689779 - IonMonkey: Eliminate negative zero checks for a*a

Status: RESOLVED FIXED

(Core :: JavaScript Engine, defect)

Description

[David Anderson [:dvander] - inactive, e-mail if emergency]

Attachment: benchmark

In the attached benchmark, we could get a significant speedup from removing the negative zero check on the multiplication. We should be able to do this in MIR by seeing that (a * a) will never be negative.

Comment 1

[Terrence Cole [:terrence]]

Attachment: First attempt.

Speedup is 1.12x, but I need some feedback on the patch's specifics:

1) I think isCongruent is a more specific (and costly) test that we want for this, but I could be quite wrong.

2) Is there a way to implement this check without having to copy the body of MBinaryArithInstruction::foldsTo?

3) Should I add a setter for canBeNegativeZero_?

Comment 2

[Terrence Cole [:terrence]]

Attachment: v2

Shorter patch using congruentTo and calling out to super (requires us to re-acquire lhs/rhs).  It seems to be epsilon slower than v1 in the benchmark, but it's well within the error bars.

Comment 3

[David Anderson [:dvander] - inactive, e-mail if emergency]

Comment on attachment 563524 [details] [diff] [review]
v2

Nice.

Comment 4

[Terrence Cole [:terrence]]

Attachment: rebased

I still don't have commit access, but that's no reason to let this bitrot.

Comment 5

[David Anderson [:dvander] - inactive, e-mail if emergency]

https://hg.mozilla.org/projects/ionmonkey/rev/80f0dce7aa2e