On Tue, Sep 22, 2015 at 4:04 PM, Matt Turner <matts...@gmail.com> wrote: > On Fri, Sep 18, 2015 at 12:49 AM, Eduardo Lima Mitev <el...@igalia.com> wrote: >> When both fadd and fmul instructions have at least one operand that is a >> constant and it is only used once, the total number of instructions can >> be reduced from 3 (1 ffma + 2 load_const) to 2 (1 fmul + 1 fadd); because >> the constants will be progagated as immediate operands of fmul and fadd. >> >> This patch modifies opt_peephole_ffma pass to detect this situation and >> bails-out fusing fmul+fadd into ffma. >> >> As shown in shader-db results below, it seems to help a good bunch. However, >> there are some caveats: >> >> * It seems i965 specific, so I'm not sure if modifying the NIR pass >> directly is desired, as opposed to moving this to the backend. >> >> * There are still a high number of HURTs, but these could be reduced by being >> more specific in the conditions to bailout. >> >> total instructions in shared programs: 1683959 -> 1677447 (-0.39%) >> instructions in affected programs: 604918 -> 598406 (-1.08%) >> helped: 4633 >> HURT: 804 >> GAINED: 0 >> LOST: 0 >> --- > > Interesting -- yeah, I've thought about doing this as well. It was > more difficult before because with GLSL IR (where I was trying to do > it) it wasn't possible to determine if the constant was used by > multiple 3-src instructions. Actually, your check might be able to be > more refined to consider only uses of 3-src instructions. > > But that's getting kind of hardware-specific.
If we want to move this into the i965 driver we can. I think we're the only users. That would completely get rid of hardware-specificness issues. --Jason > Perhaps another approach would be to modify the > opt_combine_constants() pass to split MADs under some circumstances -- > e.g., it accounts for the only use of a constant we would otherwise > have to promote. But of course we don't have that pass for the vec4 > backend. > > In the mean time, I've sent a related patch that may be of interest: > "[PATCH] nir: Don't fuse fmul into ffma if used by more than 4 fadds." > > This patch, applied on top of mine gives these results on Haswell: > > Total: > total instructions in shared programs: 6595563 -> 6584885 (-0.16%) > instructions in affected programs: 1183608 -> 1172930 (-0.90%) > helped: 8074 > HURT: 842 > GAINED: 4 > > FS: > total instructions in shared programs: 4863484 -> 4859884 (-0.07%) > instructions in affected programs: 554042 -> 550442 (-0.65%) > helped: 3072 > HURT: 38 > GAINED: 4 > > VS: > total instructions in shared programs: 1729224 -> 1722146 (-0.41%) > instructions in affected programs: 629566 -> 622488 (-1.12%) > total loops in shared programs: 221 -> 221 (0.00%) > helped: 5002 > HURT: 804 > > Another thing to consider for the vec4 backend is that vec4 uniforms > have to be unpacked for use by 3-src instructions (see the > VEC4_OPCODE_UNPACK_UNIFORM opcode). We CSE the unpacking operations, > but they often do account for increases in instruction counts. > _______________________________________________ > mesa-dev mailing list > mesa-dev@lists.freedesktop.org > http://lists.freedesktop.org/mailman/listinfo/mesa-dev _______________________________________________ mesa-dev mailing list mesa-dev@lists.freedesktop.org http://lists.freedesktop.org/mailman/listinfo/mesa-dev