773859 - Math.random() needs to inlined in JM

Status: RESOLVED WONTFIX

(Core :: JavaScript Engine, defect)

Description

[(no longer active)]

Let's say that you have a function in an emscripted program which returns a random signed int between 0 and n.  Here's how the original code looks like:

 int rand(int max) {
   return (unsigned long)(emscripten_random() * 0x8fffffff) % max;
 }

Here's how the js function with -O2 --closure 0 looks like:

 function _emscripten_random() {
   return Math.random();
 }

 function _rand($max) {
   var $1 = _emscripten_random();
   var $2 = $1 * 2415919104;
   var $3 = $2 >= 0 ? Math.floor($2) : Math.ceil($2);
   return ($3 >>> 0) % ($max >>> 0);
 }

In my profile, I see a lot of call overhead to Math.random().  Looking at the implementation, it seems like it could possibly be inlined in the jitted code, which would avoid the call overhead (which in my profile takes about 50% of the entire _rand() function.)

Comment 1

[Luke Wagner [:luke]]

In general, we've stopped adding optimizations to JM that don't also help IM.  This sounds like a good IM bug, though.

Comment 2

[Sean Stangl [:sstangl]]

Filed 774364 for tracking in Ion.

Comment 3

[Paul Wright]

Should this bug be closed and/or duped to bug 774364 per comments 1 and 2?

Comment 4

[Vladimir Vukicevic [:vlad] [:vladv] (needinfo me, slow to respond)]

Don't care about this in JM any more; the IM bug is in 774364.