888237 - IonMonkey: (ARM) Use hardware divide instruction to calculate modulus

Status: RESOLVED FIXED

(Core :: JavaScript Engine, defect)

Description

[Jon Coppeard (:jonco)]

Following on from bug 872824, we can use the divide instruction to calculate modulus too, on appropriate hardware.

Comment 1

[Jon Coppeard (:jonco)]

Attachment: Proposed change

Very similar to the divide case.  Uses the combination of divide and then multiply-subtract to calculate the modulus.

Comment 2

[Marty Rosenberg [:mjrosenb]]

Comment on attachment 777179 [details] [diff] [review]
Proposed change

Review of attachment 777179 [details] [diff] [review]:
-----------------------------------------------------------------

::: js/src/ion/arm/CodeGenerator-arm.cpp
@@ +706,5 @@
> +    // save the lhs in case we end up with a 0 that should be a -0.0 because lhs < 0.
> +    masm.ma_mov(lhs, callTemp);
> +
> +    Label done;
> +    if (!modICommon(mir, lhs, rhs, output, ins->snapshot(), done))

I suspect that we can eliminate this.  The ARM ARM says that INT_MIN / -1 produces INT_MIN, and INT_MIN - (INT_MIN * -1) == INT_MIN + INT_MIN == 0, which is the result that we want.  The following negative checks should ensure that a non-truncated result bails, and truncated results become 0.

Comment 3

[Jon Coppeard (:jonco)]

https://hg.mozilla.org/integration/mozilla-inbound/rev/594fc2c953b6

Comment 4

[Ryan VanderMeulen [:RyanVM]]

https://hg.mozilla.org/mozilla-central/rev/594fc2c953b6