[PATCH] D107766: [InstCombine] Get rid of `hasOneUses()` when swapping `lshr` and `zext`

[Anton Afanasyev via Phabricator via llvm-commits]

anton-afanasyev added a comment.

In D107766#2936935 <https://reviews.llvm.org/D107766#2936935>, @lebedev.ri wrote:

> Some observations for logical right-shift.
> this will have a hard time with variable shift amounts.
> You need to avoid creating out-of-bounds shifts, there are two obvious options:
>
> 1. https://alive2.llvm.org/ce/z/XShcju <- shift amount needs to be less than target width, and truncation should only drop zeros.
> 2. https://alive2.llvm.org/ce/z/QiDPV7 <- could saturate the shift amount if you know that `%x` has more leading zeros than the number of bits to be truncated
>
> We might already have this logic somewhere, not sure.

Yes, thanks for your observations, I'm already working on it: https://alive2.llvm.org/ce/z/XcCJ9Q
There is also special care for the vector case to make a transform not being more poisonous.
`TruncInstCombine` already has appropriate logic but needs to be tweaked. 
For now I'm supposing that shift amout is constant (int or vector). Not sure that transform adding check for variable shift amount is good.


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