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

[Quentin Colombet]

On Oct 4, 2013, at 12:59 PM, Quentin Colombet <qcolombet at apple.com> wrote:

> Good point.
> 
> Let me gather some numbers.
> 
> -Quentin
> 
> On Oct 4, 2013, at 12:50 PM, Evan Cheng <evan.cheng at apple.com> wrote:
> 
>> The patch looks fine to me. Is there any measurable compile time impact?
No, nothing measurable.

I benchmarked the compiler with and without the patch in release mode on the whole llvm suite + SPEC and did not see any measurable impact.

-Quentin

>> 
>> 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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>>>>> 
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