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

[Philip Reames]

On 11/20/2014 12:07 PM, Mark Heffernan wrote:
> Rolling back this CL which just improves analysis precision (trip 
> count computation specifically) is probably not the right approach.
Strongly, strongly agreed with this point.
>
> Mark
>
> On Thu, Nov 20, 2014 at 11:13 AM, Hans Wennborg <hans at chromium.org 
> <mailto:hans at chromium.org>> wrote:
>
>     Sorry for reviving an old thread.
>
>     On Fri, Oct 10, 2014 at 10:39 AM, Mark Heffernan
>     <meheff at google.com <mailto: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
>
>
>
>
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