[AsmPrinter] Fix handling in emitGlobalConstantImpl for AIX (#116255)

When GlobalMerge creates a MergedGlobal of statics all initialized to
zero, emitGlobalConstantImpl sees a ConstantAggregateZero. This results
in just emitting zeros followed by labels for the aliases. We need to
handle it more like how emitGlobalConstantStruct does by emitting each
global inside the aggregate.

---------

Co-authored-by: Hubert Tong <[email protected]>

Author: syzaara

llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp

@@ -3849,6 +3849,8 @@ static void emitGlobalConstantImpl(const DataLayout &DL, const Constant *CV,
                                    AsmPrinter &AP, const Constant *BaseCV,
                                    uint64_t Offset,
                                    AsmPrinter::AliasMapTy *AliasList) {
+  assert(!AliasList || AP.TM.getTargetTriple().isOSBinFormatXCOFF() &&
+                           "AliasList only expected for XCOFF");
   emitGlobalAliasInline(AP, Offset, AliasList);
   uint64_t Size = DL.getTypeAllocSize(CV->getType());
 
@@ -3858,7 +3860,27 @@ static void emitGlobalConstantImpl(const DataLayout &DL, const Constant *CV,
   if (!BaseCV && CV->hasOneUse())
     BaseCV = dyn_cast<Constant>(CV->user_back());
 
-  if (isa<ConstantAggregateZero>(CV) || isa<UndefValue>(CV))
+  if (isa<ConstantAggregateZero>(CV)) {
+    StructType *structType;
+    if (AliasList && (structType = llvm::dyn_cast<StructType>(CV->getType()))) {
+      // Handle cases of aliases to direct struct elements
+      const StructLayout *Layout = DL.getStructLayout(structType);
+      uint64_t SizeSoFar = 0;
+      for (unsigned int i = 0, n = structType->getNumElements(); i < n - 1;
+           ++i) {
+        uint64_t GapToNext = Layout->getElementOffset(i + 1) - SizeSoFar;
+        AP.OutStreamer->emitZeros(GapToNext);
+        SizeSoFar += GapToNext;
+        emitGlobalAliasInline(AP, Offset + SizeSoFar, AliasList);
+      }
+      AP.OutStreamer->emitZeros(Size - SizeSoFar);
+      return;
+    } else {
+      return AP.OutStreamer->emitZeros(Size);
+    }
+  }
+
+  if (isa<UndefValue>(CV))
     return AP.OutStreamer->emitZeros(Size);
 
   if (const ConstantInt *CI = dyn_cast<ConstantInt>(CV)) {

llvm/test/CodeGen/PowerPC/global-merge-aix-sections.ll

@@ -0,0 +1,52 @@
+; RUN: rm -rf %t
+; RUN: mkdir -p %t
+
+; RUN: llc -verify-machineinstrs -mtriple powerpc64-ibm-aix-xcoff -mcpu=pwr8 < %s | FileCheck %s
+
+; RUN: llc -verify-machineinstrs -mtriple powerpc64-ibm-aix-xcoff -mcpu=pwr8 --filetype=obj -o %t/global-merge-aix-sections.o < %s
+; RUN; llvm-objdump --syms %t/global-merge-aix-sections.o | FileCheck %s --check-prefix=DATA
+
+%struct.Example = type { i32, i8 }
+
+@y = internal global i32 0, section "mycsect", align 4
+@z = internal global i32 0, section "mycsect", align 4
+@l = internal global i32 0, align 4
+@u = internal global i16 0, section "mycsect", align 2
+@myStruct1 = internal global %struct.Example zeroinitializer, section "mycsect", align 4
+
+; Function Attrs: nounwind
+define void @g() {
+entry:
+  tail call void @f(ptr noundef nonnull @y, ptr noundef nonnull @z)
+  tail call void @f(ptr noundef nonnull @l, ptr noundef nonnull @z)
+  tail call void @h(ptr noundef nonnull @u)
+  tail call void @s(ptr noundef nonnull @myStruct1)
+  ret void
+}
+
+declare void @f(ptr noundef, ptr noundef)
+declare void @h(ptr noundef)
+declare void @s(ptr noundef)
+
+; CHECK: .csect mycsect[RW],2
+; CHECK-NEXT: .lglobl u # @_MergedGlobals
+; CHECK-NEXT: .lglobl y
+; CHECK-NEXT: .lglobl z
+; CHECK-NEXT: .lglobl myStruct1
+; CHECK-NEXT: .align  2
+; CHECK-NEXT: L.._MergedGlobals:
+; CHECK-NEXT: u:
+; CHECK-NEXT:        .space  2
+; CHECK-NEXT:        .space  2
+; CHECK-NEXT: y:
+; CHECK-NEXT:        .space  4
+; CHECK-NEXT: z:
+; CHECK-NEXT:        .space  4
+; CHECK-NEXT: myStruct1:
+; CHECK-NEXT:        .space  8
+
+; DATA: 00000078 l     O .data  00000014 mycsect
+; DATA-NEXT: 00000078 l     O .data (csect: mycsect)         00000000 u
+; DATA-NEXT: 0000007c l     O .data (csect: mycsect)         00000000 y
+; DATA-NEXT: 00000080 l     O .data (csect: mycsect)         00000000 z
+; DATA-NEXT: 00000084 l     O .data (csect: mycsect)         00000000 myStruct1