[PATCH] D40480: MemorySSA backed Dead Store Elimination.

[Daniel Berlin via llvm-commits]

On Thu, Dec 7, 2017 at 6:43 AM, Dave Green via Phabricator <
reviews at reviews.llvm.org> wrote:

> dmgreen added a comment.
>
> OK. I have some performance numbers. I'm compiling clang ("ninja clang")
> and using
> -ftime-report/-stat to get info (with some extra precision for decimal
> places) and
> summing the results for all the compiled files. The total runtime is a
> little noisy on
> this machine, but these sub-numbers seem pretty stable between runs.
>
> Firstly the good news. With this version we now remove more dead store.
>  Old: 41310   New: 51660
> With my "MemSSA can enable us to remove more stores" hat on, this is good
> stuff.
>
> Some more good news is that DSE is now quicker, for the sum of time for
> each file:
>  Old: ~26s   New: ~19s
>
> The bad news is that we also need to add in the MemorySSA passes. I think
> we now
> calc this twice in the pipeline, not once as before, so times roughly
> double.
>  Old: ~35s   New: ~69s
> I'm hoping that in the long run we can shared the cost of this between
> other passes.
> NewGVN is a couple of hops earlier in the LTO pass pipeline, LICM also
> quite close
> in the normal one. Hopefully this cost can be shared out.
>

Yeah, this is always the case when we introduce new stateful
infrastructure.  We can just preserve it.


>
> The other bad news is we use a post-dom tree (again, maybe sharable?):
>  Old: ~15s   New: ~27s
>

This is surprising actually.
This would imply that post-dom is costing more than the pass?
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