[GlobalOpt] Emit fragments using field offsets from struct layout

[dstenb]

Summary

When creating the debug fragments for a SRA'd struct, use the fields'
offsets, taken from the struct layout, as the offsets for the resulting
fragments. This fixes an issue where GlobalOpt would emit fragments with
incorrect offsets for padded fields.

This should solve PR36016.

[aprantl]

Querying the the layout for the offset is definitely the right way to go. Thanks a lot!

[uabelho]

I submitted this for David since he doesn't have commit rights yet.

llvm/trunk/lib/Transforms/IPO/GlobalOpt.cpp

static GlobalVariable *SRAGlobal(GlobalVariable *GV, const DataLayout &DL) {
   Module::GlobalListType &Globals = GV->getParent()->getGlobalList();
 
   // Get the alignment of the global, either explicit or target-specific.
   unsigned StartAlignment = GV->getAlignment();
   if (StartAlignment == 0)
     StartAlignment = DL.getABITypeAlignment(GV->getType());
 
   if (StructType *STy = dyn_cast<StructType>(Ty)) {
-    uint64_t FragmentOffset = 0;
     unsigned NumElements = STy->getNumElements();
     NewGlobals.reserve(NumElements);
     const StructLayout &Layout = *DL.getStructLayout(STy);
     for (unsigned i = 0, e = NumElements; i != e; ++i) {
       Constant *In = Init->getAggregateElement(i);
       assert(In && "Couldn't get element of initializer?");
       GlobalVariable *NGV = new GlobalVariable(STy->getElementType(i), false,
                                                GlobalVariable::InternalLinkage,
for (unsigned i = 0, e = NumElements; i != e; ++i) {
       // had 256 byte alignment for example, something might depend on that:
       // propagate info to each field.
       uint64_t FieldOffset = Layout.getElementOffset(i);
       unsigned NewAlign = (unsigned)MinAlign(StartAlignment, FieldOffset);
       if (NewAlign > DL.getABITypeAlignment(STy->getElementType(i)))
         NGV->setAlignment(NewAlign);
 
       // Copy over the debug info for the variable.
-      FragmentOffset = alignTo(FragmentOffset, NewAlign);
       uint64_t Size = DL.getTypeSizeInBits(NGV->getValueType());
-      transferSRADebugInfo(GV, NGV, FragmentOffset, Size, NumElements);
-      FragmentOffset += Size;
+      uint64_t FragmentOffsetInBits = Layout.getElementOffsetInBits(i);
+      transferSRADebugInfo(GV, NGV, FragmentOffsetInBits, Size, NumElements);
     }
   } else if (SequentialType *STy = dyn_cast<SequentialType>(Ty)) {
     unsigned NumElements = STy->getNumElements();
     if (NumElements > 16 && GV->hasNUsesOrMore(16))
       return nullptr; // It's not worth it.
     NewGlobals.reserve(NumElements);
     auto ElTy = STy->getElementType();
     uint64_t EltSize = DL.getTypeAllocSize(ElTy);

llvm/trunk/test/DebugInfo/Generic/global-sra-struct.ll

 ;     return static_struct.a + static_struct.b;
 ; }
 
 ; CHECK: @static_struct.0 = internal unnamed_addr global i32 0, align 8, !dbg ![[EL0:.*]]
 ; CHECK: @static_struct.1 = internal unnamed_addr global i64 0, align 8, !dbg ![[EL1:.*]]
 
 ; CHECK: ![[EL0]] = !DIGlobalVariableExpression(var: ![[VAR:.*]], expr: !DIExpression(DW_OP_LLVM_fragment, 0, 32))
 ; CHECK: ![[VAR]] = distinct !DIGlobalVariable(name: "static_struct"
-; CHECK: ![[EL1]] = !DIGlobalVariableExpression(var: ![[VAR]], expr: !DIExpression(DW_OP_LLVM_fragment, 32, 64))
+; CHECK: ![[EL1]] = !DIGlobalVariableExpression(var: ![[VAR]], expr: !DIExpression(DW_OP_LLVM_fragment, 64, 64))
 
 @static_struct = internal global %struct.mystruct zeroinitializer, align 8, !dbg !0
 
 ; Function Attrs: nounwind optsize ssp uwtable
 define void @foo(i32 %in) #0 {
 entry:
   store i32 %in, i32* getelementptr inbounds (%struct.mystruct, %struct.mystruct* @static_struct, i32 0, i32 0), align 8, !tbaa !17
   ret void