return false;
}
-// Return true if we can prove that Neg == OpSize - Pos. This means that
+// Return true if we can prove that, whenever Neg and Pos are both in the
+// range [0, OpSize), Neg == (Pos == 0 ? 0 : OpSize - Pos). This means that
// for two opposing shifts shift1 and shift2 and a value X with OpBits bits:
//
// (or (shift1 X, Neg), (shift2 X, Pos))
//
// reduces to a rotate in direction shift2 by Pos and a rotate in direction
-// shift1 by Neg. Note that the (or ...) then invokes undefined behavior
-// if Pos == 0 (and consequently Neg == OpSize).
+// shift1 by Neg. The range [0, OpSize) means that we only need to consider
+// shift amounts with defined behavior.
static bool matchRotateSub(SDValue Pos, SDValue Neg, unsigned OpSize) {
+ // If OpSize is a power of 2 then:
+ //
+ // (a) (Pos == 0 ? 0 : OpSize - Pos) == (OpSize - Pos) & (OpSize - 1)
+ // (b) Neg == Neg & (OpSize - 1) whenever Neg is in [0, OpSize).
+ //
+ // So if OpSize is a power of 2 and Neg is (and Neg', OpSize-1), we check
+ // for the stronger condition:
+ //
+ // Neg & (OpSize - 1) == (OpSize - Pos) & (OpSize - 1) [A]
+ //
+ // for all Neg and Pos. Since Neg & (OpSize - 1) == Neg' & (OpSize - 1)
+ // we can just replace Neg with Neg' for the rest of the function.
+ //
+ // In other cases we check for the even stronger condition:
+ //
+ // Neg == OpSize - Pos [B]
+ //
+ // for all Neg and Pos. Note that the (or ...) then invokes undefined
+ // behavior if Pos == 0 (and consequently Neg == OpSize).
+ //
+ // We could actually use [A] whenever OpSize is a power of 2, but the
+ // only extra cases that it would match are those uninteresting ones
+ // where Neg and Pos are never in range at the same time. E.g. for
+ // OpSize == 32, using [A] would allow a Neg of the form (sub 64, Pos)
+ // as well as (sub 32, Pos), but:
+ //
+ // (or (shift1 X, (sub 64, Pos)), (shift2 X, Pos))
+ //
+ // always invokes undefined behavior for 32-bit X.
+ //
+ // Below, Mask == OpSize - 1 when using [A] and is all-ones otherwise.
+ unsigned LoBits = 0;
+ if (Neg.getOpcode() == ISD::AND &&
+ isPowerOf2_64(OpSize) &&
+ Neg.getOperand(1).getOpcode() == ISD::Constant &&
+ cast<ConstantSDNode>(Neg.getOperand(1))->getAPIntValue() == OpSize - 1) {
+ Neg = Neg.getOperand(0);
+ LoBits = Log2_64(OpSize);
+ }
+
// Check whether Neg has the form (sub NegC, NegOp1) for some NegC and NegOp1.
if (Neg.getOpcode() != ISD::SUB)
return 0;
return 0;
SDValue NegOp1 = Neg.getOperand(1);
- // The condition we need to prove is now NegC - NegOp1 == OpSize - Pos.
- // Check whether the terms match directly.
+ // The condition we need is now:
+ //
+ // (NegC - NegOp1) & Mask == (OpSize - Pos) & Mask
+ //
+ // If NegOp1 == Pos then we need:
+ //
+ // OpSize & Mask == NegC & Mask
+ //
+ // (because "x & Mask" is a truncation and distributes through subtraction).
+ APInt Width;
if (Pos == NegOp1)
- return NegC->getAPIntValue() == OpSize;
-
+ Width = NegC->getAPIntValue();
// Check for cases where Pos has the form (add NegOp1, PosC) for some PosC.
// Then the condition we want to prove becomes:
- // NegC - NegOp1 == OpSize - (NegOp1 + PosC)
- // NegC == OpSize - PosC
//
- // Because NegC and PosC are APInts, this is easier to test as:
- // OpSize == NegC + PosC
- if (Pos.getOpcode() == ISD::ADD && Pos.getOperand(0) == NegOp1) {
- ConstantSDNode *PosC = dyn_cast<ConstantSDNode>(Pos.getOperand(1));
- return PosC && OpSize == NegC->getAPIntValue() + PosC->getAPIntValue();
- }
+ // (NegC - NegOp1) & Mask == (OpSize - (NegOp1 + PosC)) & Mask
+ //
+ // which, again because "x & Mask" is a truncation, becomes:
+ //
+ // NegC & Mask == (OpSize - PosC) & Mask
+ // OpSize & Mask == (NegC + PosC) & Mask
+ else if (Pos.getOpcode() == ISD::ADD &&
+ Pos.getOperand(0) == NegOp1 &&
+ Pos.getOperand(1).getOpcode() == ISD::Constant)
+ Width = (cast<ConstantSDNode>(Pos.getOperand(1))->getAPIntValue() +
+ NegC->getAPIntValue());
+ else
+ return false;
- return false;
+ // Now we just need to check that OpSize & Mask == Width & Mask.
+ if (LoBits)
+ return Width.getLoBits(LoBits) == 0;
+ return Width == OpSize;
}
// A subroutine of MatchRotate used once we have found an OR of two opposite