[llvm-dev] Nested ADJCALLSTACK UP/DOWN allowed? (original) (raw)

Tim Neumann via llvm-dev llvm-dev at lists.llvm.org
Mon Oct 22 07:41:16 PDT 2018

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, ADJCALLSTACKUPs can't interleave probably because ADJCALLSTACKUPs 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! _> ________________________ > LLVM Developers mailing list > llvm-dev at lists.llvm.org > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

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