Bug 160073

Summary:

[B3] Fusing immediates into test instructions should work again

Product:

WebKit

Reporter:

Filip Pizlo <fpizlo>

Component:

JavaScriptCore

Assignee:

Filip Pizlo <fpizlo>

Status:

RESOLVED FIXED

Severity:

Normal

CC:

benjamin, commit-queue, keith_miller, mark.lam, msaboff, saam

Priority:

Version:

WebKit Nightly Build

Hardware:

All

OS:

All

Attachments:

Description	Flags
I don't know if I'm doing this right	none
the patch	none
the patch	sam: review+
performance	none

Filip Pizlo

Reported 2016-07-21 23:05:37 PDT

When we introduced BitImm, we forgot to change the Branch(BitAnd(value, constant)) fusion. This emits test instructions, so it should use BitImm for the constant. But it was still using Imm!

Attachments
I don't know if I'm doing this right (19.23 KB, patch) 2016-07-22 00:29 PDT, Filip Pizlo	no flags	Details Formatted Diff Diff
the patch (26.83 KB, patch) 2016-07-22 15:20 PDT, Filip Pizlo	no flags	Details Formatted Diff Diff
the patch (25.04 KB, patch) 2016-07-22 21:42 PDT, Filip Pizlo	sam: review+	Details Formatted Diff Diff
performance (80.42 KB, text/plain) 2016-07-22 21:46 PDT, Filip Pizlo	no flags	Details
Show Obsolete (2) View All Add attachment proposed patch, testcase, etc.

Filip Pizlo

Comment 1 2016-07-22 00:29:08 PDT

Created attachment 284312 [details] I don't know if I'm doing this right The logic for deciding what sorts of immediates can be used for Branch(BitAnd) fusion was broken pretty badly. I'm trying to fix it.

Filip Pizlo

Comment 2 2016-07-22 15:20:23 PDT

Created attachment 284378 [details] the patch NOTE: I haven't yet had a chance to perf test this latest version, which is why it's not r? yet.

Filip Pizlo

Comment 3 2016-07-22 21:42:17 PDT

Created attachment 284400 [details] the patch

Filip Pizlo

Comment 4 2016-07-22 21:46:22 PDT

Created attachment 284401 [details] performance It's neutral, except on LongSpider, where it's a regression on math-cordic! I investigated this regression in detail. It's hilarious. The benchmark is very small, so it's sensitive to weird stuff. And how weird this is. Previously, there was code like this: 0x4ea9fda0347e: test %edx, %r10d Where %r10 is initialized to a constant, which gets hoisted. Normally this would be bad codegen. And this patch fixes it to be: 0x365b91a03457: test $0x7fffffff, %edx But by doing this, we perturb regalloc throughout the entire function. Lots of things change in subtle ways. None of the other effects look, on the surface, like either good or bad. But in this case, occupying %r10 and so perturbing everything is worth a 9% speed-up. To me this means that we should ignore the math-cordic regression. The reason why we're seeing such a weird effect on this test is that this test contains only one hot function. This function doesn't even do a particularly good job of staying in FTL anyway.

Filip Pizlo

Comment 5 2016-07-24 10:38:13 PDT

Landed in https://trac.webkit.org/changeset/203666

Filip Pizlo

Comment 6 2016-07-24 12:01:26 PDT

Follow-up fix: https://trac.webkit.org/changeset/203668 Late in the game I removed the Imm->BitImm changes in a few places to recover perf regressions, and I went too far by removing it from BranchTest8. Also, I forgot to spin the load-branch fusion tests on not-x86.

Saam Barati

Comment 7 2016-07-26 19:34:38 PDT

Rolled out in: http://trac.webkit.org/changeset/203758

Filip Pizlo

Comment 8 2016-08-01 16:23:52 PDT

Relanded in https://trac.webkit.org/changeset/203996

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