1365928 - Annotate style struct getters as always returning a non-null pointer

Status: RESOLVED FIXED

(Core :: CSS Parsing and Computation, enhancement)

Description

[Jonathan Kew [:jfkthame]]

The nsStyleXXX* getters in nsStyleContext and nsIFrame will never return a null pointer (see comment at the declaration of StyleData() in nsStyleContext.h). If we annotate these methods with the returns_nonnull attribute (supported by gcc/clang), this may enable the compiler to optimize better around call sites.

Comment 1

[Jonathan Kew [:jfkthame]]

Attachment: patch 1 - Add a MOZ_NONNULL_RETURN annotation to mfbt/Attributes.h

Comment 2

[Jonathan Kew [:jfkthame]]

Attachment: patch 2 - Annotate style-struct getters with MOZ_NONNULL_RETURN for better optimization

This appears to affect code-gen in a few places in my build; I haven't analyzed them individually but presumably the optimizer knows what it's doing....

Comment 3

[Nathan Froyd [:froydnj]]

Comment on attachment 8869013 [details] [diff] [review]
patch 1 - Add a MOZ_NONNULL_RETURN annotation to mfbt/Attributes.h

Review of attachment 8869013 [details] [diff] [review]:
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Thanks!  What does this enable for your particular use case?  Better codegen for placement new?

Comment 4

[David Baron :dbaron: (⌚️UTC-4, no longer working on Mozilla)]

I think this came from bug 1365356 comment 7 and bug 1365356 comment 8.

Comment 5

[Jonathan Kew [:jfkthame]]

(In reply to David Baron :dbaron: ⌚️UTC-7 from comment #4)
> I think this came from bug 1365356 comment 7 and bug 1365356 comment 8.

Indeed. It looks like in some cases, it's enabling the compiler to optimize away calls to StyleDisplayWithOptionalParam(), by recognizing that the pointer being passed must be non-null and therefore the method collapses to simply returning its parameter.

Comment 6

[Jonathan Kew [:jfkthame]]

https://hg.mozilla.org/integration/mozilla-inbound/rev/3d3190803408c7605fbb52f8215a757fffedcad5
Bug 1365928 - patch 1 - Add a MOZ_NONNULL_RETURN annotation to mfbt/Attributes.h. r=froydnj

https://hg.mozilla.org/integration/mozilla-inbound/rev/75cd3397298ad5c0f02129a87f465059018a9f63
Bug 1365928 - patch 2 - Annotate style-struct getters with MOZ_NONNULL_RETURN for better optimization. r=mats

Comment 7

[Wes Kocher (:KWierso) (Not reading bugmail; email directly if needed)]

https://hg.mozilla.org/mozilla-central/rev/3d3190803408
https://hg.mozilla.org/mozilla-central/rev/75cd3397298a