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

[Nikita Popov via llvm-commits]

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@@ -6552,10 +6552,25 @@ static bool SwitchToLookupTable(SwitchInst *SI, IRBuilder<> &Builder,
     // Note: We call removeProdecessor later since we need to be able to get the
     // PHI value for the default case in case we're using a bit mask.
   } else {
-    Value *Cmp = Builder.CreateICmpULT(
-        TableIndex, ConstantInt::get(MinCaseVal->getType(), TableSize));
-    RangeCheckBranch =
-        Builder.CreateCondBr(Cmp, LookupBB, SI->getDefaultDest());
+    int KeepDefaultBranch = true;
+    if (UseSwitchConditionAsTableIndex) {
+      ConstantRange CR = computeConstantRange(TableIndex, /* ForSigned */ false,
+                                              /* UseInstrInfo*/ true);
+      if (DL.fitsInLegalInteger(CR.getUpper().getZExtValue())) {
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nikic wrote:

fitsInLegalInteger() accepts a bit width, while you are passing the value itself. Should be `CR.getUpper().getActiveBits()`.

Also this should be `getUnsignedMax()` rather than `getUpper()`.

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