[llvm] r308847 - [SCEV] Limit max size of AddRecExpr during evolving

[Hans Wennborg via llvm-commits]

Should this be merged to 5.0?

On Sun, Jul 23, 2017 at 8:40 AM, Max Kazantsev via llvm-commits
<llvm-commits at lists.llvm.org> wrote:
> Author: mkazantsev
> Date: Sun Jul 23 08:40:19 2017
> New Revision: 308847
>
> URL: http://llvm.org/viewvc/llvm-project?rev=308847&view=rev
> Log:
> [SCEV] Limit max size of AddRecExpr during evolving
>
> When SCEV calculates product of two SCEVAddRecs from the same loop, it
> tries to combine them into one big AddRecExpr. If the sizes of the initial
> SCEVs were `S1` and `S2`, the size of their product is `S1 + S2 - 1`, and every
> operand of the resulting SCEV is combined from operands of initial SCEV and
> has much higher complexity than they have.
>
> As result, if we try to calculate something like:
>   %x1 = {a,+,b}
>   %x2 = mul i32 %x1, %x1
>   %x3 = mul i32 %x2, %x1
>   %x4 = mul i32 %x3, %x2
>   ...
> The size of such SCEVs grows as `2^N`, and the arguments
> become more and more complex as we go forth. This leads
> to long compilation and huge memory consumption.
>
> This patch sets a limit after which we don't try to combine two
> `SCEVAddRecExpr`s into one. By default, max allowed size of the
> resulting AddRecExpr is set to 16.
>
> Differential Revision: https://reviews.llvm.org/D35664
>
> Added:
>     llvm/trunk/test/Analysis/ScalarEvolution/max-addrec-size.ll
> Modified:
>     llvm/trunk/lib/Analysis/ScalarEvolution.cpp