618690 - JM: TypeInference: Hoist array bounds checks

Status: RESOLVED FIXED

(Core :: JavaScript Engine, defect)

Description

[Brian Hackett [Laid off!]]

For loops like:

for (i = 0; i < n; i++)
  arr[i]

We can hoist the initialized-length check to the loop's preamble code provided we can determine that arr is not modified in the loop, that i and n are integers and that the length of arr cannot shrink.

This should generalize but not too broadly, as array-accessing loops tend to be written in similar ways and doing a full symbolic range analysis is complicated and takes time.  It should, however, handle loop forms like that in v8-crypto's am3, which looks like:

while(--n >= 0)
  arr[i++];

Comment 1

[Brian Hackett [Laid off!]]

Attachment: WIP

Determines when bounds checks can be hoisted for a few access patterns: x[n], x[i] where i is loop invariant, and x[i] where i is <= something as determined from a linear relationship in the loop condition.  Also factors loop register stuff out of FrameState and into a new LoopState file (paving the way to make FrameState a per-frame thing).

Comment 2

[Brian Hackett [Laid off!]]

Attachment: patch

Patch landed on JM.  This is mostly about adding the machinery so we can hoist array bounds checks, will circle back and improve robustness once LICM and property accesses are improved and we have a good corpus of the bounds checks we want to be able to hoist.

http://hg.mozilla.org/projects/jaegermonkey/rev/7928f2dc3d4d

function foo(x, n) {
  for (var i = 0; i < n; i++)
    x[i] = i;
  var q = 0;
  for (var i = 0; i < 100000; i++) {
    for (var j = 0; j < n; j++)
      q = x[j];
  }
  return q;
}
foo([], 1000);

js -m -n (old): 145
js -m -n (new): 110
js -m: 577
js -j: 374
d8: 150

Comment 3

[Tom S [:evilpie]]

We merged Type Inference.