[ValueTracking] Teach computeKnownBits about [su]min/max
Reasoning about a select in terms of a min or max allows us to derive a
tigher bound on the result.
llvm-svn: 277914
diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index f2b4078..c3a5e6a 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -950,14 +950,63 @@
KnownZero = APInt::getHighBitsSet(BitWidth, LeadZ);
break;
}
- case Instruction::Select:
+ case Instruction::Select: {
computeKnownBits(I->getOperand(2), KnownZero, KnownOne, Depth + 1, Q);
computeKnownBits(I->getOperand(1), KnownZero2, KnownOne2, Depth + 1, Q);
+ Value *LHS, *RHS;
+ SelectPatternFlavor SPF = matchSelectPattern(I, LHS, RHS).Flavor;
+ if (SelectPatternResult::isMinOrMax(SPF)) {
+ computeKnownBits(RHS, KnownZero, KnownOne, Depth + 1, Q);
+ computeKnownBits(LHS, KnownZero2, KnownOne2, Depth + 1, Q);
+ } else {
+ computeKnownBits(I->getOperand(2), KnownZero, KnownOne, Depth + 1, Q);
+ computeKnownBits(I->getOperand(1), KnownZero2, KnownOne2, Depth + 1, Q);
+ }
+
+ unsigned MaxHighOnes = 0;
+ unsigned MaxHighZeros = 0;
+ if (SPF == SPF_SMAX) {
+ // If both sides are negative, the result is negative.
+ if (KnownOne[BitWidth - 1] && KnownOne2[BitWidth - 1])
+ // We can derive a lower bound on the result by taking the max of the
+ // leading one bits.
+ MaxHighOnes =
+ std::max(KnownOne.countLeadingOnes(), KnownOne2.countLeadingOnes());
+ // If either side is non-negative, the result is non-negative.
+ else if (KnownZero[BitWidth - 1] || KnownZero2[BitWidth - 1])
+ MaxHighZeros = 1;
+ } else if (SPF == SPF_SMIN) {
+ // If both sides are non-negative, the result is non-negative.
+ if (KnownZero[BitWidth - 1] && KnownZero2[BitWidth - 1])
+ // We can derive an upper bound on the result by taking the max of the
+ // leading zero bits.
+ MaxHighZeros = std::max(KnownZero.countLeadingOnes(),
+ KnownZero2.countLeadingOnes());
+ // If either side is negative, the result is negative.
+ else if (KnownOne[BitWidth - 1] || KnownOne2[BitWidth - 1])
+ MaxHighOnes = 1;
+ } else if (SPF == SPF_UMAX) {
+ // We can derive a lower bound on the result by taking the max of the
+ // leading one bits.
+ MaxHighOnes =
+ std::max(KnownOne.countLeadingOnes(), KnownOne2.countLeadingOnes());
+ } else if (SPF == SPF_UMIN) {
+ // We can derive an upper bound on the result by taking the max of the
+ // leading zero bits.
+ MaxHighZeros =
+ std::max(KnownZero.countLeadingOnes(), KnownZero2.countLeadingOnes());
+ }
+
// Only known if known in both the LHS and RHS.
KnownOne &= KnownOne2;
KnownZero &= KnownZero2;
+ if (MaxHighOnes > 0)
+ KnownOne |= APInt::getHighBitsSet(BitWidth, MaxHighOnes);
+ if (MaxHighZeros > 0)
+ KnownZero |= APInt::getHighBitsSet(BitWidth, MaxHighZeros);
break;
+ }
case Instruction::FPTrunc:
case Instruction::FPExt:
case Instruction::FPToUI: