Shader ALU improvement not having expected impact


We've analysed our main shader (which presumably accounts for most of the pixels from the 3D pass). The shader is largely ALU bound in most architectures (see trimmed malioc's report below):

Before optimization (Mali G-71):

                               A     LS      V      T
Total instruction cycles:    6.8    0.0    4.0    2.0

After optimization: 

                               A     LS      V      T
Total instruction cycles:    4.7    0.0    4.8    2.0  

This optimization was driven by the fact that we were convinced, perhaps wrongly, that the Shader Core Unit was ALU bound (image attached).

(LEFT: after optimization; RIGHT: before optimization)

After the optimization being applied though, we didn't notice any significant improvement in ALU (both in the total span of a frame but also within the region I show above, which I believe to correspond to the 3D pass): ~69% from ~70%. 
My suspicion is that this might be related with the Partial Coverage Rate values - according to your blog, this could be due to sliver/micro triangles. The execution core utilization drops significantly midway and I can't flag any other culprit. So, if we're really eroding the performance due to that kind of geometry, would that explain the ineffective optimization? 


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