[turbofan] Remove redundant cmp operands swap.

[turbofan] Remove redundant cmp operands swap.

Remove redundant operands swap for compare operation.

BUG=

Committed: https://crrev.com/86af70afb01d778897cd0aac6c3da8337acab859
Cr-Commit-Position: refs/heads/master@{#41385}

[shiyu.zhang, 2016-11-16 09:05:38]

We found TurboFan has additional register spills compared with Crankshaft in
Octane2-Navier-stokes. The hot spot of Navier-stokes includes many array
accesses:

    for (var i=1; i<=width; i++)
                lastX = x[currentRow] = (x0[currentRow] +
a*(lastX+x[++currentRow]+x[++lastRow]+x[++nextRow])) * invC;

Thus, the GIT code contains a lot of bound check. Part of the TurboFan's
additional register spills happen in these bound check cmp. For example:

Bound check in CrankShaft

                cmp eax,[ebp-0xa4]
                jnc 3218  (0x4a01e612)

Bound check in TurboFan

                mov [ebp-0x34],edi
                mov edi,[ebp-0x3c]
                cmp edi,edx
                jna 4297  (0x2bdf6f49)

For this case, one of the cmp operand is register, the other is pre-spilled in
memory. TurboFan needs additional spill because it has swapped the cmp operands
in instruction selector and used the pre-spilled operand as left operand, which
must occupy one more register.

If one operand will not be used any more after current instruction, it's
reasonable to use it as a left operand, which will be overwritten in
bin-operations like add/sub/div/mul. But for cmp, since it will not overwrite
left operand, could you please tell the motivation of swapping operands?
Furthermore, two swaps with different conditions in VisitWordCompare and
VisitCompare looks confusing, maybe one of them is redundant?
After removing one cmp operands swap as we did in this patch,
Octane2-Navier-stokes shows ~3% improvement on ia32 Atom, this CL is just for
address the problem and experiment, appreciate your comments.

Below is some reference patches related to this problem:
https://codereview.chromium.org/667933002
https://codereview.chromium.org/1706763002
https://codereview.chromium.org/1845603002

[Benedikt Meurer, 2016-11-30 12:08:43]

bmeurer@chromium.org changed reviewers:
+ bmeurer@chromium.org, jarin@chromium.org

[Benedikt Meurer, 2016-11-30 12:08:44]

Ah, very awesome. I was wondering about these weird moves already, but never
found the time to investigate where they come from. LGTM!

[Benedikt Meurer, 2016-11-30 12:08:49]

The CQ bit was checked by bmeurer@chromium.org

[commit-bot: I haz the power, 2016-11-30 12:08:55]

CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2016-11-30 12:32:26]

CQ is committing da patch.

Bot data: {"patchset_id": 1, "attempt_start_ts": 1480507729383190, "parent_rev":
"340c629bf830fdabd76059bcc0849f1db842212a", "commit_rev":
"36162b66e01218e1ce07d9bc94aa10dcc5b51572"}

[commit-bot: I haz the power, 2016-11-30 12:32:29]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2016-11-30 12:33:03]

Description was changed from

==========
[turbofan] Remove redundant cmp operands swap.

Remove redundant operands swap for compare operation.

BUG=
==========

to

==========
[turbofan] Remove redundant cmp operands swap.

Remove redundant operands swap for compare operation.

BUG=

Committed: https://crrev.com/86af70afb01d778897cd0aac6c3da8337acab859
Cr-Commit-Position: refs/heads/master@{#41385}
==========

[commit-bot: I haz the power, 2016-11-30 12:33:04]

Patchset 1 (id:??) landed as
https://crrev.com/86af70afb01d778897cd0aac6c3da8337acab859
Cr-Commit-Position: refs/heads/master@{#41385}

[bradn, 2016-11-30 18:40:37]

bradnelson@google.com changed reviewers:
+ bradnelson@google.com

[bradn, 2016-11-30 18:40:38]

This seems to have regressed ia32 slightly.