1995488 - Optimise clamp{Double}IntToUint8 on riscv64

Status: RESOLVED FIXED

(Core :: JavaScript Engine: JIT, enhancement, P3)

Comment 1

[André Bargull [:anba]]

Attachment: Bug 1995488 - Part 1: Avoid comparison against non-zero in clampIntToUint8 for riscv64. r=#spidermonkey-reviewers!

ClampIToUint8 was compiled to:

slti      t4, t0, 0
li        t5, 1
bne       t4, t5, 0 -> skip
mv        t0, zero
j         0 -> skip2
li        t5, 255
bge       t5, t0, 0 -> skip2
li        t0, 255

Loading the immediate 1 into t5 is unnecessary when instead comparing
against zero. But we can go even further and use the same approach as used
in MacroAssemblerRiscv64Compat::minMax32 to check for negative values. That
saves another branch and ClampIToUint8 is now compiled to:

sgtz      t4, t0
neg       t4, t4
and       t0, t0, t4
li        t4, 255
bge       t4, t0, 0 -> skip
li        t0, 255

Comment 2

[André Bargull [:anba]]

Attachment: Bug 1995488 - Part 2: Remove branches from clampDoubleToUint8 on riscv64. r=#spidermonkey-reviewers!

Improves the generated code from:

feq.d     t4, ft1, ft1
beq       t4, zero, 0 -> nan
addi      zero, t4, 17
fcvt.w.d  [RNE] t0, ft1
sgtz      t5, t0
neg       t5, t5
and       t0, t0, t5
li        t5, 255
bge       t5, t0, 0 -> skip
li        t0, 255
j         0 -> done
mv        t0, zero

To:

fcvt.w.d  [RNE] t0, ft1
feq.d     t4, ft1, ft1
neg       t4, t4
and       t0, t0, t4
sgtz      t4, t0
neg       t4, t4
and       t0, t0, t4
li        t4, 255
bge       t4, t0, 0 -> skip
li        t0, 255

Two less instructions and only a single conditional jump.

Comment 3

[Pulsebot]

Pushed by [e-mail]:
https://github.com/mozilla-firefox/firefox/commit/f6e86e9a5e2b
https://hg.mozilla.org/integration/autoland/rev/b7f1cbaf21f0
Part 1: Avoid comparison against non-zero in clampIntToUint8 for riscv64. r=spidermonkey-reviewers,iain
https://github.com/mozilla-firefox/firefox/commit/9aa3304e2fa4
https://hg.mozilla.org/integration/autoland/rev/c28e2144cb69
Part 2: Remove branches from clampDoubleToUint8 on riscv64. r=spidermonkey-reviewers,iain

Comment 4

[Norisz Fay [:noriszfay]]

https://hg.mozilla.org/mozilla-central/rev/b7f1cbaf21f0
https://hg.mozilla.org/mozilla-central/rev/c28e2144cb69