[COFF] Keep using associative comdats for xdata/pdata for MinGW

[mstorsjo]

Summary

Using the symbol name as suffix for the section name is enough to keep things working for ld.bfd, but making them proper associative comdats helps LLD to bring them in when necessary.

[pcc]

I'd prefer if we did something like this rather than changing the linker. I see gcc's non-conformance with the coff spec as a bug and if there aren't any real users of gcc+lld I'd prefer us not to have workarounds for that case.

[mstorsjo]

As tested by @rnk in D49700 (in the comment https://reviews.llvm.org/D49700#1177295), this doesn't actually cause these sections to be deduplicated when using ld.bfd, so this doesn't work as intended/hoped.

lib/MC/MCStreamer.cpp

static MCSection *getWinCFISection(MCContext &Context, unsigned *NextWinCFIID,
   unsigned UniqueID = TextSecCOFF->getOrAssignWinCFISectionID(NextWinCFIID);
 
   // If this section is COMDAT, this unwind section should be COMDAT associative
   // with its group.
   const MCSymbol *KeySym = nullptr;
   if (TextSecCOFF->getCharacteristics() & COFF::IMAGE_SCN_LNK_COMDAT) {
     KeySym = TextSecCOFF->getCOMDATSymbol();
 
-    // In a GNU environment, we can't use associative comdats. Instead, do what
-    // GCC does, which is to make plain comdat selectany section named like
-    // ".[px]data$_Z3foov".
+    // In a GNU environment, we can't rely on associative comdats. Instead, do
+    // almost what GCC does, which is to make plain comdat selectany section
+    // named like ".[px]data$_Z3foov". We make them associative though, to help
+    // LLD keep them if the function itself is kept, while the symbol suffixed
+    // section name is enough for make things work for ld.bfd.
     if (!Context.getAsmInfo()->hasCOFFAssociativeComdats()) {
       std::string SectionName =
           (MainCFISecCOFF->getSectionName() + "$" +
            TextSecCOFF->getSectionName().split('$').second)
               .str();
       return Context.getCOFFSection(
           SectionName,
           MainCFISecCOFF->getCharacteristics() | COFF::IMAGE_SCN_LNK_COMDAT,
-          MainCFISecCOFF->getKind(), "", COFF::IMAGE_COMDAT_SELECT_ANY);
+          MainCFISecCOFF->getKind(), KeySym->getName(),
+          COFF::IMAGE_COMDAT_SELECT_ASSOCIATIVE);
     }
   }
 
   return Context.getAssociativeCOFFSection(MainCFISecCOFF, KeySym, UniqueID);
 }
 
 MCSection *MCStreamer::getAssociatedPDataSection(const MCSection *TextSec) {
   return getWinCFISection(getContext(), &NextWinCFIID,

test/CodeGen/X86/mingw-comdats-xdata.ll

return:                                           ; preds = %lpad, %invoke.cont
   ret i32 %retval.0
 }
 
 ; The .xdata section below doesn't have the usual comdat symbol attached, which requires
 ; a different syntax for the assembly output.
 
 ; GNU: .section        .text$_Z3fooi,"xr",discard,_Z3fooi
 ; GNU: _Z3fooi:
-; GNU: .section        .xdata$_Z3fooi,"dr"
-; GNU: .linkonce       discard
+; GNU: .section        .xdata$_Z3fooi,"dr",associative,_Z3fooi
 ; GNU: GCC_except_table1:
 ; GNU: .section        .data$gv,"dw",discard,gv
 ; GNU: gv:
 ; GNU: .long 42
 
 ; Make sure the assembler puts the .xdata and .pdata in sections with the right
 ; names.
 ; GNUOBJ: .text$_Z3fooi
-; GNUOBJ: .xdata$_Z3fooi
 ; GNUOBJ: .data$gv
+; GNUOBJ: .xdata$_Z3fooi
 ; GNUOBJ: .pdata$_Z3fooi
 
 declare dso_local i32 @_Z3bari(i32)
 
 declare dso_local i32 @__gxx_personality_seh0(...)
 
 declare dso_local i8* @__cxa_begin_catch(i8*) local_unnamed_addr
 
 declare dso_local void @__cxa_end_catch() local_unnamed_addr
 
 attributes #0 = { norecurse uwtable }
 attributes #1 = { inlinehint uwtable }
 attributes #3 = { nounwind }

test/CodeGen/X86/mingw-comdats.ll

 ; GNU32: .section        .data$_gv,"dw",discard,_gv
 ; GNU32: _gv:
 ; GNU32: .long 42
 
 
 ; Make sure the assembler puts the .xdata and .pdata in sections with the right
 ; names.
 ; GNUOBJ: .text$_Z3fooi
-; GNUOBJ: .xdata$_Z3fooi
 ; GNUOBJ: .data$gv
+; GNUOBJ: .xdata$_Z3fooi
 ; GNUOBJ: .pdata$_Z3fooi
 
 declare dso_local i32 @_Z3bari(i32)
 
 attributes #0 = { norecurse uwtable }
 attributes #1 = { inlinehint uwtable }