[llvm] r219517 - This patch de-pessimizes the calculation of loop trip counts in

[Hans Wennborg]

Sorry for reviving an old thread.

On Fri, Oct 10, 2014 at 10:39 AM, Mark Heffernan <meheff at google.com> wrote:
> Author: meheff
> Date: Fri Oct 10 12:39:11 2014
> New Revision: 219517
>
> URL: http://llvm.org/viewvc/llvm-project?rev=219517&view=rev
> Log:
> This patch de-pessimizes the calculation of loop trip counts in
> ScalarEvolution in the presence of multiple exits. Previously all
> loops exits had to have identical counts for a loop trip count to be
> considered computable. This pessimization was implemented by calling
> getBackedgeTakenCount(L) rather than getExitCount(L, ExitingBlock)
> inside of ScalarEvolution::getSmallConstantTripCount() (see the FIXME
> in the comments of that function). The pessimization was added to fix
> a corner case involving undefined behavior (pr/16130). This patch more
> precisely handles the undefined behavior case allowing the pessimization
> to be removed.

This caused a 50 k regression in Chromium's binary size due to more
unrolling. I suspect it's something we'll have to take, but was
wondering if you have any numbers showing performance benefit here?

Thanks,
Hans