commit | bcdede0c53ab6c0cb6912111f599435cd8f6929f | [log] [tgz] |
---|---|---|
author | Benedikt Meurer <bmeurer@chromium.org> | Wed Sep 26 14:47:25 2018 |
committer | Commit Bot <commit-bot@chromium.org> | Sun Oct 07 12:00:01 2018 |
tree | 0619dc5d12152f840781d894bb1ec4c9c8da50fd | |
parent | 248fd5ffe045f583ee493634d64cf9b3b068c106 [diff] |
[turbofan] Eliminate redundant Smi checks around array accesses. As identified in the web-tooling-benchmark, there are specific code patterns involving array indexed property accesses and subsequent comparisons of those indices that lead to repeated Smi checks in the optimized code, which in turn leads to high register pressure and generally bad register allocation. An example of this pattern is code like this: ```js function f(a, n) { const i = a[n]; if (n >= 1) return i; } ``` The `a[n]` property access introduces a CheckBounds on `n`, which later lowers to a `CheckedTaggedToInt32[dont-check-minus-zero]`, however the `n >= 1` comparison has collected `SignedSmall` feedback and so it introduces a `CheckedTaggedToTaggedSigned` operation. This second Smi check is redundant and cannot easily be combined with the earlier tagged->int32 conversion, since that also deals with heap numbers and even truncates -0 to 0. So we teach the RedundancyElimination to look at the inputs of these speculative number comparisons and if there's a leading bounds check on either of these inputs, we change the input to the result of the bounds check. This avoids the redundant Smi checks later and generally allows the SimplifiedLowering to do a significantly better job on the number comparisons. We only do this in case of SignedSmall feedback and only for inputs that are not already known to be in UnsignedSmall range, to avoid doing too many (unnecessary) expensive lookups during RedundancyElimination. All of this is safe despite the fact that CheckBounds truncates -0 to 0, since the regular number comparisons in JavaScript identify 0 and -0 (unlike Object.is()). This also adds appropriate tests, especially for the interesting cases where -0 is used only after the code was optimized. Bug: v8:6936, v8:7094 Change-Id: Ie37114fb6192e941ae1a4f0bfe00e9c0a8305c07 Reviewed-on: https://chromium-review.googlesource.com/c/1246181 Reviewed-by: Sigurd Schneider <sigurds@chromium.org> Commit-Queue: Benedikt Meurer <bmeurer@chromium.org> Cr-Commit-Position: refs/heads/master@{#56428}
V8 is Google's open source JavaScript engine.
V8 implements ECMAScript as specified in ECMA-262.
V8 is written in C++ and is used in Google Chrome, the open source browser from Google.
V8 can run standalone, or can be embedded into any C++ application.
V8 Project page: https://github.com/v8/v8/wiki
Checkout depot tools, and run
fetch v8
This will checkout V8 into the directory v8
and fetch all of its dependencies. To stay up to date, run
git pull origin gclient sync
For fetching all branches, add the following into your remote configuration in .git/config
:
fetch = +refs/branch-heads/*:refs/remotes/branch-heads/* fetch = +refs/tags/*:refs/tags/*
Please follow the instructions mentioned on the V8 wiki.