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Avoid undefined behavior in glsl-routing test

Pierre-Eric Pelloux-Prayer requested to merge pepp/piglit:fix_glsl_routing into master

This test reads attribute in the FS (e.g: gl_SecondaryColor) without writing them in the VS. It shouldn't affect the output since in this case the eq() function returns 0.0, so it's equivalent to: r += 0.0 * gl_SecondaryColor.

The problems are:

  • if gl_SecondaryColor is unwritten, its value is allowed to be NaN, in which case it will affect the output color and test will fail even if the driver is valid
  • the GLSL 1.20 specs says (7.6 Varying variables):

"The values in gl_Color and gl_SecondaryColor will be derived automatically by the system from gl_FrontColor, gl_BackColor, gl_FrontSecondaryColor, and gl_BackSecondaryColor based on which face is visible."

And the paragraph before says:

"A particular [varying variable] should be written to if any functionality in a corresponding fragment shader or fixed pipeline uses it or state derived from it. Otherwise, behavior is undefined."

This commit replaces the 'r += eq(...) * gl_SecondaryColor' lines with 'if (eq(...)) r += gl_SecondaryColor' ones which aren't affected by the above issues.

Fixes the test on my AMD Vega10 system.

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