#define DEBUG_TYPE "instcombine"
+bool canTryToConstantAddTwoShiftAmounts(Value *Sh0, Value *ShAmt0, Value *Sh1,
+ Value *ShAmt1) {
+ // We have two shift amounts from two different shifts. The types of those
+ // shift amounts may not match. If that's the case let's bailout now..
+ if (ShAmt0->getType() != ShAmt1->getType())
+ return false;
+
+ // As input, we have the following pattern:
+ // Sh0 (Sh1 X, Q), K
+ // We want to rewrite that as:
+ // Sh x, (Q+K) iff (Q+K) u< bitwidth(x)
+ // While we know that originally (Q+K) would not overflow
+ // (because 2 * (N-1) u<= iN -1), we have looked past extensions of
+ // shift amounts. so it may now overflow in smaller bitwidth.
+ // To ensure that does not happen, we need to ensure that the total maximal
+ // shift amount is still representable in that smaller bit width.
+ unsigned MaximalPossibleTotalShiftAmount =
+ (Sh0->getType()->getScalarSizeInBits() - 1) +
+ (Sh1->getType()->getScalarSizeInBits() - 1);
+ APInt MaximalRepresentableShiftAmount =
+ APInt::getAllOnesValue(ShAmt0->getType()->getScalarSizeInBits());
+ return MaximalRepresentableShiftAmount.uge(MaximalPossibleTotalShiftAmount);
+}
+
// Given pattern:
// (x shiftopcode Q) shiftopcode K
// we should rewrite it as
if (!match(Sh1, m_Shift(m_Value(X), m_ZExtOrSelf(m_Value(ShAmt1)))))
return nullptr;
- // We have two shift amounts from two different shifts. The types of those
- // shift amounts may not match. If that's the case let's bailout now..
- if (ShAmt0->getType() != ShAmt1->getType())
- return nullptr;
-
- // As input, we have the following pattern:
- // Sh0 (Sh1 X, Q), K
- // We want to rewrite that as:
- // Sh x, (Q+K) iff (Q+K) u< bitwidth(x)
- // While we know that originally (Q+K) would not overflow
- // (because 2 * (N-1) u<= iN -1), we have looked past extensions of
- // shift amounts. so it may now overflow in smaller bitwidth.
- // To ensure that does not happen, we need to ensure that the total maximal
- // shift amount is still representable in that smaller bit width.
- unsigned MaximalPossibleTotalShiftAmount =
- (Sh0->getType()->getScalarSizeInBits() - 1) +
- (Sh1->getType()->getScalarSizeInBits() - 1);
- APInt MaximalRepresentableShiftAmount =
- APInt::getAllOnesValue(ShAmt0->getType()->getScalarSizeInBits());
- if (MaximalRepresentableShiftAmount.ult(MaximalPossibleTotalShiftAmount))
+ // Verify that it would be safe to try to add those two shift amounts.
+ if (!canTryToConstantAddTwoShiftAmounts(Sh0, ShAmt0, Sh1, ShAmt1))
return nullptr;
// We are only looking for signbit extraction if we have two right shifts.