Consider:
u0 = foo()
if (divergent) {
u0 = bar()
r0 = baz(u0)
} else {
r0 = quux(u0)
}
Logically, this is fine, there is no interference between bar() and u0. But
physically, both sides of the if execute so the bar() write to u0 overwrites the
variable the else reads. So this is a miscompile.
The solution is to model the extra edges in the physical control flow graph,
which lives next to the existing logical control flow graph. Liveness for UGPRs
now follows the physical CFG, while liveness for GPRs continues to follow the
logical CFG. That models the interference properly, while still allowing phis to
work as before (since phis writing UGPRs follow uniform bits of control flow
that are necessarily critical edge free for the same reason the logical CFG is).
Because our RA copies shuffled registers back at block ends (following
Colombet), there's no issue with live range splits here (unlike aco which
inserts phis for this case and then needs to worry about critical edges around
those phis).
There might still be an extremely-challenging-to-hit bug here with UGPR spilling
which I need to think more about. It might be fine as-is? Not convinced though.
But this is big enough and strictly less broken than what we have right now and
the full solution will build on this, so here we are.
Fixes artefating in SuperTuxKart and Celestia knows what else.
Totals:
Instrs: 2770938 -> 2771269 (+0.01%); split: -0.00%, +0.02%
CodeSize: 40133712 -> 40138480 (+0.01%); split: -0.01%, +0.02%
Totals from 158 (5.97% of 2647) affected shaders:
Instrs: 514523 -> 514854 (+0.06%); split: -0.02%, +0.09%
CodeSize: 7603040 -> 7607808 (+0.06%); split: -0.03%, +0.09%
Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/41215>
