JIT Optimization: Eliminate duplicate bswap

[deeprobin]

An bswap swaps the endianess. If we emit an bswap twice we get back to the initial state, so this can be optimized (similar to what GCC and Clang do).

Reproduction: https://godbolt.org/z/KMEn8bYsc

Expected diff

    mov eax, edx
-   bswap eax
-   bswap eax
    ret

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Issue Details

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An bswap swaps the endianess. If we emit an bswap twice we get back to the initial state, so this can be optimized (similar to what GCC and Clang do).

Reproduction: https://godbolt.org/z/KMEn8bYsc

Expected diff

    mov eax, edx
-   bswap eax
-   bswap eax
    ret

Author:	deeprobin
Assignees:	-
Labels:	area-CodeGen-coreclr, untriaged
Milestone:	-

[EgorBo]

As we discussed in the Discord - this optimization looks super niche
if it's two lines of code and it actually helps with some real-world code - we can accept it I'd guess, but overall I'd personally not bother.

[deeprobin]

@EgorBo

I asked in the LLVM reviews what the real-world case is (See D6407: [InstCombine] Minor optimization for bswap with binary ops).
Maybe this will give us some insight.

[stephentoub]

I asked in the LLVM reviews what the real-world case is

In the future, please only open issues when the real-world use case is known. Thanks.

[JulieLeeMSFT]

As we discussed in the Discord - this optimization looks super niche

Closing now. Let's reopen this issue when there is a real world case.

[deeprobin]

The main reason to merge this into LLVM was the ARGB - RGBA Pixel Code Swizzling (see LLVM Phabricator conversation)

I could imagine that this problem might occur with the API proposal #48615.

Tanner do you think the ARGB-RGB Pixel Swizzling could be a hot path for certain use cases (e.g. WinForms)? If so, the optimization would not be so bad in my opinion.

/cc @RKSimon
/cc @JeremyKuhne
/cc @tannergooding

[davidbolvansky]

The main reason to merge this into LLVM was the ARGB - RGBA Pixel Code Swizzling (see LLVM Phabricator conversation)

no, “this” has nothing common with that LLVM review. Two different transformations.