1484908 - Intermittent failure in/css/css-images/tiled-gradients.html on Haswell

Status: NEW

(Core :: Graphics: WebRender, defect, P3)

Description

[Robert O'Callahan (:roc) (email my personal email if necessary)]

You don't see this on Mozilla CI because the Amazon instances are pre-Haswell. By manipulating CPUID I was able to show that the failures occur when FMA3 is enabled and disappear when FMA3 is disabled. There's noise along the diagonal of the gradient which varies between test and reference. Skia has some Haswell-specific code paths that are probably relevant.

Comment 1

[Robert O'Callahan (:roc) (email my personal email if necessary)]

This seems to only affect WebRender rendering.

Comment 2

[Robert O'Callahan (:roc) (email my personal email if necessary)]

I guess it's more likely to be the Gallium/LLVM JIT that's creating FMA-dependent code paths here.

Comment 3

[Robert O'Callahan (:roc) (email my personal email if necessary)]

Debugging: https://pernos.co/debug/_PKRP2-vC_AXVb9dTkFlfA/index.html
This intermittent is very frequent >= Haswell.