[PATCH] D109280: [WIP][DSE] Memory intrinsics like memset, memcpy, memmove are removed if they are overwritten by a store in a loop

Wed Feb 2 11:34:01 PST 2022

fhahn added a comment.

@vdsered  are you still planning on pushing this forward?

In D109280#3043563 <https://reviews.llvm.org/D109280#3043563>, @nikic wrote:

> In D109280#3043179 <https://reviews.llvm.org/D109280#3043179>, @xbolva00 wrote:
>
>> I think this would have nontrivial impact on compile time (@nikic ?) and the results from testsuite do not look so promising - I would expect more “hits” to justify (small if possible) compile time regression.
>
> Yes, this has a large negative effect: https://llvm-compile-time-tracker.com/compare.php?from=64eaffb613d0cb7fa7542fa48281a2e617ad8ee9&to=6e7452757e16c4260fa9a5862761a68ed778dbf9&stat=instructions

Agreed that the compile-time impact looks very large, but I *think* it may not be as bad as it looks at the moment. There seems to be a bug in the code that means we effectively run the main DSE loop twice if `EnableOptimizationAcrossLoops = false` (as in this patch).

Thee patch unconditionally runs the whole main DSE loop again, as below. `prepareStateForAcrossLoopOptimization` has en early exit if `!EnableOptimizationAcrossLoops`. Otherwise clears `MemDeps` and roughly only adds MemDeps with loop ranges. So as a consequence, if `!EnableOptimizationAcrossLoops` we process *all* MemoryDeps again.

  MadeChange |= State.prepareStateForAcrossLoopOptimization();
  MadeChange |= runDSEOptimizationLoop(State);

I tried to rebase the patches and collect compile-time numbers with that issue fixed: https://llvm-compile-time-tracker.com/compare.php?from=f622c7b7d33b211517d8fe4f725d1028d786fc08&to=00a8810b123e606c19d9926d11183318323a8752&stat=instructions

NewPM-O3: +0.24%
NewPM-ReleaseThinLTO: +0.39%
NewPM-ReleaseLTO-g: +0.26%

We should still see if we can get those down further, but I think they look more encouraging to start with.

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D109280/new/

https://reviews.llvm.org/D109280