[Mesa-dev] [PATCH] i965: Use NIR by default for vertex shaders on GEN8+

[Matt Turner]

On Fri, May 8, 2015 at 3:27 AM, Kenneth Graunke <kenneth at whitecape.org> wrote:
> Looking at a couple of the shaders that are still worse off...it looks
> like a ton of Source shaders used to do MUL/ADD with an attribute and
> two immediates, and now are doing MOV/MOV/MAD.

I just looked, and thought that too for a minute, but it actually
shouldn't be doing that. Take for instance:

shaders/closed/steam/dota-2/498.shader_test VS SIMD8: 47 -> 53 (12.77%)

It indeed replaces 6x MUL/ADD pairs with MOV/MAD (introducing 6 extra
MOVs), but....

Without NIR we have

mul(8)          g15<1>F         g6<8,8,1>F      6F
...
add(8)          g16<1>F         g15<8,8,1>F     2.1F
add(8)          g35<1>F         g15<8,8,1>F     3.1F
add(8)          g42<1>F         g15<8,8,1>F     4.1F
add(8)          g45<1>F         g15<8,8,1>F     5.1F
add(8)          g48<1>F         g15<8,8,1>F     0.1F
add(8)          g51<1>F         g15<8,8,1>F     1.1F

That is, one multiply is consumed by 6 adds.

With NIR we have

mov(1)          g22<1>F         2.1F
mov(1)          g22.1<1>F       6F
mad(8)          g16<1>F         g22<0,1,0>.xF   g22.1<0,1,0>.xF g6<4,4,1>F
mov(1)          g22.2<1>F       3.1F
mad(8)          g23<1>F         g22.2<0,1,0>.xF g22.1<0,1,0>.xF g6<4,4,1>F
mov(1)          g22.3<1>F       4.1F
mad(8)          g30<1>F         g22.3<0,1,0>.xF g22.1<0,1,0>.xF g6<4,4,1>F
mov(1)          g22.4<1>F       5.1F
mad(8)          g33<1>F         g22.4<0,1,0>.xF g22.1<0,1,0>.xF g6<4,4,1>F
mov(1)          g22.5<1>F       0.1F
mad(8)          g36<1>F         g22.5<0,1,0>.xF g22.1<0,1,0>.xF g6<4,4,1>F
mov(1)          g22.6<1>F       1.1F
mad(8)          g39<1>F         g22.6<0,1,0>.xF g22.1<0,1,0>.xF g6<4,4,1>F

So we're doing the g6 * 6F operation 6 times! We see this in the NIR as well:

        vec1 ssa_419 = ffma ssa_384, ssa_132, ssa_133
        vec1 ssa_423 = ffma ssa_384, ssa_132, ssa_135
        vec1 ssa_427 = ffma ssa_384, ssa_132, ssa_137
        vec1 ssa_428 = ffma ssa_384, ssa_132, ssa_139
        vec1 ssa_429 = ffma ssa_384, ssa_132, ssa_141
        vec1 ssa_430 = ffma ssa_384, ssa_132, ssa_144

Whoops. Ideas for fixing that? I'm guessing that this accounts for
nearly all of the remaining 1120 hurt programs.