1995490 - Optimise left-rotate for riscv64

Status: RESOLVED FIXED

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

Description

[André Bargull [:anba]]

Left-rotate is implemented by performing 32 - (count % 32)) in assembly code, which is fairly inefficient.

Comment 1

[André Bargull [:anba]]

Attachment: Bug 1995490 - Part 1: Optimise left-rotate for riscv64. r=#spidermonkey-reviewers!

i32.rotl was compiled to:

li        t5, 32
remw      t4, a1, t5
negw      t4, t4
addi      t4, t4, 32
negw      t5, t4
sllw      t5, a0, t5
srlw      a0, a0, t4
or        a0, t5, a0
sext.w    a0, a0

And now it's directly:

negw      t4, a1
srlw      t4, a0, t4
sllw      a0, a0, a1
or        a0, t4, a0
sext.w    a0, a0

Comment 2

[André Bargull [:anba]]

Attachment: Bug 1995490 - Part 2: Replace modulus with bit-and for shift/rotate with immediate. r=#spidermonkey-reviewers!

Shift and rotate instructions are defined as using the low five resp. six bits.
This is more accurately implemented when using bit-and instead of (signed)
integer modulus.

Comment 3

[Pulsebot]

Pushed by [e-mail]:
https://github.com/mozilla-firefox/firefox/commit/f8e4fb61645e
https://hg.mozilla.org/integration/autoland/rev/9ae44eb6b083
Part 1: Optimise left-rotate for riscv64. r=spidermonkey-reviewers,iain
https://github.com/mozilla-firefox/firefox/commit/46af4d9a893a
https://hg.mozilla.org/integration/autoland/rev/6c2445409807
Part 2: Replace modulus with bit-and for shift/rotate with immediate. r=spidermonkey-reviewers,iain

Comment 4

[Norisz Fay [:noriszfay]]

https://hg.mozilla.org/mozilla-central/rev/9ae44eb6b083
https://hg.mozilla.org/mozilla-central/rev/6c2445409807