[CodeGen] [WinException] Only produce handler data at the end of the function if needed

If we are going to write handler data (that is written as variable
length data following after the unwind info in .xdata), we need to
emit the handler data immediately, but for cases where no such
info is going to be written, skip emitting it right away. (Unwind
info for all remaining functions that hasn't gotten it emitted
directly is emitted at the end.)

This does slightly change the ordering of sections (triggering a
bunch of updates to DebugInfo/COFF tests), but the change should be
benign.

This also matches GCC's assembly output, which doesn't output
.seh_handlerdata unless it actually is needed.

For ARM64, the unwind info can be packed into the runtime function
entry itself (leaving no data in the .xdata section at all), but
that can only be done if there's no follow-on data in the .xdata
section. If emission of the unwind info is triggered via
EmitWinEHHandlerData (or the .seh_handlerdata directive), which
implicitly switches to the .xdata section, there's a chance of the
caller wanting to pass further data there, so the packed format
can't be used in that case.

Differential Revision: https://reviews.llvm.org/D87448

1 files changed, 16 insertions, 3 deletions

diff --git a/llvm/lib/CodeGen/AsmPrinter/WinException.cpp b/llvm/lib/CodeGen/AsmPrinter/WinException.cpp
index c47ac7e..08f259c 100644
--- a/llvm/lib/CodeGen/AsmPrinter/WinException.cpp
+++ b/llvm/lib/CodeGen/AsmPrinter/WinException.cpp
@@ -258,11 +258,11 @@ void WinException::endFuncletImpl() {
     if (F.hasPersonalityFn())
       Per = classifyEHPersonality(F.getPersonalityFn()->stripPointerCasts());
 
-    // Emit an UNWIND_INFO struct describing the prologue.
-    Asm->OutStreamer->EmitWinEHHandlerData();
-
     if (Per == EHPersonality::MSVC_CXX && shouldEmitPersonality &&
         !CurrentFuncletEntry->isCleanupFuncletEntry()) {
+      // Emit an UNWIND_INFO struct describing the prologue.
+      Asm->OutStreamer->EmitWinEHHandlerData();
+
       // If this is a C++ catch funclet (or the parent function),
       // emit a reference to the LSDA for the parent function.
       StringRef FuncLinkageName = GlobalValue::dropLLVMManglingEscape(F.getName());
@@ -271,9 +271,22 @@ void WinException::endFuncletImpl() {
       Asm->OutStreamer->emitValue(create32bitRef(FuncInfoXData), 4);
     } else if (Per == EHPersonality::MSVC_Win64SEH && MF->hasEHFunclets() &&
                !CurrentFuncletEntry->isEHFuncletEntry()) {
+      // Emit an UNWIND_INFO struct describing the prologue.
+      Asm->OutStreamer->EmitWinEHHandlerData();
+
       // If this is the parent function in Win64 SEH, emit the LSDA immediately
       // following .seh_handlerdata.
       emitCSpecificHandlerTable(MF);
+    } else if (shouldEmitPersonality || shouldEmitLSDA) {
+      // Emit an UNWIND_INFO struct describing the prologue.
+      Asm->OutStreamer->EmitWinEHHandlerData();
+      // In these cases, no further info is written to the .xdata section
+      // right here, but is written by e.g. emitExceptionTable in endFunction()
+      // above.
+    } else {
+      // No need to emit the EH handler data right here if nothing needs
+      // writing to the .xdata section; it will be emitted for all
+      // functions that need it in the end anyway.
     }
 
     // Switch back to the funclet start .text section now that we are done