After trying multiple times to merge if-statements with phis between them I've come to the conclusion that it cannot be done without regressions. The problem is for some shaders we end up with a whole bunch of phis for the merged ifs resulting in increased register pressure.
So this patch just merges ifs that have no phis between them. This seems to be consistent with what LLVM does so for radeonsi we only see a change (although its a large change) in a single shader.
Shader-db results i965 (SKL):
total instructions in shared programs: 13098176 -> 13098152 (<.01%) instructions in affected programs: 1326 -> 1302 (-1.81%) helped: 4 HURT: 0
total cycles in shared programs: 332032989 -> 332037583 (<.01%) cycles in affected programs: 60665 -> 65259 (7.57%) helped: 0 HURT: 4
The cycles estimates reported by shader-db for i965 seem inaccurate as the only difference in the final code is the removal of the redundent condition evaluations and jumps.
Also the biggest code reduction (~7%) for radeonsi was in a tomb raider tressfx shader but for some reason this does not get merged for i965.
Shader-db results radeonsi (VEGA):
Totals from affected shaders: SGPRS: 232 -> 232 (0.00 %)
VGPRS: 164 -> 164 (0.00 %) Spilled SGPRs: 59 -> 59 (0.00 %) Spilled VGPRs: 0 -> 0 (0.00 %) Private memory VGPRs: 0 -> 0 (0.00 %) Scratch size: 0 -> 0 (0.00 %) dwords per thread Code Size: 14584 -> 13520 (-7.30 %) bytes LDS: 0 -> 0 (0.00 %) blocks Max Waves: 13 -> 13 (0.00 %) Wait states: 0 -> 0 (0.00 %)
4d3f6cb973 nir: merge some basic consecutive ifs
src/compiler/nir/nir_opt_if.c | 93 +++++++++++++++++++++++++++++++++++++++++++
1 file changed, 93 insertions(+)