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/*
* Copyright © 2015 Connor Abbott
*
* Permission is hereby granted, free of charge, to any person obtaining a
* copy of this software and associated documentation files (the "Software"),
* to deal in the Software without restriction, including without limitation
* the rights to use, copy, modify, merge, publish, distribute, sublicense,
* and/or sell copies of the Software, and to permit persons to whom the
* Software is furnished to do so, subject to the following conditions:
*
* The above copyright notice and this permission notice (including the next
* paragraph) shall be included in all copies or substantial portions of the
* Software.
*
* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
* FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
* THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
* LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
* FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
* IN THE SOFTWARE.
*
* Authors:
* Connor Abbott (cwabbott0@gmail.com)
*
*/
#include "nir.h"
/*
* This is a pass for removing phi nodes that look like:
* a = phi(b, b, b, ...)
*
* Note that we can't ignore undef sources here, or else we may create a
* situation where the definition of b isn't dominated by its uses. We're
* allowed to do this since the definition of b must dominate all of the
* phi node's predecessors, which means it must dominate the phi node as well
* as all of the phi node's uses. In essence, the phi node acts as a copy
* instruction. b can't be another phi node in the same block, since the only
* time when phi nodes can source other phi nodes defined in the same block is
* at the loop header, and in that case one of the sources of the phi has to
* be from before the loop and that source can't be b.
*/
static bool
remove_phis_block(nir_block *block, void *state)
{
bool *progress = state;
nir_foreach_instr_safe(block, instr) {
if (instr->type != nir_instr_type_phi)
break;
nir_phi_instr *phi = nir_instr_as_phi(instr);
nir_ssa_def *def = NULL;
bool srcs_same = true;
nir_foreach_phi_src(phi, src) {
assert(src->src.is_ssa);
/* For phi nodes at the beginning of loops, we may encounter some
* sources from backedges that point back to the destination of the
* same phi, i.e. something like:
*
* a = phi(a, b, ...)
*
* We can safely ignore these sources, since if all of the normal
* sources point to the same definition, then that definition must
* still dominate the phi node, and the phi will still always take
* the value of that definition.
*/
if (src->src.ssa == &phi->dest.ssa)
continue;
if (def == NULL) {
def = src->src.ssa;
} else {
if (src->src.ssa != def) {
srcs_same = false;
break;
}
}
}
if (!srcs_same)
continue;
/* We must have found at least one definition, since there must be at
* least one forward edge.
*/
assert(def != NULL);
assert(phi->dest.is_ssa);
nir_ssa_def_rewrite_uses(&phi->dest.ssa, nir_src_for_ssa(def));
nir_instr_remove(instr);
*progress = true;
}
return true;
}
static bool
remove_phis_impl(nir_function_impl *impl)
{
bool progress = false;
nir_foreach_block(impl, remove_phis_block, &progress);
if (progress) {
nir_metadata_preserve(impl, nir_metadata_block_index |
nir_metadata_dominance);
}
return progress;
}
bool
nir_opt_remove_phis(nir_shader *shader)
{
bool progress = false;
nir_foreach_overload(shader, overload)
if (overload->impl)
progress = remove_phis_impl(overload->impl) || progress;
return progress;
}
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