Fix DBC double comparisons for NaN.

Fix DBC double comparisons for NaN.

Because NaN is not handled in a separate branch (like on other architetures -
e.g. branch-on-parity-flag on ia32), and every comparison with NaN returns false,
except "!=" we can't flip the branch condition in the same way for double
compares like we do for integer compares.

We could re-order blocks to always make the false-successor the fall-through, but
for now I just disable flipping the branch condition for double compares.

BUG=#27481
R=zra@google.com

Committed: https://github.com/dart-lang/sdk/commit/e013aaa511fcd4cf1f593e801845d2c30220896e

[Florian Schneider, 2016-10-06 23:59:57]

fschneider@google.com changed reviewers:
+ zra@google.com

[zra, 2016-10-07 02:14:43]

lgtm w/ question

https://codereview.chromium.org/2400103002/diff/1/runtime/vm/intermediate_lan...
File runtime/vm/intermediate_language_dbc.cc (right):

https://codereview.chromium.org/2400103002/diff/1/runtime/vm/intermediate_lan...
runtime/vm/intermediate_language_dbc.cc:1791: // For double comparisons we can't
flip the condition like with smi
Can we flip for all but == and/or != ?

[Florian Schneider, 2016-10-07 16:49:33]

https://codereview.chromium.org/2400103002/diff/1/runtime/vm/intermediate_lan...
File runtime/vm/intermediate_language_dbc.cc (right):

https://codereview.chromium.org/2400103002/diff/1/runtime/vm/intermediate_lan...
runtime/vm/intermediate_language_dbc.cc:1791: // For double comparisons we can't
flip the condition like with smi
On 2016/10/07 02:14:43, zra wrote:
> Can we flip for all but == and/or != ?

As discussed offline, we could do inline assembly to handle the nan-case like in
other architectures. Or we could reorder blocks so that the fall-through is
always the false-label.

I'll do some performance measurements to figure out the impact of this change.

[Florian Schneider, 2016-10-07 21:42:51]

On 2016/10/07 16:49:33, Florian Schneider wrote:
>
https://codereview.chromium.org/2400103002/diff/1/runtime/vm/intermediate_lan...
> File runtime/vm/intermediate_language_dbc.cc (right):
> 
>
https://codereview.chromium.org/2400103002/diff/1/runtime/vm/intermediate_lan...
> runtime/vm/intermediate_language_dbc.cc:1791: // For double comparisons we
can't
> flip the condition like with smi
> On 2016/10/07 02:14:43, zra wrote:
> > Can we flip for all but == and/or != ?
> 
> As discussed offline, we could do inline assembly to handle the nan-case like
in
> other architectures. Or we could reorder blocks so that the fall-through is
> always the false-label.
> 
> I'll do some performance measurements to figure out the impact of this change.

The performance impact of my change seems small enough. I'll land this as is for
now with the TODO.

[Florian Schneider, 2016-10-07 22:30:25]

Description was changed from

==========
Fix DBC double comparisons for NaN.

Because NaN is not handled in a separate branch (like on other architetures -
e.g. branch-on-parity-flag on ia32), and every comparison with NaN returns
false,
except "!=" we can't flip the branch condition in the same way for double
compares like we do for integer compares.

We could re-order blocks to always make the false-successor the fall-through,
but
for now I just disable flipping the branch condition for double compares.

BUG=#27481
==========

to

==========
Fix DBC double comparisons for NaN.

Because NaN is not handled in a separate branch (like on other architetures -
e.g. branch-on-parity-flag on ia32), and every comparison with NaN returns
false,
except "!=" we can't flip the branch condition in the same way for double
compares like we do for integer compares.

We could re-order blocks to always make the false-successor the fall-through,
but
for now I just disable flipping the branch condition for double compares.

BUG=#27481
R=zra@google.com

Committed:
https://github.com/dart-lang/sdk/commit/e013aaa511fcd4cf1f593e801845d2c30220896e
==========

[Florian Schneider, 2016-10-07 22:30:27]

Committed patchset #1 (id:1) manually as
e013aaa511fcd4cf1f593e801845d2c30220896e (presubmit successful).