[llvm] cb09b5f - [MC] Disable MCAssembler based constant folding for compact unwind and emitJumpTableEntry

[Fangrui Song via llvm-commits]

Author: Fangrui Song
Date: 2024-06-04T15:06:12-07:00
New Revision: cb09b5f3d53e5b7b4452bb3db78dca79fc9b3f17

URL: https://github.com/llvm/llvm-project/commit/cb09b5f3d53e5b7b4452bb3db78dca79fc9b3f17
DIFF: https://github.com/llvm/llvm-project/commit/cb09b5f3d53e5b7b4452bb3db78dca79fc9b3f17.diff

LOG: [MC] Disable MCAssembler based constant folding for compact unwind and emitJumpTableEntry

Similar to commit 245491a9f384e4c53421196533c2a2b693efaf8d for DwarfDebug.

This completely disables the expensive MCFragment walk code in
`AttemptToFoldSymbolOffsetDifference` when compiling sqlite3.i for
macOS.

In the future, we should try enabling the MCFragment walk only for
constructs like `.if . -_start == 1` and `.subsection a-b` and
remove these `setUseAssemblerInfoForParsing`.

Added: 
    

Modified: 
    llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp
    llvm/lib/MC/MCDwarf.cpp

Removed: 
    


################################################################################
diff  --git a/llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp b/llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp
index c5755b9bdc8d0..e8bab26907b7e 100644
--- a/llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp
+++ b/llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp
@@ -2801,8 +2801,12 @@ void AsmPrinter::emitJumpTableInfo() {
     MCSymbol* JTISymbol = GetJTISymbol(JTI);
     OutStreamer->emitLabel(JTISymbol);
 
+    // Defer MCAssembler based constant folding due to a performance issue. The
+    // label 
diff erences will be evaluated at write time.
+    OutStreamer->setUseAssemblerInfoForParsing(false);
     for (const MachineBasicBlock *MBB : JTBBs)
       emitJumpTableEntry(MJTI, MBB, JTI);
+    OutStreamer->setUseAssemblerInfoForParsing(true);
   }
   if (!JTInDiffSection)
     OutStreamer->emitDataRegion(MCDR_DataRegionEnd);

diff  --git a/llvm/lib/MC/MCDwarf.cpp b/llvm/lib/MC/MCDwarf.cpp
index aba4071e6b910..f7bc3a6bbe538 100644
--- a/llvm/lib/MC/MCDwarf.cpp
+++ b/llvm/lib/MC/MCDwarf.cpp
@@ -1864,6 +1864,12 @@ void MCDwarfFrameEmitter::Emit(MCObjectStreamer &Streamer, MCAsmBackend *MAB,
   FrameEmitterImpl Emitter(IsEH, Streamer);
   ArrayRef<MCDwarfFrameInfo> FrameArray = Streamer.getDwarfFrameInfos();
 
+  // Disable AttemptToFoldSymbolOffsetDifference folding of EmitCompactUnwind
+  // and fdeStart-cieStart for EmitFDE due to the the performance issue. The
+  // label 
diff erences will be evaluate at write time.
+  assert(Streamer.getUseAssemblerInfoForParsing());
+  Streamer.setUseAssemblerInfoForParsing(false);
+
   // Emit the compact unwind info if available.
   bool NeedsEHFrameSection = !MOFI->getSupportsCompactUnwindWithoutEHFrame();
   if (IsEH && MOFI->getCompactUnwindSection()) {
@@ -1910,11 +1916,6 @@ 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 
diff erences 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;