[llvm] [InstCombine] Fold chained GEP with constant base into single GEP (PR #170439)

[Nikita Popov via llvm-commits]

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@@ -2803,6 +2803,33 @@ Instruction *InstCombinerImpl::visitGEPOfGEP(GetElementPtrInst &GEP,
   if (Src->getResultElementType() != GEP.getSourceElementType())
     return nullptr;
 
+  // Fold chained GEP with constant base into single GEP:
+  // gep i8, (gep i8, %base, C1), (select Cond, C2, C3)
+  // -> gep i8, %base, (select Cond, C1+C2, C1+C3)
+  if (GEP.getNumIndices() == 1 && Src->getNumIndices() == 1) {
+    Value *SrcIdx = *Src->idx_begin();
+    Value *GEPIdx = *GEP.idx_begin();
+    const APInt *ConstOffset, *TrueVal, *FalseVal;
+    Value *Cond;
+    if (match(SrcIdx, m_APInt(ConstOffset)) &&
+        match(GEPIdx,
+              m_Select(m_Value(Cond), m_APInt(TrueVal), m_APInt(FalseVal)))) {
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nikic wrote:

Please also test GEP with vector offsets (and the variant where one has scalar and one has vector). I think this is currently not handled correctly.

https://github.com/llvm/llvm-project/pull/170439