[llvm] r258971 - [SimplifyCFG] limit recursion depth when speculating instructions (PR26308)

[Hans Wennborg via llvm-commits]

On Wed, Jan 27, 2016 at 11:22 AM, Sanjay Patel via llvm-commits
<llvm-commits at lists.llvm.org> wrote:
> Author: spatel
> Date: Wed Jan 27 13:22:45 2016
> New Revision: 258971
>
> URL: http://llvm.org/viewvc/llvm-project?rev=258971&view=rev
> Log:
> [SimplifyCFG] limit recursion depth when speculating instructions (PR26308)
>
> This is a fix for:
> https://llvm.org/bugs/show_bug.cgi?id=26308
>
> With the switch to using the TTI cost model in:
> http://reviews.llvm.org/rL228826
> ...it became possible to hit a zero-cost cycle of instructions (gep -> phi -> gep...),
> so we need a cap for the recursion in DominatesMergePoint().
>
> A recursion depth parameter was already added for a different reason in:
> http://reviews.llvm.org/rL255660
> ...so we can just set a limit for it.
>
> I pulled "10" out of the air and made it an independent parameter that we can play with.
> It might be higher than it needs to be given the currently low default value of
> PHINodeFoldingThreshold (2). That's the starting cost value that we enter the recursion
> with, and most instructions have cost set to TCC_Basic (1), so I don't think we're going
> to speculate more than 2 instructions with the current parameters.
>
> As noted in the review and the TODO comment, we can do better than just limiting recursion
> depth.
>
> Differential Revision: http://reviews.llvm.org/D16637

Should this be merged to 3.8?