This pass attempts to dectect code sequences like
if (x < y) {
z = y - x;
...
}
and replace them with sequences like
t = x - y;
if (t < 0) {
z = -t;
...
}
On architectures where the subtract can generate the flags used by the
if-statement, this saves an instruction. It's also possible that moving
an instruction out of the if-statement will allow
nir_opt_peephole_select to convert the whole thing to a bcsel.
Currently only floating point compares and adds are supported. Adding
support for integer will be a challenge due to integer overflow. There
are a couple possible solutions, but they may not apply to all
architectures.
v2: Fix a typo in the commit message and a couple typos in comments.
Fix possible NULL pointer deref from result of push_block(). Add
missing (-A + B) case. Suggested by Caio.
v3: Fix is_not_const_zero to work correctly with types other than
nir_type_float32. Suggested by Ken.
v4: Add some comments explaining how this works. Suggested by Ken.
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
