[llvm-dev] Nested ADJCALLSTACK UP/DOWN allowed?

[Tim Neumann via llvm-dev]

Hi Shiva,

thanks for the info and the patch! It at the very least fixes my
minimized example.
On Mon, Oct 22, 2018 at 8:03 AM Shiva Chen <shiva0217 at gmail.com> wrote:
>
> Hi Tim,
>
> ADJCALLSTACKUP`s can't interleave probably because ADJCALLSTACKUP`s
> may lower to SP adjustment instructions by
> eliminateCallFramePseudoInstr when there exist variable sized objects.
> Interleaving ADJCALLSTACKUP`s may cause the instructions accessing
> variable sized objects with the incorrect offset.
> So instruction scheduler creates CallResource artificial registers as
> a glue to avoid interleaving while scheduling.
> We encounter the same issue and find out that one of the
> ADJCALLSTACKUP has been pre-scheduled by
> PrescheduleNodesWithMultipleUses as you mention in
> https://github.com/avr-rust/rust/issues/111#issuecomment-431603807.
> The ADJCALLSTACKUP hoist too early in the code sequence and hold
> CallResource too long which make other ADJCALLSTACKUP/DOWN can't be
> scheduled. When there is no other nodes can be scheduled, the
> scheduler will try to rename the register by creating copies, but
> CallResource isn't a real register which triggers the error.
> It seems that PrescheduleNodesWithMultipleUses is a heuristic
> optimization method for better scheduling result, but hoisting
> ADJCALLSTACKUP won't have benefit. There're several conditions that
> PrescheduleNodesWithMultipleUses won't pre-schedule the node. I think
> ADJCALLSTACKUP should be one of them because it will increase the
> lifetime of CallResource. I have tried to push a patch on
> https://reviews.llvm.org/D53485. Hopefully, it could be helpful.
>
> Best,
> Shiva
> Tim Neumann via llvm-dev <llvm-dev at lists.llvm.org> 於 2018年10月22日 週一 上午5:51寫道：
> >
> > > The reason that brought me looking into the whole issue was some pattern getting turned into a compiler library call late. Back then I sidestepped the whole discussion by stopping to emit ADJCALLSTACK UP/DOWN of zero byte call frames (https://reviews.llvm.org/D42006) so they wouldn’t nest anymore for the helpers functions in question…
> >
> > Adding a similar patch to AVR fixes at least my minimizedreproduction,
> > I haven't gotten around to trying it on other code yet.
> >
> > Thanks for all the info!
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