Bug 86161

uniform float ux;
uniform float uy;

void main()
{
  float x = ux * uy;
  gl_FragColor = vec4(x * 0.5);
}



This fragment shader yields the following assembly:

SIMD8 shader: 7 instructions. Compacted 112 to 80 bytes (29%)
   START B0
   (assign  (xyzw) (var_ref gl_FragColor)  (swiz xxxx (expression float * (expression float * (var_ref ux) (var_ref uy) ) (constant float (0.500000)) ) )) 
mul(8)          g3<1>F          g2<0,1,0>F      g2.1<0,1,0>F    { align1 1Q compacted };
mul(8)          g4<1>F          g3<8,8,1>F      0.5F            { align1 1Q };
   FB write target 0
mov(8)          g113<1>F        g4<8,8,1>F                      { align1 1Q compacted };
mov(8)          g114<1>F        g4<8,8,1>F                      { align1 1Q compacted };
mov(8)          g115<1>F        g4<8,8,1>F                      { align1 1Q compacted };
mov(8)          g116<1>F        g4<8,8,1>F                      { align1 1Q compacted };
nop                                                             ;
sendc(8)        null            g113<8,8,1>F
                            render RT write SIMD8 LastRT Surface = 0 mlen 4 rlen 0 { align1 1Q EOT };
   END B0


In particular, the first instruction:
mul(8)          g3<1>F          g2<0,1,0>F      g2.1<0,1,0>F    { align1 1Q compacted };

The operands are the two uniform values in subregisters g2.0 and g2.1. The written register region is the whole g3 with exec size 8, where a single float with exec size 1 would suffice.

Of course just changing the above instruction to exec size 1 would achieve nothing if the register allocator is not improved to benefit from the leftover space...