[llvm] [InstCombine] Convert load from LUT into a select (PR #98339)

[Nikita Popov via llvm-commits]

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@@ -998,6 +998,93 @@ static bool canSimplifyNullLoadOrGEP(LoadInst &LI, Value *Op) {
   return false;
 }
 
+static Value *foldLoadFromIndexedGlobal(LoadInst &LI, IRBuilderBase &Builder) {
+  if (LI.isVolatile())
+    return nullptr;
+
+  auto *GEP = dyn_cast<GetElementPtrInst>(LI.getPointerOperand());
+  if (!GEP || LI.getType() != GEP->getResultElementType())
+    return nullptr;
+
+  auto *GV = dyn_cast<GlobalVariable>(GEP->getPointerOperand());
+  if (!GV || !GV->isConstant() || !GV->hasDefinitiveInitializer() ||
+      GV->getValueType() != GEP->getSourceElementType())
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nikic wrote:

As usual, type based reasoning is not allowed. Convert to offsets instead.

https://github.com/llvm/llvm-project/pull/98339