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

[Eric Christopher]

>
> 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?

> 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.

> 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?

-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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