[llvm] [AMDGPU] Promote nestedGEP allocas to vectors (PR #141199)
Harrison Hao via llvm-commits
llvm-commits at lists.llvm.org
Fri May 23 04:02:13 PDT 2025
================
@@ -437,8 +437,57 @@ static Value *GEPToVectorIndex(GetElementPtrInst *GEP, AllocaInst *Alloca,
unsigned BW = DL.getIndexTypeSizeInBits(GEP->getType());
SmallMapVector<Value *, APInt, 4> VarOffsets;
APInt ConstOffset(BW, 0);
- if (GEP->getPointerOperand()->stripPointerCasts() != Alloca ||
- !GEP->collectOffset(DL, BW, VarOffsets, ConstOffset))
+
+ // Walk backwards through nested GEPs to collect both constant and variable
+ // offsets, so that nested vector GEP chains can be lowered in one step.
+ //
+ // Given this IR fragment as input:
+ //
+ // %0 = alloca [10 x <2 x i32>], align 8, addrspace(5)
+ // %1 = getelementptr [10 x <2 x i32>], ptr addrspace(5) %0, i32 0, i32 %j
+ // %2 = getelementptr i8, ptr addrspace(5) %1, i32 4
+ // %3 = load i32, ptr addrspace(5) %2, align 4
+ //
+ // Combine both GEP operations in a single pass, producing:
+ // BasePtr = %0
+ // ConstOffset = 4
+ // VarOffsets = { %j → element_size(<2 x i32>) }
+ //
+ // That lets us emit a single buffer_load directly into a VGPR, without ever
+ // allocating scratch memory for the intermediate pointer.
+ Value *CurPtr = GEP;
+ while (auto *CurGEP = dyn_cast<GetElementPtrInst>(CurPtr)) {
+ SmallMapVector<Value *, APInt, 4> LocalVarsOffsets;
+ APInt LocalConstOffset(BW, 0);
----------------
harrisonGPU wrote:
Thanks, I’ve added a new lit test that includes a three-level nested GEP example.
https://github.com/llvm/llvm-project/pull/141199
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