[AIX] Minor cleanup in AsmPrinter. [NFC]

[sfertile]

Summary

• Extends the comments related to function descriptors, noting how they are only used on AIX.

• Changes the condition used to gate the creation of the current function symbol in AsmPrinter::SetupMachineFunction to reflect being AIX specific. The creation of the symbol is different because of AIXs linkage conventions, not because AIX uses function descriptors.

[Xiangling_L]

llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp

670

s/functions/function?

671

I kinda feel this comment is a little misleading. The linkage of function descriptor symbol and function entry point symbol must be same all the time, no matter the linkage is internal or not. This comment here seems imply that If the function linkage is internal, then the function descriptor symbol can have different linkage as the function symbol.? And I vaguely recall that why we choose to omit .lglobl foo{DS} when foo is internal is because by default, for csect name, it would be .lglobl linkage if we don't explicitly set '.globl as assembler manual state:

The assembler automatically generates the symbol table entry for any csect name with a class of C_HIDEXT unless there is an
explicit .globl pseudo-op applied to the csect name.

Please correct me if I am wrong.

[hubert.reinterpretcast]

llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp

670

The most clear would be "function's".

1664

s/functions/function's/;

[sfertile]

llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp

671

Thanks, You are right. If we can always emit the linkage why not do so? I understand its redundant because the default is correct anyway, but it slightly simplifies the code.

[daltenty]

This change regresses a whole bunch of lit tests when running on AIX, which change just the march and then stumble into the assert(MAI->needsFunctionDescriptors() && "AIX ABI is descriptor based.");, for example test/CodeGen/XCore/section-name.ll. Perhaps this should live in the PPC target printer instead?

[sfertile]

Sorry about that David. I've posted https://reviews.llvm.org/D74622 to revert back to using the MCASmInfo hook as the check.

llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp

void AsmPrinter::EmitFunctionHeader() {
 
   // Print out constants referenced by the function
   EmitConstantPool();
 
   // Print the 'header' of function.
   OutStreamer->SwitchSection(getObjFileLowering().SectionForGlobal(&F, TM));
   EmitVisibility(CurrentFnSym, F.getVisibility());
 
+  // If the functions linkage is not internal then the function descriptor
+  // symbol must have same linkage as the function symbol.
   if (MAI->needsFunctionDescriptors() &&
       F.getLinkage() != GlobalValue::InternalLinkage)
     EmitLinkage(&F, CurrentFnDescSym);
 
   EmitLinkage(&F, CurrentFnSym);
   if (MAI->hasFunctionAlignment())
     EmitAlignment(MF->getAlignment(), &F);
 
if (MAI->hasSubsectionsViaSymbols()) {
       // Emit an .alt_entry directive for the actual function symbol.
       OutStreamer->EmitSymbolAttribute(CurrentFnSym, MCSA_AltEntry);
     } else {
       EmitGlobalConstant(F.getParent()->getDataLayout(), F.getPrefixData());
     }
   }
 
   // Emit the function descriptor. This is a virtual function to allow targets
-  // to emit their specific function descriptor.
+  // to emit their specific function descriptor. Right now it is only used by
+  // the AIX target. The PowerPC 64-bit V1 ELF target also uses function
+  // descriptors and should be converted to use this hook as well.
   if (MAI->needsFunctionDescriptors())
     EmitFunctionDescriptor();
 
   // Emit the CurrentFnSym. This is a virtual function to allow targets to do
   // their wild and crazy things as required.
   EmitFunctionEntryLabel();
 
   if (CurrentFnBegin) {
MCSymbol *AsmPrinter::getCurExceptionSym() {
   return CurExceptionSym;
 }
 
 void AsmPrinter::SetupMachineFunction(MachineFunction &MF) {
   this->MF = &MF;
   const Function &F = MF.getFunction();
 
   // Get the function symbol.
-  if (MAI->needsFunctionDescriptors()) {
-    assert(TM.getTargetTriple().isOSAIX() && "Function descriptor is only"
-                                             " supported on AIX.");
+  if (TM.getTargetTriple().isOSAIX()) {
+    // AIX is unique here in that the name of the symbol emitted for the
+    // function body does not have the same name as the source functions
+    // C-linkage name.
+    assert(MAI->needsFunctionDescriptors() && "AIX ABI is descriptor based.");
     assert(CurrentFnDescSym && "The function descriptor symbol needs to be"
                                " initalized first.");
 
     // Get the function entry point symbol.
     CurrentFnSym =
         OutContext.getOrCreateSymbol("." + CurrentFnDescSym->getName());
 
+    // Set the containing csect.
     MCSectionXCOFF *FnEntryPointSec =
         cast<MCSectionXCOFF>(getObjFileLowering().SectionForGlobal(&F, TM));
-    // Set the containing csect.
     cast<MCSymbolXCOFF>(CurrentFnSym)->setContainingCsect(FnEntryPointSec);
   } else {
     CurrentFnSym = getSymbol(&MF.getFunction());
   }
 
   CurrentFnSymForSize = CurrentFnSym;
   CurrentFnBegin = nullptr;
   CurExceptionSym = nullptr;