[llvm] [SimplifyCFG] Transform switch to select when common bits uniquely identify one case (PR #145233)
Gábor Spaits via llvm-commits
llvm-commits at lists.llvm.org
Tue Jul 1 02:10:36 PDT 2025
================
@@ -6234,10 +6234,33 @@ static Value *foldSwitchToSelect(const SwitchCaseResultVectorTy &ResultVector,
// case 0,2,8,10 -> Cond & 0b1..0101 == 0 ? result : default
if (isPowerOf2_32(CaseCount)) {
ConstantInt *MinCaseVal = CaseValues[0];
- // Find mininal value.
- for (auto *Case : CaseValues)
+ // In case, there are bits, that can only be present in the CaseValues we
+ // can transform the switch into a select if the conjunction of
+ // all the values uniquely identify the CaseValues.
+ APInt AndMask = APInt::getAllOnes(MinCaseVal->getBitWidth());
+
+ for (auto *Case : CaseValues) {
if (Case->getValue().slt(MinCaseVal->getValue()))
MinCaseVal = Case;
+ AndMask &= Case->getValue();
+ }
+
+ KnownBits Known = computeKnownBits(Condition, DL);
+ unsigned int ConditionWidth = Condition->getType()->getIntegerBitWidth();
+ APInt ActiveBits =
+ APInt(ConditionWidth, Known.countMaxActiveBits(), false);
+
+ APInt One(ConditionWidth, 1, false);
+ // To make sure, that the representation of the accepted values is
+ // actually unique we check, wheter the conjucted bits and the another
+ // conjuction with the input value will only be true for exactly CaseCount
+ // number times.
+ if ((One << (ActiveBits - AndMask.popcount())) == CaseCount) {
----------------
spaits wrote:
`FreeBits` could onyl be bigger than `ConditionWidth`, if the `Known.countMaxActiveBits() - AndMask.popcount()` overflows, so we could just cut of all the possible 1s, that are outside of active bits.
I think this would be a legal move. It doesn't matter what is in `AndMask` before the active bits. That part of the parameters would be 0 anyway.
What do you think?
https://github.com/llvm/llvm-project/pull/145233
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