[LLVMdev] Switching the new block placement pass on by default for 3.1

[Chandler Carruth]

On Mon, Apr 16, 2012 at 9:00 AM, Bill Wendling <wendling at apple.com> wrote:

> On Apr 15, 2012, at 6:42 PM, Chandler Carruth wrote:
>
> > Hello folks,
> >
> > I think I've fixed everything left to be fixed in block placement, and I
> would really like for it to be on by default in 3.1:
> > 1) I've been testing this as thoroughly as I can across a large amount
> of real-world code, so it shouldn't be too flaky.
> > 2) The code has been checked in and reviewed previously.
> > 3) It completes a significant missing feature in LLVM with
> builtin_expect.
> > 4) It "fixes" some of the regressions introduced with the inlining work
> by more effectively ensuring the loops are laid out correctly. (yacr2
> specifically exhibited this problem)
> > 5) I see more positive movement in performance than negative, but that
> may be an artifact of the testing I've been doing.
> >
> > I'm planning to check in a change to enable it by default later tonight
> (PST) and hopefully allow nightly testers to run through it and report any
> significant problems it introduces. It should be relatively easy to revert
> if folks are opposed to it being in 3.1, or if the testers turn up any
> problems*.
> >
> > Thoughts? Others agree with turning it on? Strong desire for it to be
> delayed until after the branch? I do understand the desire to avoid last
> minute features... I just wasn't able to isolate the single big performance
> regression I saw until today, despite working on it for two weeks... ::
> sigh ::
> >
> Sooner rather than later is obviously better. I would say turn it on for a
> nightly test to make sure it works for everyone.


I've enabled it in r154816. I tried to get it done sooner, but updating the
regression tests was a nightmare. That said, they uncovered two super easy
missed optimizations implemented along the way.

I'll be around for quite some time and will be watching build bots. I've
already started a local nightly test run on my end as well.
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