On 2013/04/02 06:44:21, Zheng Liu wrote:
X64 already had it because I accidentally included it in a previous code
review
http://code.google.com/p/v8/source/detail?r=13289
(oops, sorry!)
This change is that for some processor, sequence like:
divsd XMM0, xmm?
(movaps XMM0, XMM0)
mulsd xmm? ,XMM0
Will prevent div and mul execute in parallel even if they are indepedent
(e.g.
belong to two loop iterations)
Adding [*anymov* XMM0, XMM0] can workaround it.
I tested it on Nehalem/Westmere (e.g. i7-9**/8**, Xeon X56**)
Not sure about newer processors.
Case to reproduce:
~12% faster on affected processors
var darr1 = new Float64Array([0.5, 0.5]);
var darr2 = new Float64Array([0.5, 0.5]);
function fn1(a, b, n) {
var c;
for (var i = 0; i < n; i++)
c = b[i&1] * (1 / a[i&1]);
}
fn1(darr1, darr2, 10);
fn1(darr1, darr2, 10);
%OptimizeFunctionOnNextCall(fn1);
var start = Date.now();
fn1(darr1, darr2, 1000*1000*100);
print(Date.now() - start);
Sorry that it took this long.
Could you add a comment to the inserted instruction (also on x64) so that it
makes some sense when somebody looks at the code later on and won't
accidentally
remove it?
https://codereview.chromium.org/13447003/
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