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When expanding a SMULFIXSAT ISD node (usually originating from a smul.fix.sat intrinsic) we've applied some optimizations for the special case when the scale is zero. The idea has been that it would be cheaper to use an SMULO instruction (if legal) to perform the multiplication and at the same time detect any overflow. And in case of overflow we could use some SELECT:s to replace the result with the saturated min/max value. The only tricky part is to know if we overflowed on the min or max value, i.e. if the product is positive or negative. Unfortunately the implementation has been incorrect as it has looked at the product returned by the SMULO to determine the sign of the product. In case of overflow that product is truncated and won't give us the correct sign bit. This patch is adding an extra XOR of the multiplication operands, which is used to determine the sign of the non truncated product. This patch fixes PR51677. Reviewed By: lebedev.ri Differential Revision: https://reviews.llvm.org/D108938 (cherry picked from commit 789f01283d52065b10049b58a3288c4abd1ef351) |
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