[Bug 90901] [regression] dEQP: "Detected variance between two invariant values"

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=90901

--- Comment #4 from Wayne Boyer <wayne.boyer at intel.com> ---
Kristian Kristensen looked into this and reported the following:

Ok, know what's going on with this bug now. The test tests the
invariance keyword by trying to provoke rounding errors coming from
different code paths computing the same result. Different levels of
optimization may reuse common subexpression results or rewrite a
mul+add to a mad, resulting in the same GLSL code computing getting
compiled to different hw instructions. A driver that support the GLSL
invariant keyword will make sure that different shaders always compute
the same value the same way. On drivers (like mesa) that don't support
invariance, the test may still pass, and as such, the fact that the
test passed in the past doesn't make this a regression. When we
switched mesa to use SIMD8 VS (the commit in quesstion), we started
using a different optimizer for the vertex shader code, which
optimizes the shaders enough that we get different results for the
depth values.

In short: not a regression, the test passed previously because it's
hard to consistently trigger the error condition.

Kristian

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