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Using unsigned compares for uint can be significantly faster

NEW
Unassigned

Status

Tamarin
Baseline JIT (CodegenLIR)
7 years ago
6 years ago

People

(Reporter: Werner Sharp, Unassigned)

Tracking

unspecified
Future
x86
Windows 7
Bug Flags:
flashplayer-qrb +

Details

(Whiteboard: PACMAN)

(Reporter)

Description

7 years ago
asmicro/global_var_write test case was showing some very strange performance gains with my simple function analyzer change...50% faster on my Xeon.  Simple test case below shows these results:

With method overhead skipped: 230 msecs (INT or UINT)
With prologue code and i:int var: 280 msecs
With prologue and i:uint var: 430 msecs.

Why the big difference?  The method overhead skip just shifts the hot loop in memory...some sort of cache alignment?  The uint/int change changes one instruction in the loop...a JL vs. a JB.  Why a 50% slowdown.


var global_var:int = 1;

function writeloop():int {
    var v:int = 2;
    for ( var i:int=0; i < 100000000 ; i++ )
        global_var = v;
    return i;
}

function foo()
{
	var d1:Number = new Date().getTime() ;
	for (var i:int = 1; i < 2; i++)
	{
		writeloop();
	}
	var d2:Number = new Date().getTime() ;
	trace("elapsed time is " + (d2-d1));
}
foo();
(Reporter)

Updated

7 years ago
Whiteboard: PACMAN
(Reporter)

Comment 1

7 years ago
I answer one question.  If I change nativei386.cpp to emit a JL instead of a JB for the unsigned compare, I get the 280 msecs from 430.  So that single instruction at the end of the loop is responsible for the time difference. Otherwise the JITed code looks identical.

Comment 2

7 years ago
FYI, some future mitigate techniques might conflict with this work.

Updated

6 years ago
Flags: flashplayer-qrb+
Summary: Investigate code alignment with this simple test case → Using unsigned compares for uint can be significantly faster
Target Milestone: --- → Future
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