On Sep 22, 2015 10:01 PM, "Jason Ekstrand" <<a href="mailto:jason@jlekstrand.net">jason@jlekstrand.net</a>> wrote: > > It's possible that, if a vecN operation is involved in a phi node, that we > could end up moving from a register to itself. If swizzling is involved, > we need to emit the move but. However, if there is no swizzling, then the > mov is a no-op and we might as well not bother emitting it. > > Shader-db results on Haswell: > > total instructions in shared programs: 6262536 -> 6259558 (-0.05%) > instructions in affected programs: 184780 -> 181802 (-1.61%) > helped: 838 > HURT: 0 By the way, I have absolutely no idea why this helps more than Matt's patch to delete these in register coalesce. > --- > src/glsl/nir/nir_lower_vec_to_movs.c | 19 ++++++++++++++++++- > 1 file changed, 18 insertions(+), 1 deletion(-) > > diff --git a/src/glsl/nir/nir_lower_vec_to_movs.c b/src/glsl/nir/nir_lower_vec_to_movs.c > index 287f2bf..2039891 100644 > --- a/src/glsl/nir/nir_lower_vec_to_movs.c > +++ b/src/glsl/nir/nir_lower_vec_to_movs.c > @@ -83,7 +83,24 @@ insert_mov(nir_alu_instr *vec, unsigned start_idx, nir_shader *shader) > } > } > > - nir_instr_insert_before(&vec->instr, &mov->instr); > + /* In some situations (if the vecN is involved in a phi-web), we can end > + * up with a mov from a register to itself. Some of those channels may end > + * up doing nothing and there's no reason to have them as part of the mov. > + */ > + if (src_matches_dest_reg(&mov->dest.dest, &mov->src[0].src) && > + !mov->src[0].abs && !mov->src[0].negate) { > + for (unsigned i = 0; i < 4; i++) { > + if (mov->src[0].swizzle[i] == i) > + mov->dest.write_mask &= ~(1 << i); > + } > + } > + > + /* Only emit the instruction if it actually does something */ > + if (mov->dest.write_mask) { > + nir_instr_insert_before(&vec->instr, &mov->instr); > + } else { > + ralloc_free(mov); > + } > > return mov->dest.write_mask; > } > -- > 2.5.0.400.gff86faf >