[SimplifyCFG] eliminate switch cases based on known range of switch condition
This was noted in PR24766:
https://llvm.org/bugs/show_bug.cgi?id=24766#c2
We may not know whether the sign bit(s) are zero or one, but we can still
optimize based on knowing that the sign bit is repeated.
Differential Revision: http://reviews.llvm.org/D20275
llvm-svn: 270222
diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index 16e8865..fb6a13f 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -3914,14 +3914,20 @@
APInt KnownZero(Bits, 0), KnownOne(Bits, 0);
computeKnownBits(Cond, KnownZero, KnownOne, DL, 0, AC, SI);
+ // We can also eliminate cases by determining that their values are outside of
+ // the limited range of the condition based on how many significant (non-sign)
+ // bits are in the condition value.
+ unsigned ExtraSignBits = ComputeNumSignBits(Cond, DL, 0, AC, SI) - 1;
+ unsigned MaxSignificantBitsInCond = Bits - ExtraSignBits;
+
// Gather dead cases.
SmallVector<ConstantInt *, 8> DeadCases;
for (auto &Case : SI->cases()) {
- if ((Case.getCaseValue()->getValue() & KnownZero) != 0 ||
- (Case.getCaseValue()->getValue() & KnownOne) != KnownOne) {
+ APInt CaseVal = Case.getCaseValue()->getValue();
+ if ((CaseVal & KnownZero) != 0 || (CaseVal & KnownOne) != KnownOne ||
+ (CaseVal.getMinSignedBits() > MaxSignificantBitsInCond)) {
DeadCases.push_back(Case.getCaseValue());
- DEBUG(dbgs() << "SimplifyCFG: switch case '" << Case.getCaseValue()
- << "' is dead.\n");
+ DEBUG(dbgs() << "SimplifyCFG: switch case " << CaseVal << " is dead.\n");
}
}