1690538 - SIMD optimization x64/x86: Better code for float->int conversion

Status: RESOLVED FIXED

(Core :: JavaScript: WebAssembly, enhancement, P3)

Description

[Lars T Hansen [:lth]]

We have fairly elaborate code for int->float conversion, see eg MacroAssemblerX86Shared::truncSatFloat32x4Toint32x2 or the even worse unsigned variant right below it. We can do better: https://github.com/WebAssembly/simd/issues/173, in the third comment, "option 2" is appealing. It's also possible v8 has updated its code since I let myself be inspired by it for these functions, and we should let it inspire us again.

(Also: https://github.com/WebAssembly/simd/issues/190, though it mostly points to #173.)

Comment 1

[Yury Delendik (:yury)]

Looked into "option 2" (vmaxps dst, src, [rip + OFFSET_THAT_CONTAINS_INT_MAX_AS_FLOAT] looks incorrect):

• even with vminps...
• there is no exact representation for INT_MAX as float: 2147483520.f as INT_MAX produces 0x7FFFFF80, and 2147483647.f gives 0x80000000f

Comment 2

[Lars T Hansen [:lth]]

Surely there is a canonical representation for INT_MAX as float on the architecture even if it is not exact? Indeed our own unsignedTruncSatFloat32x4ToInt32x4 uses such a representation.

Happy to delay this until we have time to concentrate on those details.

Comment 3

[Yury Delendik (:yury)]

Attachment: Bug 1690538 - Better code for i32x4.trunc_sat_f32x4_s conversion. r?jseward

For non-AVX, the approach saves one instruction.

For AVX, the savings are 3 instructions.

Comment 4

[Yury Delendik (:yury)]

Attachment: Bug 1690538 - Revise unsignedTruncSatFloat32x4ToInt32x4 code. r?jseward

Fix alu vs fpu xor, reword comment.

Depends on D144463

Comment 5

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/8bd424227b76
Better code for i32x4.trunc_sat_f32x4_s conversion. r=jseward
https://hg.mozilla.org/integration/autoland/rev/4ee04d98cd3d
Revise unsignedTruncSatFloat32x4ToInt32x4 code. r=jseward

Comment 6

[Iulian Moraru]

https://hg.mozilla.org/mozilla-central/rev/8bd424227b76
https://hg.mozilla.org/mozilla-central/rev/4ee04d98cd3d