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Commit 6c3bf364 authored by Noah Goldstein's avatar Noah Goldstein
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[X86] Invert transforming `(x * (Pow2_Ceil(C1) - (1 << C0))) & C1` -> `(-x << C0) & C1`

We can detect the case under the following circumstances:
Take `(Pow2_Ceil(C1) - (1 << C0))` as `C2`.
    1) `C2` is NOT a power of 2.
    2) `C2 + LeastSignificantBit(C2)` is a nonzero power of 2.
    3) `C2 u>= C1`

The motivation is the middle end transforms:
    `(-x << C0) & C1`
to
    `(x * (Pow2_Ceil(C1) - (1 << C2))) & C1`

As it saves IR instructions. On X86 the two instruction, `sub` and
`shl`, and better than the `mul` so we want to undo the transform.

This comes up when shifting a bit-mask by a byte-misalignment i.e:
    `y << ((-(uintptr)x * 8) & 63)`

Alive2 Proofs (including all cases with undefs in the vector):
https://alive2.llvm.org/ce/z/f-65b6

Reviewed By: RKSimon, pengfei

Differential Revision: https://reviews.llvm.org/D150294
parent 36cfea5a
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