[llvm] [InstCombine] Canonicalise SextADD + GEP (PR #69581)

[via llvm-commits]

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@@ -2335,6 +2335,20 @@ Instruction *InstCombinerImpl::visitGetElementPtrInst(GetElementPtrInst &GEP) {
       return GetElementPtrInst::Create(GEP.getResultElementType(), NewPtr,
                                        Idx2);
     }
+
+    ConstantInt *C;
+    if (match(GEP.getOperand(1), m_OneUse(m_SExt(m_OneUse(m_NSWAdd(
+                                     m_Value(Idx1), m_ConstantInt(C))))))) {
+      // %add = add nsw i32 %idx1, idx2
+      // %sidx = sext i32 %add to i64
+      // %gep = getelementptr i32, i32* %ptr, i64 %sidx
+      // as :
+      // %newptr = getelementptr i32, i32* %ptr, i32 %idx1
+      // %newgep = getelementptr i32, i32* %newptr, i32 idx2
+      auto *NewPtr = Builder.CreateGEP(GEP.getResultElementType(),
+                                       GEP.getPointerOperand(), Idx1);
+      return GetElementPtrInst::Create(GEP.getResultElementType(), NewPtr, C);
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LiqinWeng wrote:

done

https://github.com/llvm/llvm-project/pull/69581