[PATCH][Peephole] Rewrite copies to avoid cross register banks copies.

[Evan Cheng]

The patch looks fine to me. Is there any measurable compile time impact?

Evan

On Sep 9, 2013, at 9:20 AM, Quentin Colombet <qcolombet at apple.com> wrote:

> Ping^2?
> 
> -Quentin
> 
> On Sep 3, 2013, at 9:41 AM, Quentin Colombet <qcolombet at apple.com> wrote:
> 
>> Ping?
>> -Quentin
>> 
>> On Aug 28, 2013, at 5:29 PM, Quentin Colombet <qcolombet at apple.com> wrote:
>> 
>>> Hi Evan,
>>> 
>>> Eric suggested that you may be the right person to review this patch.
>>> Could you have a look, please?
>>> 
>>> This fixes <rdar://problem/14742333>.
>>> 
>>> Thanks,
>>> -Quentin
>>> <PeepholeCopy.svndiff>
>>> 
>>> On Aug 28, 2013, at 5:25 PM, Quentin Colombet <qcolombet at apple.com> wrote:
>>> 
>>>> 
>>>> On Aug 28, 2013, at 4:43 PM, Eric Christopher <echristo at gmail.com> wrote:
>>>> 
>>>>>> 
>>>>>> The patch is not triggered by any of the tests that are in the llvm test
>>>>>> suite (tested on both ARM and X86).
>>>>>> 
>>>>> 
>>>>> *nod* I figured, otherwise you'd have said it. :)
>>>>> 
>>>>>> Like I said in my original email, I could rewrite/eliminate more
>>>>>> aggressively copies (and in that case it does kick in in the llvm test
>>>>>> suite) but then, the register allocator does in a few cases a poor
>>>>>> coalescing/live-range splitting job.
>>>>>> 
>>>>> 
>>>>> *nod*
>>>>> 
>>>>>> That said, as it is, when the patch kicks in, we observed with our internal
>>>>>> tests: no regressions and up to 25% runtime speed-up.
>>>>>> 
>>>>> 
>>>>> Pretty nice. Can you give a general idea of the type of code that this
>>>>> is triggering on?
>>>> Basically it was some code using a tone of intrinsics, in particular mixing vector, integer, and floating point code.
>>>> For isel, this means, tone of bitcast, vector insert, and vector extract, across different basic blocks.
>>>> 
>>>>> 
>>>>>> Perhaps an
>>>>>> example of what you're optimizing as a comment in the code?
>>>>>> 
>>>>>> I have added one in the file header. What do you think?
>>>>>> 
>>>>> 
>>>>> Helps. I was hoping for more code details, but the comment provides
>>>>> some helpful bit.
>>>> Do you think I should put more comments?
>>>> 
>>>>> 
>>>>>> I have moved the assignment inside the loop.
>>>>>> Is it better?
>>>>>> (I do like predicated code!)
>>>>>> 
>>>>> 
>>>>> Looks better to me from a readability perspective.
>>>>> 
>>>>>> 
>>>>>> I'm not necessarily comfortable approving, but the code looks reasonable.
>>>>>> 
>>>>>> What do you miss to be comfortable?
>>>>>> Could you point me someone that may be comfortable to approve this, please?
>>>>>> (So that I can through him under the bus!)
>>>>>> 
>>>>> 
>>>>> Ultimately Jakob or Evan. It looks pretty simple so unless you get an
>>>>> objection by tomorrow morning I guess submit?
>>>> I will do that.
>>>> 
>>>> Thanks,
>>>> Quentin
>>>>> 
>>>>> -eric
>>>>> 
>>>>>> Thanks again.
>>>>>> 
>>>>>> -Quentin
>>>>>> 
>>>>>> 
>>>>>> -eric
>>>>>> 
>>>>>> 
>>>>>> On Fri, Aug 23, 2013 at 1:04 PM, Quentin Colombet <qcolombet at apple.com>
>>>>>> wrote:
>>>>>> 
>>>>>> Hi,
>>>>>> 
>>>>>> Here is a patch to rewrite copies to avoid cross register banks copies when
>>>>>> possible.
>>>>>> Thanks for your reviews.
>>>>>> 
>>>>>> ** Context **
>>>>>> 
>>>>>> By definition copies across register bank are not coalescable. Still, it may
>>>>>> be possible to get rid of such a copy when the value is available in another
>>>>>> register of the same register file.
>>>>>> Consider the following example, where capital and lower letters denote
>>>>>> different register file:
>>>>>> b = copy A <-- cross-bank copy
>>>>>> …
>>>>>> C = copy b <-- cross-bank copy
>>>>>> 
>>>>>> This could have been optimized this way:
>>>>>> b = copy A  <-- cross-bank copy
>>>>>> …
>>>>>> C = copy A <-- same-bank copy
>>>>>> 
>>>>>> Note: b and C's definitions may be in different basic blocks.
>>>>>> 
>>>>>> 
>>>>>> ** Proposed Solution **
>>>>>> 
>>>>>> Add a peephole optimization that looks through a chain of copies leading to
>>>>>> a cross-bank copy and reuses a source that is on the same register file if
>>>>>> available.
>>>>>> 
>>>>>> This solution could also be used to get rid of some copies (e.g., A could
>>>>>> have been used instead of C). However, we do not do so because:
>>>>>> - It may over constrain the coloring of the source register for coalescing.
>>>>>> - The register allocator may not be able to find a nice split point for the
>>>>>> longer live-range, leading to more spill.
>>>>>> 
>>>>>> 
>>>>>> ** Testcase?! **
>>>>>> 
>>>>>> This patch does not include a test case, because it is very difficult to
>>>>>> reproduce this behavior with a reasonably small input.
>>>>>> 
>>>>>> Cheers,
>>>>>> -Quentin
>>>>>> 
>>>>>> _______________________________________________
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