The primary motivation here is to avoid needing to worry about instructions
potentially pointing to the preallocated error instruction in the case of
allocation failure.
This doesn't cover all passes, but none of the other passes make assumptions
about instruction sources.
This is because lower_nonconstant_array_loads() can potentially turn
nonconstant loads into constant loads, allowing copy-prop to turn these
loads into previous instructions, which might help other passes as well.
This patch lowers the number of required temps for the following ps_2_0
shader from 19 to 16:
int i;
float3x3 mats[4];
float4 main() : sv_target
{
return mul(mats[i], float3(1, 2, 3)).xyzz;
}
This can save a significant amount of temp registers because it allows to
avoid referencing the temp (and having to store it) when not needed.
For instance, this patch lowers the number of required temps for the
following ps_2_0 shader from 24 to 19:
int i;
float3x3 mats[4];
float4 main() : sv_target
{
return mul(mats[i], float3(1, 2, 3)).xyzz;
}
Also, it is needed for SM1 vertex shader relative addressing since
non-constant loads are required to be directly on the uniform ('c'
registers) instead of the temp, and non-constant loads cannot be
transformed by copy propagation.
