Adds javascript overflow checking to arm64.

Adds javascript overflow checking to arm64.

Also skips a test for timing out on the bot.

R=regis@google.com

Committed: https://code.google.com/p/dart/source/detail?r=36232

[regis, 2014-05-15 17:18:24]

LGTM

Although the range check seems quite expensive (not specific to your cl).

[regis, 2014-05-15 17:36:46]

On 2014/05/15 17:18:24, regis wrote:
> LGTM
> 
> Although the range check seems quite expensive (not specific to your cl).

Instead of two signed compare:

overflow = result < -2^52 || result > 2^52

we could do it with one unsigned compare:
overflow = (result + 2^52) > 2^53

Something like this:

TMP1 = 2^52
TMP2 = result + TMP1
cmp TMP2, TMP1 lsl 1
branch if higher to overflow

This should work fine on ARM64, but this adds register pressure on X64.
Of course, this gets a bit more involved on 32-bit platforms.

But this requires one more register

[zra, 2014-05-15 17:38:16]

Committed patchset #1 manually as r36232 (presubmit successful).

[zra, 2014-05-15 17:45:19]

On 2014/05/15 17:36:46, regis wrote:
> On 2014/05/15 17:18:24, regis wrote:
> > LGTM
> > 
> > Although the range check seems quite expensive (not specific to your cl).
> 
> Instead of two signed compare:
> 
> overflow = result < -2^52 || result > 2^52
> 
> we could do it with one unsigned compare:
> overflow = (result + 2^52) > 2^53
> 
> Something like this:
> 
> TMP1 = 2^52
> TMP2 = result + TMP1
> cmp TMP2, TMP1 lsl 1
> branch if higher to overflow
> 
> This should work fine on ARM64, but this adds register pressure on X64.
> Of course, this gets a bit more involved on 32-bit platforms.
> 
> But this requires one more register

Sorry, didn't see your comment before committing. At the moment, I'd only be
able to try to find a performance improvement on x64 where we might not have an
extra register available. I'll add a TODO in my next CL.

[regis, 2014-05-15 17:49:43]

No worries. I sent the comment after my LGTM and this is not critical. Running
fast with overflow checks is probably not our highest priority. A TODO to
document possible improvement is good enough for now, if you can remember to add
it. Thanks.