[llvm] [SimplifyCFG] Delete the unnecessary range check for small mask operation (PR #65835)

[Nikita Popov via llvm-commits]

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@@ -6631,6 +6630,23 @@ static bool SwitchToLookupTable(SwitchInst *SI, IRBuilder<> &Builder,
 
   BranchInst *RangeCheckBranch = nullptr;
 
+  // Grow the table to cover all possible index values to avoid the range check.
+  if (UseSwitchConditionAsTableIndex) {
+    ConstantRange CR = computeConstantRange(TableIndex, /* ForSigned */ false);
+    // Grow the table shouldn't have any size impact by checking
+    // WouldFitInRegister.
+    // TODO: Consider growing the table also when it doesn't fit in a register
+    // if no optsize is specified.
+    if (SwitchLookupTable::WouldFitInRegister(
+            DL, CR.getUpper().getLimitedValue(), PHIs[0]->getType())) {
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nikic wrote:

This is checking the type of just one phi. Compare with the use here: https://github.com/llvm/llvm-project/blob/7d66f9b5aa8b15602b2b345b36af00a28b5a4f9d/llvm/lib/Transforms/Utils/SimplifyCFG.cpp#L6390-L6394

https://github.com/llvm/llvm-project/pull/65835