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[SDAG] Allow scalable vectors in ComputeNumSignBits
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Authored by reames on Oct 31 2022, 6:25 PM.

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Summary

This is a continuation of the series of patches adding lane wise support for scalable vectors in various knownbit-esq routines.

The basic idea here is that we track a single lane for scalable vectors which corresponds to an unknown number of lanes at runtime. This is enough for us to perform lane wise reasoning on many arithmetic operations.

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Event Timeline

reames created this revision.Oct 31 2022, 6:25 PM
Herald added a project: Restricted Project. · View Herald TranscriptOct 31 2022, 6:25 PM
reames requested review of this revision.Oct 31 2022, 6:25 PM
reames updated this revision to Diff 476483.Nov 18 2022, 8:06 AM

rebase and ping

This revision is now accepted and ready to land.Nov 18 2022, 9:41 AM
This revision was automatically updated to reflect the committed changes.

Realized I hadn't included a bailout before ComputeNumSignBitsForTargetNode in this patch. This was unintentional, but there are few enough implementations of this that a quick audit was easy. I didn't see anything concerning. So I'm going to leave this as is instead of inserting the bailout and then removing it again. :)

llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
4358–4359

This can never be true can it?

More relevant to the other exit paths but is return 1; what we want here rather than break;? I asked because the existing scalable vector early exits do the latter (e.g. a couple of lines down) and so I'm wondering if we're loosing information or if the other exit paths are wrong.

reames added inline comments.Nov 18 2022, 12:19 PM
llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
4358–4359

I think this one is dead. Extract_vector_elt should always be returning a vector. We have extract_subvector for vector extract from vector.

As for the 1 vs break thing, both are correct, but 1 is possible non-optimal. The code below the switch uses compute known bits, so if our compute known bits handles one of these and the sign bits are known, then 1 is unnecessarily conservative.

I'll fix both in a follow up commit.

reames added inline comments.Nov 18 2022, 12:30 PM
llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
4358–4359

Addressed in 3fb08d1

reames reopened this revision.Nov 21 2022, 8:28 AM

I reverted this on Friday due to a reported problem with an earlier dependent change. Reopening so that I remember to reland once the underlying issue is resolved. No action needed from reviewers.

Note: When I reland, I will fold in the post-commit review comment and fixes that originally landed separately.

This revision is now accepted and ready to land.Nov 21 2022, 8:28 AM