[llvm] r291609 - CodeGen: Allow small copyable blocks to "break" the CFG.

[Xinliang David Li via llvm-commits]

Kyle, I looked at Matt's case in more details. For this target, bb2 in is
selected as fallthrough of bb in the final layout, so bb4  should not be a
taildup'ed. Can you take a look what went wrong?

By comparison, on x86, the cloned bb4 is the layout successor of bb which
is expected.

David

On Wed, Jan 11, 2017 at 12:06 AM, Matt Arsenault <arsenm2 at gmail.com> wrote:

>
> On Jan 10, 2017, at 19:13, Kyle Butt <iteratee at google.com> wrote:
>
>>
>>
> I looked at the code in question. There are more compare instructions, but
> no codepath should execute more of them. Which codepath are you concerned
> about?
>
> For the compare, and 1 of the branches, it occurs due to tail duplication,
> and so for those, this is not a regression, it is WAI.
>
> Are you worried about the code size, or did this actually cause a
> performance regression?
> If it did cause a regression, can you tell me which path is the hot path?
>
>> -Matt
>>
>>
> Thanks,
> Kyle.
>
>
> This changes from having a path where no branch occurs, to ensuring that a
> branch will occur, and branches are expensive. I noticed this from the code
> size changes, but I’m mostly surprised by replacing a fall through with a
> branch.
>
> Looking at the expected cycle counts on all paths in the artificial
> testcase, the loads + waits are always skipped, which is good. I think if
> the waitcnts were inserted smarter, the original code CFG would be slightly
> better. I need to look more at the full testcase.
>
> -Matt
>
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