2052528 - Metal ANGLE cannot draw a single vertex/instance if attribute size is not aligned to 4 bytes

Status: RESOLVED FIXED

(Core :: Graphics: WebRender, defect)

Description

[Jamie Nicol [:jnicol]]

We run into this issue when attempting to run webrender on Metal ANGLE (bug 2052515).

Metal does not support binding vertex attributes where the offset + stride > buffer_len. To work around this, ANGLE will in such cases set the stride to the size of the single attribute. However, Metal also requires that the strides are aligned to 4 bytes.

When we have instance attributes that are not 4-byte aligned, and attempt to draw a single instance, we therefore hit this assertion.

Specifically, this happens when drawing a single instance with cs_svg_filter_node, which has 2x u16 attributes.

If we pack these into a u16x2 we can avoid the crash.

Comment 1

[Jamie Nicol [:jnicol]]

Attachment: Bug 2052528 - Add missing uvec2 support to SWGL. r?#gfx-reviewers

Comment 2

[Jamie Nicol [:jnicol]]

Attachment: Bug 2052528 - Ensure instance attributes have 4 byte alignment. r?#gfx-reviewers

Metal requires that vertex attributes' offsets + strides do not extend
beyond the end of their buffer. When drawing a single vertex or
instance, ANGLE therefore clamps the stride to the size of the single
attribute. Unfortunately, however, Metal also requires that vertex
attribute strides are aligned to 4 bytes.

This makes it impossible to use non-4-byte-aligned instance attributes
on Metal ANGLE, as attempting to draw a single instance will fail due
to invalid alignment.

To avoid this, we ensure instance attributes are always aligned to 4
bytes. Specifically, this means packing cs_svg_filter_node's filter
kind and input count attributes into a u16x2 vector, rather than as
2 separate u16 scalars.

Comment 3

[Pulsebot]

Pushed by [e-mail]:
https://github.com/mozilla-firefox/firefox/commit/e54c2da87957
https://hg.mozilla.org/integration/autoland/rev/b487125e2ffa
Add missing uvec2 support to SWGL. r=gfx-reviewers,lsalzman
https://github.com/mozilla-firefox/firefox/commit/d154a3eb4311
https://hg.mozilla.org/integration/autoland/rev/2cf4824879e0
Ensure instance attributes have 4 byte alignment. r=gfx-reviewers,lsalzman

Comment 4

[Norisz Fay [:noriszfay]]

https://hg.mozilla.org/mozilla-central/rev/b487125e2ffa
https://hg.mozilla.org/mozilla-central/rev/2cf4824879e0