[Bug 100913] 6% performance drop in GpuTest v0.7 Volplosion with "remove GLSL IR optimisation loop"

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=100913

--- Comment #1 from Eero Tamminen <eero.t.tamminen at intel.com> ---
After the change, the main fragment shader doesn't anymore compile as SIMD16
(benchmark seems to compile shader 7 twice so you get the warning twice):
FS compile failed: Failure to register allocate.  Reduce number of live scalar
values to avoid this.

Resulting SIMD8 shader has about the same amount of instructions (nearly two
thousand) as earlier, with following changes:
- One mad()  more
- One add() & mul() less
- slightly more register bank conflicts
- at max, 71 live registers instead of earlier 61

-> somehow this change messes up the current register allocation.

Before the change, the SIMD16 variant of the shader had at max 109 live
registers.

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