1633386 - Simplify flexible-shift functions

Status: RESOLVED FIXED

(Core :: JavaScript Engine: JIT, enhancement)

Description

[André Bargull [:anba]]

flexibleLshift32 (and the right shift alternatives) can be simplified using xchg, which also leads to shorter assembly. (JSC does something similar.)

x << y was previously generating the following assembly (for CacheIR):

[Codegen] emitInt32LeftShiftResult
[Codegen] push       %rcx
[Codegen] push       %rbp
[Codegen] movq       0x8(%rsp), %rbp
[Codegen] movq       %rbp, %rcx
[Codegen] andl       $0x1f, %ebx
[Codegen] push       %rbx
[Codegen] xchgq      %rcx, %rbx
[Codegen] shll       %cl, %ebx
[Codegen] movq       %rbx, %rcx
[Codegen] pop        %rbx
[Codegen] movabsq    $0xfff8800000000000, %r11
[Codegen] orq        %r11, %rcx

and now generates:

[Codegen] emitInt32LeftShiftResult
[Codegen] push       %rcx
[Codegen] push       %rbp
[Codegen] movq       0x8(%rsp), %rbp
[Codegen] movq       %rbp, %rcx
[Codegen] andl       $0x1f, %ebx
[Codegen] xchgq      %rbx, %rcx
[Codegen] shll       %cl, %ebx
[Codegen] xchgq      %rbx, %rcx
[Codegen] movabsq    $0xfff8800000000000, %r11
[Codegen] orq        %r11, %rcx

Comment 1

[André Bargull [:anba]]

Attachment: Bug 1633386: Simplify flexible shift instructions. r=jandem!

Comment 2

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/3f8428b83795
Simplify flexible shift instructions. r=jandem

Comment 3

[Cosmin Sabou [:CosminS]]

https://hg.mozilla.org/mozilla-central/rev/3f8428b83795