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++];

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).

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

We merged Type Inference.