On Mon, Feb 22, 2016 at 1:05 PM, Ian Romanick <i...@freedesktop.org> wrote:
> On 02/19/2016 02:57 AM, Iago Toral wrote:
>> I don't know much about this, but using shader_samples_identical should
>> only give a benefit if we actually get identical samples, otherwise it
>> means more work, right?
>
> It is possible to do more work. Assuming the backend compiler does
> everything right, that amount of extra work should be trivial. The
> initial texelFetch will get the MCS data and the sample zero data. The
> textureSamplesIdenticalEXT also wants the MCS data. The backend CSE
> pass should eliminate the second fetch of MCS data. Then the added
> work is just comparing the MCS data with 0 and the flow control.
>
> Below is the BDW SIMD8 code for SAMPLES=4. I see at least 8
> instructions that could be removed (all the moves to g124-g127).
> Without GL_EXT_shader_samples_identical, none of those extra
> instructions are there. I'll play around with the GLSL to see if I can
> get the backend to generate better code. Otherwise, we'll have to see
> about fixing the backend.
>
> SIMD8 shader: 51 instructions. 0 loops. 170 cycles. 0:0 spills:fills.
> Promoted 0 constants. Compacted 816 to 544 bytes (33%)
> START B0
> pln(8) g34<1>F g4<0,1,0>F g2<8,8,1>F { align1 1Q
> compacted };
> pln(8) g35<1>F g4.4<0,1,0>F g2<8,8,1>F { align1 1Q
> compacted };
> mov(8) g6<1>F [0F, 0F, 0F, 0F]VF { align1 1Q
> compacted };
> mov(8) g7<1>F [0F, 0F, 0F, 0F]VF { align1 1Q
> compacted };
> mov(8) g8<1>D g34<8,8,1>F { align1 1Q
> compacted };
> mov(8) g9<1>D g35<8,8,1>F { align1 1Q
> compacted };
> send(8) g2<1>UW g6<8,8,1>F
> sampler ld2dms SIMD8 Surface = 1 Sampler = 0 mlen
> 4 rlen 4 { align1 1Q };
> mov(8) g124<1>F g2<8,8,1>F { align1 1Q
> compacted };
> mov(8) g125<1>F g3<8,8,1>F { align1 1Q
> compacted };
> mov(8) g126<1>F g4<8,8,1>F { align1 1Q
> compacted };
> mov(8) g127<1>F g5<8,8,1>F { align1 1Q
> compacted };
> mov.nz.f0(8) null<1>UD 0x00000000UD { align1 1Q };
> (+f0) if(8) JIP: 32 UIP: 392 { align1 1Q };
This is a dead branch unless I'm missing something. The mov.nz 0x0
sets the flag to false.
I thought for a moment that maybe we were playing games and setting
only enabled-channels of the flag to false, but there's nothing else
that initializes the flag, so that can't be it.
Without the seemingly-dead control flow the movs to g124-g127 before
the if would be removed.
Have I misunderstood something?
> END B0 ->B1 ->B2
> START B1 <-B0
> else(8) JIP: 376 UIP: 376 { align1 1Q };
And then of course we have no code in the "then" case anyway... :)
> END B1 ->B3
> START B2 <-B0
> mov(8) g10<1>F 1.4013e-45F { align1 1Q };
> mov(8) g11<1>F [0F, 0F, 0F, 0F]VF { align1 1Q
> compacted };
> mov(8) g12<1>F g8<8,8,1>F { align1 1Q
> compacted };
> mov(8) g13<1>F g9<8,8,1>F { align1 1Q
> compacted };
> mov(8) g14<1>F 2.8026e-45F { align1 1Q };
> mov(8) g15<1>F [0F, 0F, 0F, 0F]VF { align1 1Q
> compacted };
> mov(8) g16<1>F g8<8,8,1>F { align1 1Q
> compacted };
> mov(8) g17<1>F g9<8,8,1>F { align1 1Q
> compacted };
> mov(8) g18<1>F 4.2039e-45F { align1 1Q };
> mov(8) g19<1>F [0F, 0F, 0F, 0F]VF { align1 1Q
> compacted };
> mov(8) g20<1>F g8<8,8,1>F { align1 1Q
> compacted };
> mov(8) g21<1>F g9<8,8,1>F { align1 1Q
> compacted };
> send(8) g6<1>UW g10<8,8,1>F
> sampler ld2dms SIMD8 Surface = 1 Sampler = 0 mlen
> 4 rlen 4 { align1 1Q };
> send(8) g10<1>UW g14<8,8,1>F
> sampler ld2dms SIMD8 Surface = 1 Sampler = 0 mlen
> 4 rlen 4 { align1 1Q };
> send(8) g14<1>UW g18<8,8,1>F
> sampler ld2dms SIMD8 Surface = 1 Sampler = 0 mlen
> 4 rlen 4 { align1 1Q };
> add(8) g18<1>F g2<8,8,1>F g6<8,8,1>F { align1 1Q
> compacted };
> add(8) g22<1>F g3<8,8,1>F g7<8,8,1>F { align1 1Q
> compacted };
> add(8) g26<1>F g4<8,8,1>F g8<8,8,1>F { align1 1Q
> compacted };
> add(8) g30<1>F g5<8,8,1>F g9<8,8,1>F { align1 1Q
> compacted };
> add(8) g19<1>F g10<8,8,1>F g14<8,8,1>F { align1 1Q
> compacted };
> add(8) g23<1>F g11<8,8,1>F g15<8,8,1>F { align1 1Q
> compacted };
> add(8) g27<1>F g12<8,8,1>F g16<8,8,1>F { align1 1Q
> compacted };
> add(8) g31<1>F g13<8,8,1>F g17<8,8,1>F { align1 1Q
> compacted };
> add(8) g20<1>F g18<8,8,1>F g19<8,8,1>F { align1 1Q
> compacted };
> add(8) g24<1>F g22<8,8,1>F g23<8,8,1>F { align1 1Q
> compacted };
> add(8) g28<1>F g26<8,8,1>F g27<8,8,1>F { align1 1Q
> compacted };
> add(8) g32<1>F g30<8,8,1>F g31<8,8,1>F { align1 1Q
> compacted };
> mul(8) g21<1>F g20<8,8,1>F 0.25F { align1 1Q };
> mul(8) g25<1>F g24<8,8,1>F 0.25F { align1 1Q };
> mul(8) g29<1>F g28<8,8,1>F 0.25F { align1 1Q };
> mul(8) g33<1>F g32<8,8,1>F 0.25F { align1 1Q };
> mov(8) g124<1>UD g21<8,8,1>UD { align1 1Q
> compacted };
> mov(8) g125<1>UD g25<8,8,1>UD { align1 1Q
> compacted };
> mov(8) g126<1>UD g29<8,8,1>UD { align1 1Q
> compacted };
> mov(8) g127<1>UD g33<8,8,1>UD { align1 1Q
> compacted };
> END B2 ->B3
> START B3 <-B2 <-B1
> endif(8) JIP: 16 { align1 1Q };
> sendc(8) null<1>UW g124<8,8,1>F
> render RT write SIMD8 LastRT Surface = 0 mlen 4
> rlen 0 { align1 1Q EOT };
> nop ;
> END B3
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