[Mesa-dev] [PATCH 5/5] glsl: Improve the local dead code optimization to eliminate unused channels.

[Ian Romanick]

On 05/08/2012 05:59 PM, Eric Anholt wrote:
> Total instructions: 261582 ->  261316
> 135/2147 programs affected (6.3%)
> 36752 ->  36486 instructions in affected programs (0.7% reduction)
>
> This excludes a tropics shader that now gets 16-wide mode and throws
> off the numbers.  5 shaders are hurt: two extra MOVs in 4 tropics
> shaders it looks like because we don't split register names according
> to independent webs, and one gstreamer shader where it looks like
> try_rewrite_rhs_to_dst() is falling on its face.
>
> This should also help avoid a regression in VSes from idr's ARB
> programs to GLSL work.

Sort of, but not really.  It seems to change cases like

    (assign  (xyzw) (var_ref gl_TexCoord[5])  (constant vec4 (0.000000 
0.500000 0.000000 1.000000)) )
mov(8)          m10<1>.xzF      0F                              { 
align16 WE_normal 1Q };
mov(8)          m10<1>.yF       0.5F                            { 
align16 WE_normal 1Q };
mov(8)          m10<1>.wF       1F                              { 
align16 WE_normal 1Q };
    (assign  (x) (var_ref gl_TexCoord[5])  (expression float dot 
(var_ref vertex_attrib8) (var_ref gl_Env14) ) )
dp4(8)          m10<1>.xF       g12<4,4,1>F     g5<0,4,1>F      { 
align16 WE_normal 1Q };

into

    (assign  (zw) (var_ref gl_TexCoord[5])  (constant vec2 (0.000000 
1.000000)) )
mov(8)          g16<1>.xF       0F                              { 
align16 WE_normal 1Q };
mov(8)          g16<1>.yF       1F                              { 
align16 WE_normal 1Q };
mov(8)          m10<1>.zwF      g16<4,4,1>.yyxyF                { 
align16 WE_normal 1Q };
    (assign  (x) (var_ref gl_TexCoord[5])  (expression float dot 
(var_ref vertex_attrib8) (var_ref gl_Env14) ) )
dp4(8)          m10<1>.xF       g12<4,4,1>F     g5<0,4,1>F      { 
align16 WE_normal 1Q };

The extra load of 0.5 is eliminated (yay), but now the immediate values 
are loaded into an extra temporary (boo) resulting in the same total 
number of instructions.