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

[Yingwei Zheng via llvm-commits]

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@@ -998,6 +998,89 @@ 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())
+    return nullptr;
+
+  Constant *Init = GV->getInitializer();
+  if (!isa<ConstantArray>(Init) && !isa<ConstantDataArray>(Init))
+    return nullptr;
+
+  uint64_t ArrayElementCount = Init->getType()->getArrayNumElements();
+  // Don't blow up on huge arrays.
+  // This threshold is chosen based on statistics on a dataset
+  // which is collected from real-world applications.
----------------
dtcxzyw wrote:

> This scan is of number of hits on the fold (edit: assuming inf threshold)?

Yeah. There are only 1714 potential folds in my dataset.


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