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

[via llvm-commits]

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@@ -6545,6 +6544,17 @@ static bool SwitchToLookupTable(SwitchInst *SI, IRBuilder<> &Builder,
 
   BranchInst *RangeCheckBranch = nullptr;
 
+  if (UseSwitchConditionAsTableIndex) {
+    ConstantRange CR = computeConstantRange(TableIndex, /* ForSigned */ false);
+    if (DL.fitsInLegalInteger(CR.getUpper().getLimitedValue())) {
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zmodem wrote:

Why is `fitsInLegalInteger` the right check? For example, if the index is `x % 128` and we have 125 cases, shouldn't we want to grow the table to 127?

Also, since we're trading size for speed, maybe we should not do this in `optsize` functions?

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