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

[Quentin Colombet]

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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>> 
>>