[llvm-dev] MergeLoadStoreMotion and GVNHoist

[Hiroshi Yamauchi via llvm-dev]

I don't have an answer, but a small self-contained test that
demonstrates the issue may be helpful.
i

On Mon, Sep 2, 2019 at 6:10 AM Luke Drummond via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
>
> Hello
>
> I'm investigating a regression while bumping our target from llvm-4 to
> llvm7. We see that common loads are no longer being hoisted by the
> MergedLoadStoreMotion pass. We've tracked this issue down to [1]. My
> understanding of [1] is that load hoisting is performed by GVNHoist and is
> therefore no longer required to be duplicated by MLSM, but as far as I
> can tell, GVNHoist is not enabled by default at any standard
> optimization level [2][3][4] (it's hidden behind a flag). However the MLSM
> code is enabled at any (level > 1).
>
> Is this an oversight?
>
> If this _is_ an oversight, does it make sense to now enable GVNHoist in the same
> cases as MLSM i.e. at all optlevels > 1 ?
>
> If not, what is the reason for not enabling this?
>
> All the Best
>
> Luke
>
> [1] [0fbee48d](https://github.com/llvm-mirror/llvm/commit/0fbee48dc4899e045537b9118a65664f1706596c#diff-a1aa52c6b72ac5fb6e39994c431e24c2L537)
> [2] https://github.com/llvm-mirror/llvm/blob/3a58154/lib/Transforms/IPO/PassManagerBuilder.cpp#L388-L392
> [3] https://github.com/llvm-mirror/llvm/blob/3a58154/lib/Transforms/IPO/PassManagerBuilder.cpp#L325-L326
> [4] https://github.com/llvm-mirror/llvm/blob/3a58154/lib/Transforms/IPO/PassManagerBuilder.cpp#L117-L119
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