Stream: git-wasmtime

Topic: wasmtime / PR #5918 Remove the Cranelift `vselect` instru...

Wasmtime GitHub notifications bot (Mar 02 2023 at 23:12):

alexcrichton opened PR #5918 from remove-vselect to main:

This instruction is documented as selecting lanes based on the "truthy" value of the condition lane, but the current status of the implementation of this instruction is:

• x64 - uses the high bit for f32x4 and f64x2 and otherwise uses the high bit of each byte doing a byte-wise lane select rather than whatever the controlling type is.

• AArch64 - this is the same as bitselect which is a bit-wise selection rather than a lane-wise selection.

• s390x - this is the same as AArch64, a bit-wise selection rather than lane-wise.

• interpreter - the interpreter implements the documented semantics of selecting based on "truthy" values.

Coupled with the status of the implementation is the fact that this instruction is not used by WebAssembly SIMD today either. The only use of this instruction in Cranelift is the nan-canonicalization pass. By moving nan-canonicalization to bitselect, since that has the desired semantics, there's no longer any need for vselect.

Given this situation this commit subsqeuently removes vselect and all usage of it throughout Cranelift.

Closes #5917

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Wasmtime GitHub notifications bot (Mar 02 2023 at 23:23):

afonso360 submitted PR review.

Wasmtime GitHub notifications bot (Mar 02 2023 at 23:23):

afonso360 submitted PR review.

Wasmtime GitHub notifications bot (Mar 02 2023 at 23:23):

afonso360 created PR review comment:

Same here?

Wasmtime GitHub notifications bot (Mar 02 2023 at 23:23):

afonso360 created PR review comment:

I think these were removed by mistake right?

Wasmtime GitHub notifications bot (Mar 02 2023 at 23:41):

alexcrichton updated PR #5918 from remove-vselect to main.

Wasmtime GitHub notifications bot (Mar 02 2023 at 23:42):

alexcrichton submitted PR review.

Wasmtime GitHub notifications bot (Mar 02 2023 at 23:42):

alexcrichton created PR review comment:

Oh dear now I'm not sure how these leaked in...

Wasmtime GitHub notifications bot (Mar 07 2023 at 16:41):

alexcrichton updated PR #5918 from remove-vselect to main.

Wasmtime GitHub notifications bot (Mar 07 2023 at 20:41):

jameysharp submitted PR review.

Wasmtime GitHub notifications bot (Mar 07 2023 at 20:41):

jameysharp submitted PR review.

Wasmtime GitHub notifications bot (Mar 07 2023 at 20:41):

jameysharp created PR review comment:

Maybe some additional documentation along these lines:

        This instruction selects whole values. Use `bitselect` to choose each bit according to a mask.

Wasmtime GitHub notifications bot (Mar 07 2023 at 20:41):

jameysharp created PR review comment:

Might as well fix the rest of the comment while we're here I guess.

;; we can use x64_blend.

Wasmtime GitHub notifications bot (Mar 07 2023 at 20:41):

jameysharp created PR review comment:

I think all of these vselect/icmp rewrites are still valid if we match on bitselect instead, right? as_bool on a typeset containing vectors of integers produces the same typeset, so the result of icmp should be the same as its arguments on vectors, I think.

Wasmtime GitHub notifications bot (Mar 07 2023 at 20:41):

jameysharp created PR review comment:

I'm hoping we can replace vselect with bitselect in this file (and rename it) and have all these tests still pass.

Wasmtime GitHub notifications bot (Mar 07 2023 at 20:41):

jameysharp created PR review comment:

I think replacing vselect with bitselect almost works for fcmp as well, except that typesets containing vectors of floats turn into vectors of ints of the same size, but bitselect requires all the types to be the same.

Maybe we should extend bitselect to accept int vectors of the right width even for selecting between float vectors? I think that would remove the need for this PR's changes to NaN canonicalization, and probably be more useful in general.

