[AIX] Emit AvailableExternally Linkage on AIX

Since on AIX, our strategy is to not use -u to suppress any undefined
symbols, we need to emit .extern for the symbols with AvailableExternally
linkage.

Differential Revision: https://reviews.llvm.org/D80642

lib/CodeGen/AsmPrinter/AsmPrinter.cpp

@@ -438,8 +438,13 @@ void AsmPrinter::emitLinkage(const GlobalValue *GV, MCSymbol *GVSym) const {
       return;
     }
     LLVM_FALLTHROUGH;
-  case GlobalValue::AppendingLinkage:
   case GlobalValue::AvailableExternallyLinkage:
+    if (MAI->hasDotExternDirective()) {
+      OutStreamer->emitSymbolAttribute(GVSym, MCSA_Extern);
+      return;
+    }
+    LLVM_FALLTHROUGH;
+  case GlobalValue::AppendingLinkage:
     llvm_unreachable("Should never emit this");
   }
   llvm_unreachable("Unknown linkage type!");

lib/CodeGen/TargetLoweringObjectFileImpl.cpp

@@ -1983,7 +1983,7 @@ TargetLoweringObjectFileXCOFF::getTargetSymbol(const GlobalValue *GV,
   // function entry point. We choose to always return a function descriptor
   // here.
   if (const GlobalObject *GO = dyn_cast<GlobalObject>(GV)) {
-    if (GO->isDeclaration())
+    if (GO->isDeclarationForLinker())
       return cast<MCSectionXCOFF>(getSectionForExternalReference(GO, TM))
           ->getQualNameSymbol();
 
@@ -2011,7 +2011,7 @@ MCSection *TargetLoweringObjectFileXCOFF::getExplicitSectionGlobal(
 
 MCSection *TargetLoweringObjectFileXCOFF::getSectionForExternalReference(
     const GlobalObject *GO, const TargetMachine &TM) const {
-  assert(GO->isDeclaration() &&
+  assert(GO->isDeclarationForLinker() &&
          "Tried to get ER section for a defined global.");
 
   SmallString<128> Name;
@@ -2133,6 +2133,7 @@ XCOFF::StorageClass TargetLoweringObjectFileXCOFF::getStorageClassForGlobal(
     return XCOFF::C_HIDEXT;
   case GlobalValue::ExternalLinkage:
   case GlobalValue::CommonLinkage:
+  case GlobalValue::AvailableExternallyLinkage:
     return XCOFF::C_EXT;
   case GlobalValue::ExternalWeakLinkage:
   case GlobalValue::LinkOnceAnyLinkage:
@@ -2143,9 +2144,6 @@ XCOFF::StorageClass TargetLoweringObjectFileXCOFF::getStorageClassForGlobal(
   case GlobalValue::AppendingLinkage:
     report_fatal_error(
         "There is no mapping that implements AppendingLinkage for XCOFF.");
-  case GlobalValue::AvailableExternallyLinkage:
-    report_fatal_error("unhandled AvailableExternallyLinkage when mapping "
-                       "linkage to StorageClass");
   }
   llvm_unreachable("Unknown linkage type!");
 }

lib/Target/PowerPC/PPCAsmPrinter.cpp

@@ -1728,7 +1728,7 @@ bool PPCAIXAsmPrinter::doInitialization(Module &M) {
 
   auto setCsectAlignment = [this](const GlobalObject *GO) {
     // Declarations have 0 alignment which is set by default.
-    if (GO->isDeclaration())
+    if (GO->isDeclarationForLinker())
       return;
 
     SectionKind GOKind = getObjFileLowering().getKindForGlobal(GO, TM);

lib/Target/TargetLoweringObjectFile.cpp

@@ -143,7 +143,7 @@ void TargetLoweringObjectFile::emitPersonalityValue(MCStreamer &Streamer,
 /// may be overridden by the target implementation.
 SectionKind TargetLoweringObjectFile::getKindForGlobal(const GlobalObject *GO,
                                                        const TargetMachine &TM){
-  assert(!GO->isDeclaration() && !GO->hasAvailableExternallyLinkage() &&
+  assert(!GO->isDeclarationForLinker() &&
          "Can only be used for global definitions");
 
   // Functions are classified as text sections.

test/CodeGen/PowerPC/aix-available-externally-linkage.ll

@@ -0,0 +1,39 @@
+; RUN: llc -verify-machineinstrs -mtriple powerpc-ibm-aix-xcoff -mcpu=pwr4 \
+; RUN:     -mattr=-altivec < %s | \
+; RUN:   FileCheck %s
+
+; RUN: llc -verify-machineinstrs -mtriple powerpc64-ibm-aix-xcoff -mcpu=pwr4 \
+; RUN:     -mattr=-altivec < %s | \
+; RUN:   FileCheck %s
+
+; RUN: llc -verify-machineinstrs -mtriple powerpc-ibm-aix-xcoff -mcpu=pwr4 \
+; RUN:     -mattr=-altivec -filetype=obj -o %t.o < %s
+; RUN: llvm-readobj --symbols %t.o | \
+; RUN:   FileCheck --check-prefix=XCOFF32 %s
+
+; RUN: not --crash llc -verify-machineinstrs -mtriple powerpc64-ibm-aix-xcoff \
+; RUN:     -mcpu=pwr4 -mattr=-altivec -filetype=obj -o %t.o 2>&1 < %s | \
+; RUN:   FileCheck --check-prefix=XCOFF64 %s
+; XCOFF64: LLVM ERROR: 64-bit XCOFF object files are not supported yet.
+
+@_ZN3Foo1aE = available_externally constant i32 -1
+
+; CHECK: .extern  _ZN3Foo1aE[UA]
+
+; XCOFF32:          Index: [[#Index:]]{{.*}}{{[[:space:]] *}}Name: _ZN3Foo1aE
+; XCOFF32-NEXT:     Value (RelocatableAddress): 0x0
+; XCOFF32-NEXT:     Section: N_UNDEF
+; XCOFF32-NEXT:     Type: 0x0
+; XCOFF32-NEXT:     StorageClass: C_EXT (0x2)
+; XCOFF32-NEXT:     NumberOfAuxEntries: 1
+; XCOFF32-NEXT:     CSECT Auxiliary Entry {
+; XCOFF32-NEXT:       Index: [[#Index+1]]
+; XCOFF32-NEXT:       SectionLen: 0
+; XCOFF32-NEXT:       ParameterHashIndex: 0x0
+; XCOFF32-NEXT:       TypeChkSectNum: 0x0
+; XCOFF32-NEXT:       SymbolAlignmentLog2: 0
+; XCOFF32-NEXT:       SymbolType: XTY_ER (0x0)
+; XCOFF32-NEXT:       StorageMappingClass: XMC_UA (0x4)
+; XCOFF32-NEXT:       StabInfoIndex: 0x0
+; XCOFF32-NEXT:       StabSectNum: 0x0
+; XCOFF32-NEXT:     }