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[DirectX] Fix GEP flattening with 0-indexed GEPs on global variables #149211

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Merged
merged 1 commit into from
Jul 17, 2025
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[DirectX] Fix GEP flattening with 0-indexed GEPs on global variables #149211

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10 changes: 10 additions & 0 deletions llvm/lib/Target/DirectX/DXILFlattenArrays.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -343,6 +343,16 @@ bool DXILFlattenArraysVisitor::visitGetElementPtrInst(GetElementPtrInst &GEP) {
Info.RootFlattenedArrayType, Info.RootPointerOperand,
{ZeroIndex, FlattenedIndex}, GEP.getName(), GEP.getNoWrapFlags());

// If the pointer operand is a global variable and all indices are 0,
// IRBuilder::CreateGEP will return the global variable instead of creating
// a GEP instruction or GEP ConstantExpr. In this case we have to create and
// insert our own GEP instruction.
if (!isa<GEPOperator>(NewGEP))
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Is there something preventing us from just doing this directly the first time?

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@Icohedron Icohedron Jul 17, 2025 •
edited
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If I wanted to always create a GEP Instruction, then sure we don't need this if statement and can just use GetElementPtrInst::Create without using the IRBuilder.
But I am keeping the ability to let IRBuilder::CreateGEP create GEP ConstantExprs as well.

NewGEP = GetElementPtrInst::Create(
Info.RootFlattenedArrayType, Info.RootPointerOperand,
{ZeroIndex, FlattenedIndex}, GEP.getNoWrapFlags(), GEP.getName(),
Builder.GetInsertPoint());

// Replace the current GEP with the new GEP. Store GEPInfo into the map
// for later use in case this GEP was not the end of the chain
GEPChainInfoMap.insert({cast<GEPOperator>(NewGEP), std::move(Info)});
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22 changes: 22 additions & 0 deletions llvm/test/CodeGen/DirectX/flatten-array.ll
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Original file line number Diff line number Diff line change
Expand Up @@ -218,6 +218,28 @@ define void @two_index_gep_const() {
ret void
}

define void @zero_index_global() {
; CHECK-LABEL: define void @zero_index_global(
; CHECK-NEXT: [[GEP:%.*]] = getelementptr inbounds nuw [4 x float], ptr addrspace(3) @g.1dim, i32 0, i32 0
; CHECK-NEXT: load float, ptr addrspace(3) [[GEP]], align 4
; CHECK-NEXT: ret void
%1 = getelementptr inbounds nuw [2 x [2 x float]], ptr addrspace(3) @g, i32 0, i32 0, i32 0
%2 = load float, ptr addrspace(3) %1, align 4
ret void
}

; Note: A ConstantExpr GEP with all 0 indices is equivalent to the pointer
; operand of the GEP. Therefore the visitLoadInst will not see the pointer operand
; as a ConstantExpr GEP and will not create a GEP instruction to be visited.
; The later dxil-legalize pass will insert a GEP in this instance.
define void @zero_index_global_const() {
; CHECK-LABEL: define void @zero_index_global_const(
; CHECK-NEXT: load float, ptr addrspace(3) @g.1dim, align 4
; CHECK-NEXT: ret void
%1 = load float, ptr addrspace(3) getelementptr inbounds nuw ([2 x [2 x float]], ptr addrspace(3) @g, i32 0, i32 0, i32 0), align 4
ret void
}

define void @gep_4d_index_test() {
; CHECK-LABEL: gep_4d_index_test
; CHECK: [[a:%.*]] = alloca [16 x i32], align 4
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