Wasmtime GitHub notifications bot (Mar 07 2023 at 20:41):

jameysharp created PR review comment:

I see the remaining uses of this vselect helper are always fed by the result of icmp, to implement the min and max instructions. It'd be nice to clean that up. Opening a "good first issue" as a followup for this PR would be fine though.

Wasmtime GitHub notifications bot (Mar 07 2023 at 22:37):

alexcrichton updated PR #5918 from remove-vselect to main.

Wasmtime GitHub notifications bot (Mar 07 2023 at 22:41):

alexcrichton updated PR #5918 from remove-vselect to main.

Wasmtime GitHub notifications bot (Mar 07 2023 at 22:42):

alexcrichton submitted PR review.

Wasmtime GitHub notifications bot (Mar 07 2023 at 22:42):

alexcrichton created PR review comment:

Would you be ok with a follow-up issue for this?

Wasmtime GitHub notifications bot (Mar 07 2023 at 23:08):

jameysharp submitted PR review.

Wasmtime GitHub notifications bot (Mar 07 2023 at 23:08):

jameysharp submitted PR review.

Wasmtime GitHub notifications bot (Mar 07 2023 at 23:08):

jameysharp created PR review comment:

I'll feel a little better about adding this bitcast if it's only done right next to the bitselect call that it feeds into. I hope that'll make it easier to spot for somebody cleaning up the types of bitselect later. Also it reduces the diff in this file to pretty much just this closure.

    let vector_select = |pos: &mut FuncCursor, canon_nan: Value| {
        let is_nan = pos.ins().bitcast(val_type, MemFlags::new(), is_nan);

Wasmtime GitHub notifications bot (Mar 07 2023 at 23:08):

jameysharp created PR review comment:

Since scalar_select is only called with the is_nan value from the outer scope, can you revert adding this argument?

Wasmtime GitHub notifications bot (Mar 07 2023 at 23:08):

jameysharp created PR review comment:

Sure, assuming the changes I've just suggested to NaN canonicalization seem reasonable to you, we can think about bitselect semantics in more detail later. If we see a lot more cases of fcmp->bitcast->bitselect I might prioritize it higher.

For the record, I'm not concerned about losing these two specific optimization rules, since they weren't motivated by seeing this pattern in any benchmarks; we just added them because we could. But it'd be nice if you'd leave a comment behind mentioning something like "TODO: this optimization also applies to vectors but the pattern to match is more complicated".

Wasmtime GitHub notifications bot (Mar 07 2023 at 23:53):

alexcrichton updated PR #5918 from remove-vselect to main.

Wasmtime GitHub notifications bot (Mar 07 2023 at 23:57):

alexcrichton updated PR #5918 from remove-vselect to main.

Wasmtime GitHub notifications bot (Mar 07 2023 at 23:59):

alexcrichton submitted PR review.

Wasmtime GitHub notifications bot (Mar 07 2023 at 23:59):

alexcrichton created PR review comment:

Ok sounds good. I mostly want to not pile up too much on this PR and I'm not too familiar with cranelift's instruction format builde, so I'd prefer to set that aside for another time rather than doing it here-and-now. I do think it makes sense, though that bitselect always takes an integer mask. Most of these optimizations probably won't fire from wasm code though since wasm v128.bitselect forces everything through i8x16 which loses most of the type information. That being said these optimizations aren't all that important for wasm probably since if you're using SIMD you're probably using LLVM which would already have done optimizations like this ideally.

Wasmtime GitHub notifications bot (Mar 08 2023 at 00:25):

alexcrichton has enabled auto merge for PR #5918.

Wasmtime GitHub notifications bot (Mar 08 2023 at 01:24):

alexcrichton merged PR #5918.

Wasmtime GitHub notifications bot (Mar 08 2023 at 03:27):

alexcrichton submitted PR review.

Wasmtime GitHub notifications bot (Mar 08 2023 at 03:27):

alexcrichton created PR review comment:

I've opened https://github.com/bytecodealliance/wasmtime/issues/5962 for this

Last updated: Oct 23 2024 at 20:03 UTC