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. 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
