[AIX][AsmPrinter] Fix unsigned subtraction wrap-around (#122214)

Unsigned subtraction wrap-around occurs in `emitGlobalConstantImpl` on
an AIX-specific code path from 8e4423e when a structure type has
zero elements.

With assertions enabled, this manifests as:
```
TypeSize llvm::StructLayout::getElementOffset(unsigned int) const: Assertion `Idx < NumElements && "Invalid element idx!"' failed.
```

Author: hubert-reinterpretcast

llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp

@@ -3914,21 +3914,22 @@ static void emitGlobalConstantImpl(const DataLayout &DL, const Constant *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);
+      unsigned numElements = {structType->getNumElements()};
+      if (numElements != 0) {
+        // Handle cases of aliases to direct struct elements
+        const StructLayout *Layout = DL.getStructLayout(structType);
+        uint64_t SizeSoFar = 0;
+        for (unsigned int i = 0; i < numElements - 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;
       }
-      AP.OutStreamer->emitZeros(Size - SizeSoFar);
-      return;
-    } else {
-      return AP.OutStreamer->emitZeros(Size);
     }
+    return AP.OutStreamer->emitZeros(Size);
   }
 
   if (isa<UndefValue>(CV))

llvm/test/CodeGen/PowerPC/global-merge-aix-zero-size-struct.ll

@@ -0,0 +1,20 @@
+; RUN: llc -verify-machineinstrs -mtriple powerpc64-ibm-aix-xcoff -mcpu=pwr7 < %s | FileCheck %s
+
+; RUN: llc -verify-machineinstrs -mtriple powerpc64-ibm-aix-xcoff -mcpu=pwr7 --filetype=obj -o %t.o < %s
+; RUN: llvm-objdump --syms %t.o | FileCheck %s --check-prefix=OBJ
+
+%struct.anon = type {}
+
+@a = internal constant %struct.anon zeroinitializer, align 1
+@b = internal constant [6 x i8] c"hello\00", align 1
+
+; CHECK:      	.csect L.._MergedGlobals[RO],2
+; CHECK-NEXT: 	.lglobl	a                               # @_MergedGlobals
+; CHECK-NEXT: 	.lglobl	b
+; CHECK-NEXT: a:
+; CHECK-NEXT: b:
+; CHECK-NEXT: 	.string	"hello"
+
+; OBJ:      0000000000000000 l       .text	0000000000000006 L.._MergedGlobals
+; OBJ-NEXT: 0000000000000000 l       .text (csect: L.._MergedGlobals) 	0000000000000000 a
+; OBJ-NEXT: 0000000000000000 l       .text (csect: L.._MergedGlobals) 	0000000000000000 b