Hi Sam,
I don't have any insights on x64 for VisitWordCompareZero but as a quick
note, the TurboshaftAdapter instruction selection implementation for
VisitWordCompareZero for arm64 was just added yesterday by me (
https://chromium-review.googlesource.com/c/v8/v8/+/5002003).
So, just to clarify, I assume you are talking about the Turbofan {Node* /
TurbofanAdapter} based instruction selection with Turboshaft optimizations
followed by a RecreateSchedule?
If you do changes for the TurbofanAdapter instruction selection on arm64
for code that already has an implementation for the TurboshaftAdapter,
could you add a TODO(mliedtke) on the Turboshaft implementation with some
nice description and CC me on the gerrit changes, so these don't get lost
when we switch over to the TurboshaftAdapter?
Thanks and best regards,
Matthias
On Wed, Nov 8, 2023 at 1:54 PM Sam Parker-Haynes <sam.par...@arm.com> wrote:
> Hi,
>
> I've been investigating rematerialising flag setting instructions in
> VisitWordCompareZero, just by removing the CanCover check, and something is
> going wrong with turboshaft.
>
> I have an arithmetic overflow operation, Int32AddWithOverflow, and the
> overflow check is used by two branch operations. With a turbofan-only flow,
> the overflow operation and the projection will be duplicated for the second
> branch, and everything is fine. But with turboshaft, neither the overflow
> operation or projection are duplicated and the second branch uses the
> original projection[1] value. This looks fine at the IR level, but ends up
> broken after isel when the register allocator comes across a virtual
> register without a definition. This happens for both x64 and arm64, so I'm
> assuming turboshaft is making some assumptions, based on the current
> behaviour, that are non-obvious to me.
>
> Any ideas?
>
> Thanks!
> Sam
>
>
>
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