[CVP] Missed optimization for smax((a + 7) / 8, 1) where a s> 0

[XChy]

Alive2 proof: https://alive2.llvm.org/ce/z/Cyoo2Y Missed example: https://godbolt.org/z/9zW8PToPe

In this example, smax((a + 7) / 8, 1) should be folded to (a + 7) / 8 based on control flow context, but not.

Real-world motivation: This snippet of IR is derived from FFmpeg/.../extr_aacdec.c_latm_decode_audio_specific_config.c (after O3 pipeline) The example above is a reduced version. If you're interested in the original suboptimal IR and optimal IR. See also: https://godbolt.org/z/4a6K4b8hc

Let me know if you can confirm that it's an optimization opportunity, thanks.

[antoniofrighetto]

Looks a valid optimization to me. Not sure if, in principle, this should live in CVP or somewhere in ConstantPropagation.

[XChy]

Looks a valid optimization to me. Not sure if, in principle, this should live in CVP or somewhere in ConstantPropagation.

@antoniofrighetto thanks for feedback! I've changed the title until the pass to blame is specified.

[XChy]

I think it's the issue of CVP and LVI, in processMinMaxIntrinsic of CVP, we don't get the predicate between LHS and RHS at their uses, therefore becoming imprecise. We should add something like getPredicateAtUse in LVI and then use it in CVP.

[nikic]

@XChy I don't think that would be sufficient here. The issue is that %div is defined outside the branch condition and will use the value of %a without the condition. Even if you use AtUse API, it will not be able to constrain the result of %div (as the condition is on %a, not %div).

[XChy]

@nikic You're right, I missed that we can constrain only one value at a time now. For one-use example in this issue, an easy solution is to sink these one-use instructions as SinkPass does. But for multi-use scenario in real world, that's a bit tricky to handle.