Stream: git-wasmtime

Topic: wasmtime / PR #5097 Remove redundant branch and select in...

Wasmtime GitHub notifications bot (Oct 23 2022 at 20:40):

elliottt edited PR #5097 from trevor/remove-branch-instructions to main:

As discussed in the 2022/10/19 meeting, this PR removes many of the branch and select instructions that used iflags, in favor if using brz/brnz and select in their place. Additionally, it reworks selectif_spectre_guard to take an i8 input instead of an iflags input.

For reference, the removed instructions are: br_icmp, brif, brff, trueif, trueff, and selectif.
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Wasmtime GitHub notifications bot (Oct 24 2022 at 17:00):

elliottt updated PR #5097 from trevor/remove-branch-instructions to main.

Wasmtime GitHub notifications bot (Oct 24 2022 at 17:23):

elliottt updated PR #5097 from trevor/remove-branch-instructions to main.

Wasmtime GitHub notifications bot (Oct 24 2022 at 17:24):

elliottt requested cfallin for a review on PR #5097.

Wasmtime GitHub notifications bot (Oct 24 2022 at 17:25):

elliottt has marked PR #5097 as ready for review.

Wasmtime GitHub notifications bot (Oct 24 2022 at 18:46):

abrown submitted PR review.

Wasmtime GitHub notifications bot (Oct 24 2022 at 18:46):

abrown submitted PR review.

Wasmtime GitHub notifications bot (Oct 24 2022 at 18:46):

abrown created PR review comment:

This is an example where more work _must_ be done by the CPU due to this change, right?

Wasmtime GitHub notifications bot (Oct 24 2022 at 20:22):

elliottt submitted PR review.

Wasmtime GitHub notifications bot (Oct 24 2022 at 20:22):

elliottt created PR review comment:

It looks like we already generate multiple comparisons on main. I adapted the example you pointed out to work on the x64 backend (removed uses of trueff) and ran it through clif-util test as a precise-output test:

function %fflags(f32) {
block200(v0: f32):
    v1 = f32const 0x34.0p0
    v2 = ffcmp v0, v1
    brff eq v2, block201
    jump block400

block400:
    brff ord v2, block202
    jump block401

block401:
    return

block201:
    return

block202:
    trap heap_oob
}

We produce assembly that has one ucomiss for each brff, not one comparison whose flags are reused between those instructions, so the changes on this branch shouldn't affect performance.

  pushq   %rbp
  movq    %rsp, %rbp
block0:
  movl    $1112539136, %r8d
  movd    %r8d, %xmm5
  ucomiss %xmm5, %xmm0
  jp      label2
  jnz     label2; j label1
block1:
  movq    %rbp, %rsp
  popq    %rbp
  ret
block2:
  movl    $1112539136, %esi
  movd    %esi, %xmm9
  ucomiss %xmm9, %xmm0
  jnp     label3; j label4
block3:
  ud2 heap_oob
block4:
  movq    %rbp, %rsp
  popq    %rbp
  ret

For completeness, here's the assembly generated for that same example on this branch, adjusted for brff being removed:

  pushq   %rbp
  movq    %rsp, %rbp
block0:
  movl    $1112539136, %r8d
  movd    %r8d, %xmm5
  ucomiss %xmm5, %xmm0
  jp      label2
  jnz     label2; j label1
block1:
  jmp     label5
block2:
  movl    $1112539136, %esi
  movd    %esi, %xmm9
  ucomiss %xmm9, %xmm0
  jnp     label3; j label4
block3:
  ud2 heap_oob
block4:
  jmp     label5
block5:
  movq    %rbp, %rsp
  popq    %rbp
  ret

Wasmtime GitHub notifications bot (Oct 24 2022 at 20:55):

elliottt updated PR #5097 from trevor/remove-branch-instructions to main.

Wasmtime GitHub notifications bot (Oct 24 2022 at 21:02):

elliottt updated PR #5097 from trevor/remove-branch-instructions to main.

Wasmtime GitHub notifications bot (Oct 24 2022 at 21:30):

abrown submitted PR review.

Wasmtime GitHub notifications bot (Oct 24 2022 at 21:30):

abrown created PR review comment:

Ah, good to know!

Wasmtime GitHub notifications bot (Oct 24 2022 at 22:59):

cfallin submitted PR review.

Wasmtime GitHub notifications bot (Oct 24 2022 at 23:14):

elliottt merged PR #5097.

Last updated: Nov 22 2024 at 17:03 UTC