After we lowered `return` into `break` - the control flow is changed and
the block with this change has a new successor, which means that in this
new successor phis should have additional source.
Since the instructions that use phis in the successor are predicated -
it's ok for a new phi source to be undef.
If `return` is lowered in a nested loop, `break` is inserted in the outer
loops, so all new blocks with break require the same changes to phis
described above.
Examples of NIR before lowering:
block block_0:
loop {
block block_1:
if ssa_2 {
block block_2:
return
// succs: block_6
} else {
block block_2:
break;
// succs: block_5
}
block block_4:
}
block block_5:
// preds: block_3
vec1 32 ssa_4 = phi block_3: ssa_1
// succs: block_6
block block_6:
Here converting return to break should add block_2 to the phis
of block_5.
block block_0:
loop {
block block_1:
loop {
block block_2:
if ssa_2 {
block block_3:
return
// succs: block_8
} else {
block block_4:
break;
// succs: block_6
}
block block_5:
}
block block_6:
break;
// succs: block_7
}
block block_7:
// preds: block_6
vec1 32 ssa_4 = phi block_6: ssa_1
// succs: block_8
block block_8:
Here converting return to break will insert conditional break in
the outer loop, changing block_6 predcessors.
Cc: <mesa-stable@lists.freedesktop.org>
Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/3322
Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/3498
Signed-off-by: Danylo Piliaiev <danylo.piliaiev@globallogic.com>
Reviewed-by: Rhys Perry <pendingchaos02@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6186>
V2:
- updated to create a generic list clone helper nir_cf_list_clone()
- continue to assert on clone when fallback flag not set as suggested
by Jason.
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
