<div dir="ltr"> <div class="gmail_extra"> <div class="gmail_quote">On Fri, Apr 8, 2016 at 8:21 AM, Eduardo Lima Mitev <<a href="mailto:elima@igalia.com" target="_blank">elima@igalia.com</a>> wrote: <blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">On 04/08/2016 01:35 AM, Kenneth Graunke wrote: <blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"> Many shaders contain expression trees of the form: const_1 * (value * const_2) Reorganizing these to (const_1 * const_2) * value will allow constant folding to combine the constants. Sometimes, these constants are 2 and 0.5, so we can remove a multiply altogether. Other times, it can create more immediate constants, which can actually hurt. </blockquote></blockquote><div> </div><div>Further justification would be that the original was 4 NIR instructions: 2 load_const and 2 multiplies. Once constant folding takes over, we are again left with 4 NIR instructions: 2 load_const and 2 multiplies. The difference is that we now have (value * const_2) and (value * (const_1 * const_2)). So we have no more instructions in the end and, hopefully, the (value * const_2) goes away. Obviously, the emperical evidence says this actually hurts sometimes, but that's not surprising. </div><div> </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"> Finding a good balance here is tricky. While much more could be done, this simple patch seems to have a lot of positive benefit while having a low downside. </blockquote> Makes sense. Reviewed-by: Eduardo Lima Mitev <<a href="mailto:elima@igalia.com" target="_blank">elima@igalia.com</a>> <blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"> shader-db results on Broadwell: total instructions in shared programs: 8963768 -> 8961369 (-0.03%) instructions in affected programs: 438318 -> 435919 (-0.55%) helped: 1502 HURT: 245 total cycles in shared programs: 71527354 -> 71421516 (-0.15%) cycles in affected programs: 11541788 -> 11435950 (-0.92%) helped: 3445 HURT: 1224 </blockquote> FWIW, these are my results on HSW: total instructions in shared programs: 6223995 -> 6222470 (-0.02%) instructions in affected programs: 300945 -> 299420 (-0.51%) helped: 1231 HURT: 240 total cycles in shared programs: 56809432 -> 56615552 (-0.34%) cycles in affected programs: 7980160 -> 7786280 (-2.43%) helped: 2087 HURT: 634 Eduardo <blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"> Signed-off-by: Kenneth Graunke <<a href="mailto:kenneth@whitecape.org" target="_blank">kenneth@whitecape.org</a>> --- src/compiler/nir/nir_opt_algebraic.py | 8 ++++++++ 1 file changed, 8 insertions(+) diff --git a/src/compiler/nir/nir_opt_algebraic.py b/src/compiler/nir/nir_opt_algebraic.py index e72b4a7..420d9d9 100644 --- a/src/compiler/nir/nir_opt_algebraic.py +++ b/src/compiler/nir/nir_opt_algebraic.py @@ -274,6 +274,14 @@ optimizations = [ (('fmul', ('fneg', a), b), ('fneg', ('fmul', a, b))), (('imul', ('ineg', a), b), ('ineg', ('imul', a, b))), + # Reassociate constants in add/mul chains so they can be folded together. + # For now, we only handle cases where the constants are separated by + # a single non-constant. We could do better eventually. + (('~fmul', '#a', ('fmul', b, '#c')), ('fmul', ('fmul', a, c), b)), + (('imul', '#a', ('imul', b, '#c')), ('imul', ('imul', a, c), b)), + (('~fadd', '#a', ('fadd', b, '#c')), ('fadd', ('fadd', a, c), b)), + (('iadd', '#a', ('iadd', b, '#c')), ('iadd', ('iadd', a, c), b)), + # Misc. lowering (('fmod', a, b), ('fsub', a, ('fmul', b, ('ffloor', ('fdiv', a, b)))), 'options->lower_fmod'), (('uadd_carry', a, b), ('b2i', ('ult', ('iadd', a, b), a)), 'options->lower_uadd_carry'), </blockquote> <div class="HOEnZb"><div class="h5"> _______________________________________________ mesa-dev mailing list <a href="mailto:mesa-dev@lists.freedesktop.org" target="_blank">mesa-dev@lists.freedesktop.org</a> <a href="https://lists.freedesktop.org/mailman/listinfo/mesa-dev" rel="noreferrer" target="_blank">https://lists.freedesktop.org/mailman/listinfo/mesa-dev</a> </div></div></blockquote></div> </div></div>