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nir/dead_cf: Do not remove loops with loads that can't be reordered 

If a loop is followed by a barrier, the ordering between a load inside
the loop and other memory operations after the barrier may have to be
preserved depending on the type of memory involved.  This is relevant
when the memory is writeable by other invocations.  In such case, it
is not valid to completely eliminate the loop.

This commit doesn't attempt to precisely catch the barrier case, as
analysis could become too complex.  It simply assumes it can't drop
the loops that contain certain types of loads unless those are known
to be safe to reorder (via the access flag).

Fixes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/4475
Acked-by: Rhys Perry <pendingchaos02@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9938>
This commit is contained in:
Caio Marcelo de Oliveira Filho 2021-03-30 14:18:05 -07:00 committed by Marge Bot
parent 38f39cc1e2
commit 0092edfec0
1 changed files with 35 additions and 0 deletions
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35
src/compiler/nir/nir_opt_dead_cf.c
View File

@ -236,6 +236,41 @@ node_is_dead(nir_cf_node *node)
if (!(nir_intrinsic_infos[intrin->intrinsic].flags &
NIR_INTRINSIC_CAN_ELIMINATE))
return false;
switch (intrin->intrinsic) {
case nir_intrinsic_load_deref:
case nir_intrinsic_load_ssbo:
case nir_intrinsic_load_global:
/* If there's a memory barrier after the loop, a load might be
* required to happen before some other instruction after the
* barrier, so it is not valid to eliminate it -- unless we
* know we can reorder it.
*
* Consider only loads that the result can be affected by other
* invocations.
*/
if (intrin->intrinsic == nir_intrinsic_load_deref) {
nir_deref_instr *deref = nir_src_as_deref(intrin->src[0]);
if (!nir_deref_mode_may_be(deref, nir_var_mem_ssbo |
nir_var_mem_shared |
nir_var_mem_global |
nir_var_shader_out))
break;
}
if (nir_intrinsic_access(intrin) & ACCESS_CAN_REORDER)
break;
return false;
case nir_intrinsic_load_shared:
case nir_intrinsic_load_output:
case nir_intrinsic_load_per_vertex_output:
/* Same as above loads. */
return false;
default:
/* Do nothing. */
break;
}
}
if (!nir_foreach_ssa_def(instr, def_only_used_in_cf_node, node))