Rely on 'memcpy' in WTF::copyChars

Rely on 'memcpy' in WTF::copyChars

Before the change 'WTF::copyChars' contained an algorithm that was used
instead of 'memcpy' if the number of characters had been less than 20.

This however was unnecessary as 'memcpy' produces more efficient asm code
than the home-brewed algorithm did as it uses 'movq' (instead of 'movl')
doing the copying within the less number of asm commands (tested with
GCC 4.6.3).

So, this patch removes the home-brewed algorithm from 'WTF::copyChars'
and makes it always rely on 'memcpy'.

A simple testing programm doing the following code

    for (int i = 10000000; --i;) {
        // Re-create 'src' to prevent from optimizing cycle iterations out.
        char* src = new char[15];
        copyChars(dest, src, 15);
        copyChars(dest, src, 14);
        delete[] src;
    }

takes:

0m0.324s before the change
0m0.235s after the change (using 'memcpy').

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=169865

[Mikhail, 2014-03-24 14:17:45]

PTAL

[abarth-chromium, 2014-03-24 16:46:54]

lgtm

https://codereview.chromium.org/210013002/diff/1/Source/wtf/text/StringImpl.h
File Source/wtf/text/StringImpl.h (left):

https://codereview.chromium.org/210013002/diff/1/Source/wtf/text/StringImpl.h...
Source/wtf/text/StringImpl.h:443: // This number must be at least 2 to avoid
sharing empty, null as well as 1 character strings from SmallStrings.
I take it this comment is no longer accurate?  It doesn't make much sense to
me...

https://codereview.chromium.org/210013002/diff/1/Source/wtf/text/StringImpl.h
File Source/wtf/text/StringImpl.h (right):

https://codereview.chromium.org/210013002/diff/1/Source/wtf/text/StringImpl.h...
Source/wtf/text/StringImpl.h:301: }
Should we remove this branch too?

[commit-bot: I haz the power, 2014-03-24 16:47:03]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/mikhail.pozdnyakov@intel.com/210013002/1

[commit-bot: I haz the power, 2014-03-24 17:27:56]

Change committed as 169865

[Mikhail, 2014-03-25 10:30:25]

https://codereview.chromium.org/210013002/diff/1/Source/wtf/text/StringImpl.h
File Source/wtf/text/StringImpl.h (right):

https://codereview.chromium.org/210013002/diff/1/Source/wtf/text/StringImpl.h...
Source/wtf/text/StringImpl.h:301: }
On 2014/03/24 16:46:54, abarth wrote:
> Should we remove this branch too?
This makes a bit better assembly, on the other hand I do not think we can ever
feel it in the real world, so IMO branch can be removed indeed for the code
simplification, wdyt?

[abarth-chromium, 2014-03-25 15:20:47]

On 2014/03/25 10:30:25, mikhail.pozdnyakov wrote:
> This makes a bit better assembly, on the other hand I do not think we can ever
> feel it in the real world, so IMO branch can be removed indeed for the code
> simplification, wdyt?

I'd probably remove it.