmPrinter.cpp

@@ -114,12 +114,9 @@ void SPIRVAsmPrinter::emitEndOfAsmFile(Module &M) {
   // Bound is an approximation that accounts for the maximum used register
   // number and number of generated OpLabels
   unsigned Bound = 2 * (ST->getBound() + 1) + NLabels;
-  bool FlagToRestore = OutStreamer->getUseAssemblerInfoForParsing();
-  OutStreamer->setUseAssemblerInfoForParsing(true);
   if (MCAssembler *Asm = OutStreamer->getAssemblerPtr())
     Asm->setBuildVersion(static_cast<MachO::PlatformType>(0), Major, Minor,
                          Bound, VersionTuple(Major, Minor, 0, Bound));
-  OutStreamer->setUseAssemblerInfoForParsing(FlagToRestore);
 }
 
 void SPIRVAsmPrinter::emitFunctionHeader() {

llvm/test/MC/AsmParser/assembler-expressions-inlineasm.ll

@@ -1,13 +1,17 @@
-; RUN: not llc -mtriple x86_64-unknown-linux-gnu -o %t.s -filetype=asm %s 2>&1 | FileCheck %s
-; RUN: not llc -mtriple x86_64-unknown-linux-gnu -o %t.o -filetype=obj %s 2>&1 | FileCheck %s
-
-; Assembler-aware expression evaluation should be disabled in inline
-; assembly to prevent differences in behavior between object and
-; assembly output.
+; RUN: not llc -mtriple=x86_64 %s -o /dev/null 2>&1 | FileCheck %s
+; RUN: llc -mtriple=x86_64 -no-integrated-as < %s | FileCheck %s --check-prefix=GAS
+; RUN: llc -mtriple=x86_64 -filetype=obj %s -o - | llvm-objdump -d - |  FileCheck %s --check-prefix=DISASM
 
+; GAS: nop;  .if . - foo==1; nop;.endif
 
 ; CHECK: <inline asm>:1:17: error: expected absolute expression
 
+; DISASM:      <main>:
+; DISASM-NEXT:   nop
+; DISASM-NEXT:   nop
+; DISASM-NEXT:   xorl %eax, %eax
+; DISASM-NEXT:   retq
+
 define i32 @main() local_unnamed_addr {
   tail call void asm sideeffect "foo: nop;  .if . - foo==1;  nop;.endif", "~{dirflag},~{fpsr},~{flags}"()
   ret i32 0

llvm/tools/llvm-mc/llvm-mc.cpp

@@ -569,9 +569,6 @@ int main(int argc, char **argv) {
       Str->initSections(true, *STI);
   }
 
-  // Use Assembler information for parsing.
-  Str->setUseAssemblerInfoForParsing(true);
-
   int Res = 1;
   bool disassemble = false;
   switch (Action) {