[llvm] [InstCombine] Canonicalize `gep T, (gep i8, base, C1), (Index +nsw C2)` into `gep T, (gep i8, base, C1 + C2 * sizeof(T)), Index` (PR #76177)

[Nikita Popov via llvm-commits]

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@@ -2038,6 +2038,54 @@ static Instruction *foldSelectGEP(GetElementPtrInst &GEP,
   return SelectInst::Create(Cond, NewTrueC, NewFalseC, "", nullptr, Sel);
 }
 
+// Canonicalization:
+// gep T, (gep i8, base, C1), (Index +nsw C2) into
+// gep T, (gep i8, base, C1 + C2 * sizeof(T)), Index
+static Instruction *canonicalizeGEPOfConstGEPI8(GetElementPtrInst &GEP,
+                                                GEPOperator *Src,
+                                                InstCombinerImpl &IC) {
+  if (GEP.getNumIndices() != 1)
+    return nullptr;
+  auto &DL = IC.getDataLayout();
+  if (!Src->getSourceElementType()->isIntegerTy(8) ||
+      !Src->hasAllConstantIndices())
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nikic wrote:

Better to use accumulateConstantOffset() instead of making it i8 specific.

https://github.com/llvm/llvm-project/pull/76177