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[docs] Update llvm.loop metadata documentation.
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Authored by Meinersbur on Dec 4 2018, 12:24 PM.

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Reviewers
hfinkel
Summary

Loop metadata nodes do not adhere to the documented property:

(a) LoopIDs are not unique: Any pass that duplicates IR will do it including its metadata (e.g. LoopVersioning) such that multiple loops are linked with the same LoopID. There is even a test case (Transforms/LoopUnroll/unroll-pragmas-disabled.ll) for multiple loops with the same LoopID.

(b) LoopIDs are not persistent: Adding or removing an item from a LoopID can only be done by creating a new MDNode and assigning it to the loop's branch(es). Passes such as LoopUnroll (llvm.loop.unroll.disable) and LoopVectorize (llvm.loop.isvectorized) use this to mark loops to be not transformed multiple times or to avoid that a LoopVersioned original loop is transformed.

Update the documentation according to how llvm.loop is used in practice.

Diff Detail

Event Timeline

Meinersbur created this revision.Dec 4 2018, 12:24 PM
hfinkel added inline comments.Dec 4 2018, 12:33 PM
docs/LangRef.rst
5100

Really? Why is it obsolete? (in any case, if that's true, it should be mentioned elsewhere in the LangRef).

5103

It is -> They are

5103

identifiers -> unique loop identifiers

5104

nor -> nor necessarily

Meinersbur marked an inline comment as done.Dec 4 2018, 12:44 PM
Meinersbur added inline comments.
docs/LangRef.rst
5100

Obsolete only for use in llvm.loop metadata, not in general. How about this wording?

The 'distinct' keyword for loop metadata is also not necessary anymore.

hfinkel added inline comments.Dec 4 2018, 1:17 PM
docs/LangRef.rst
5100

Then why add it to the example?

I'd say:

Also, the 'distinct' keyword for loop-metadata nodes is no longer necessary.
Meinersbur updated this revision to Diff 176737.Dec 4 2018, 4:54 PM
Meinersbur updated this revision to Diff 176739.Dec 4 2018, 4:57 PM
Meinersbur marked 4 inline comments as done.
  • Move 'necessarily'
Meinersbur marked 2 inline comments as done.Dec 4 2018, 4:58 PM
Meinersbur added inline comments.
docs/LangRef.rst
5100

Then why add it to the example?

The idea was, because clang currently generates it.

hfinkel added inline comments.Dec 5 2018, 9:59 AM
docs/LangRef.rst
5088

While you're improving this, could you also add a note about the debug locations being, optionally, in the list?

Meinersbur updated this revision to Diff 177333.Dec 7 2018, 2:59 PM
Meinersbur marked an inline comment as done.
  • Add doc about DILocation nodes in LoopIDs
Herald added a project: Restricted Project. · View Herald TranscriptMar 11 2019, 2:24 PM
hfinkel added inline comments.Tue, Apr 16, 3:22 PM
docs/LangRef.rst
5099

Before the advent of the 'distinct' keyword, this forced the preservation of otherwise-identical metadata nodes.

5103

can have up to

5103

Prior to the property nodes, one or two `DILocation` (debug location) nodes can be present in the list. The first, if present, identifies the source-code location where the loop begins. The second, if present, identifies the source-code location where the loop ends.

5108

as unique identifiers

5109

through transformations