Bug 19803

Summary:

Merge multiple loads into a single instruction

Product:

WebKit

Reporter:

Cameron Zwarich (cpst) <zwarich>

Component:

JavaScriptCore

Assignee:

Nobody <webkit-unassigned>

Status:

RESOLVED FIXED

Severity:

Normal

CC:

mjs

Priority:

Version:

528+ (Nightly build)

Hardware:

All

OS:

All

Attachments:

Description	Flags
Patch that causes regression	none
Patch that improves performance	none

Cameron Zwarich (cpst)

Reported 2008-06-28 16:21:37 PDT

Since (load, load) is one of the most common instruction pairs on SunSpider and load has one of the smallest instruction bodies, this sounds like a good idea. However, all of my attempts to implement it lead to massive regressions on SunSpider, even if I don't even use the opcode. This points to a random GCC problem, so I'll post the code to see if we can fix it together.

Attachments
Patch that causes regression (6.13 KB, patch) 2008-06-28 16:24 PDT, Cameron Zwarich (cpst)	no flags	Details Formatted Diff Diff
Patch that improves performance (6.04 KB, patch) 2008-06-29 20:14 PDT, Cameron Zwarich (cpst)	no flags	Details Formatted Diff Diff
Show Obsolete (1) View All Add attachment proposed patch, testcase, etc.

Cameron Zwarich (cpst)

Comment 1 2008-06-28 16:24:21 PDT

Created attachment 21990 [details] Patch that causes regression Here is a patch that causes a performance regression of about 8% on SunSpider, despite the opcode not even being used. I originally had the opcode after op_load, but even moving it near the end didn't fix the regression.

Cameron Zwarich (cpst)

Comment 2 2008-06-29 19:47:41 PDT

The same thing happens with op_mov2.

Cameron Zwarich (cpst)

Comment 3 2008-06-29 20:14:03 PDT

Created attachment 22001 [details] Patch that improves performance I changed the code to evade GCC and now get a 0.2% progression. Maybe I could do better if I moved the opcode elsewhere?

Cameron Zwarich (cpst)

Comment 4 2008-09-04 00:58:22 PDT

This optimization was performed by different means in bug 20286.

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