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nir: Convert a bcsel with only phi node sources to a phi node

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v2: Remove the original ALU instruciton after all of its readers are
modified to read the new ALU instruction.

v3: Fix an issue where a bcsel that may not be executed on a loop
iteration due to a break statement is converted to a phi (and therefore
incorrectly "executed").  Noticed by Tim.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=109216
Fixes: 8fb8ebfbb0 ("intel/compiler: More peephole select")
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
This commit is contained in:
Ian Romanick
2019-01-17 17:53:40 -08:00
parent 0881e90c09
commit b031c64349
1 changed files with 220 additions and 0 deletions
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220
src/compiler/nir/nir_opt_if.c
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@@ -542,6 +542,225 @@ opt_split_alu_of_phi(nir_builder *b, nir_loop *loop)
return progress;
}
/**
* Get the SSA value from a phi node that corresponds to a specific block
*/
static nir_ssa_def *
ssa_for_phi_from_block(nir_phi_instr *phi, nir_block *block)
{
nir_foreach_phi_src(src, phi) {
if (src->pred == block)
return src->src.ssa;
}
assert(!"Block is not a predecessor of phi.");
return NULL;
}
/**
* Simplify a bcsel whose sources are all phi nodes from the loop header block
*
* bcsel instructions in a loop that meet the following criteria can be
* converted to phi nodes:
*
* - The loop has no continue instructions other than the "natural" continue
* at the bottom of the loop.
*
* - All of the sources of the bcsel are phi nodes in the header block of the
* loop.
*
* - The phi node representing the condition of the bcsel instruction chooses
* only constant values.
*
* The contant value from the condition will select one of the other sources
* when entered from outside the loop and the remaining source when entered
* from the continue block. Since each of these sources is also a phi node in
* the header block, the value of the phi node can be "evaluated." These
* evaluated phi nodes provide the sources for a new phi node. All users of
* the bcsel result are updated to use the phi node result.
*
* The replacement transforms loops like:
*
* vec1 32 ssa_7 = undefined
* vec1 32 ssa_8 = load_const (0x00000001)
* vec1 32 ssa_9 = load_const (0x000000c8)
* vec1 32 ssa_10 = load_const (0x00000000)
* // succs: block_1
* loop {
* block block_1:
* // preds: block_0 block_4
* vec1 32 ssa_11 = phi block_0: ssa_1, block_4: ssa_14
* vec1 32 ssa_12 = phi block_0: ssa_10, block_4: ssa_15
* vec1 32 ssa_13 = phi block_0: ssa_7, block_4: ssa_25
* vec1 32 ssa_14 = b32csel ssa_12, ssa_13, ssa_11
* vec1 32 ssa_16 = ige32 ssa_14, ssa_9
* ...
* vec1 32 ssa_15 = load_const (0xffffffff)
* ...
* vec1 32 ssa_25 = iadd ssa_14, ssa_8
* // succs: block_1
* }
*
* into:
*
* vec1 32 ssa_7 = undefined
* vec1 32 ssa_8 = load_const (0x00000001)
* vec1 32 ssa_9 = load_const (0x000000c8)
* vec1 32 ssa_10 = load_const (0x00000000)
* // succs: block_1
* loop {
* block block_1:
* // preds: block_0 block_4
* vec1 32 ssa_11 = phi block_0: ssa_1, block_4: ssa_14
* vec1 32 ssa_12 = phi block_0: ssa_10, block_4: ssa_15
* vec1 32 ssa_13 = phi block_0: ssa_7, block_4: ssa_25
* vec1 32 sss_26 = phi block_0: ssa_1, block_4: ssa_25
* vec1 32 ssa_16 = ige32 ssa_26, ssa_9
* ...
* vec1 32 ssa_15 = load_const (0xffffffff)
* ...
* vec1 32 ssa_25 = iadd ssa_26, ssa_8
* // succs: block_1
* }
*
* \note
* It may be possible modify this function to not require a phi node as the
* source of the bcsel that is selected when entering from outside the loop.
* The only restriction is that the source must be geneated outside the loop
* (since it will become the source of a phi node in the header block of the
* loop).
*/
static bool
opt_simplify_bcsel_of_phi(nir_builder *b, nir_loop *loop)
{
bool progress = false;
nir_block *header_block = nir_loop_first_block(loop);
nir_block *const prev_block =
nir_cf_node_as_block(nir_cf_node_prev(&loop->cf_node));
/* It would be insane if this were not true */
assert(_mesa_set_search(header_block->predecessors, prev_block));
/* The loop must have exactly one continue block which could be a block
* ending in a continue instruction or the "natural" continue from the
* last block in the loop back to the top.
*/
if (header_block->predecessors->entries != 2)
return false;
/* We can move any bcsel that can guaranteed to execut on every iteration
* of a loop. For now this is accomplished by only taking bcsels from the
* header_block. In the future, this could be expanced to include any
* bcsel that must come before any break.
*
* For more details, see
* https://gitlab.freedesktop.org/mesa/mesa/merge_requests/170#note_110305
*/
nir_foreach_instr_safe(instr, header_block) {
if (instr->type != nir_instr_type_alu)
continue;
nir_alu_instr *const bcsel = nir_instr_as_alu(instr);
if (bcsel->op != nir_op_bcsel &&
bcsel->op != nir_op_b32csel &&
bcsel->op != nir_op_fcsel)
continue;
bool match = true;
for (unsigned i = 0; i < 3; i++) {
/* FINISHME: The abs and negate cases could be handled by adding
* move instructions at the bottom of the continue block and more
* phi nodes in the header_block.
*/
if (!bcsel->src[i].src.is_ssa ||
bcsel->src[i].src.ssa->parent_instr->type != nir_instr_type_phi ||
bcsel->src[i].src.ssa->parent_instr->block != header_block ||
bcsel->src[i].negate || bcsel->src[i].abs) {
match = false;
break;
}
}
if (!match)
continue;
nir_phi_instr *const cond_phi =
nir_instr_as_phi(bcsel->src[0].src.ssa->parent_instr);
uint32_t entry_val = 0, continue_val = 0;
if (!phi_has_constant_from_outside_and_one_from_inside_loop(cond_phi,
prev_block,
&entry_val,
&continue_val))
continue;
/* If they both execute or both don't execute, this is a job for
* nir_dead_cf, not this pass.
*/
if ((entry_val && continue_val) || (!entry_val && !continue_val))
continue;
const unsigned entry_src = entry_val ? 1 : 2;
const unsigned continue_src = entry_val ? 2 : 1;
/* Create a new phi node that selects the value for prev_block from
* the bcsel source that is selected by entry_val and the value for
* continue_block from the other bcsel source. Both sources have
* already been verified to be phi nodes.
*/
nir_block *const continue_block = find_continue_block(loop);
nir_phi_instr *const phi = nir_phi_instr_create(b->shader);
nir_phi_src *phi_src;
phi_src = ralloc(phi, nir_phi_src);
phi_src->pred = prev_block;
phi_src->src =
nir_src_for_ssa(ssa_for_phi_from_block(nir_instr_as_phi(bcsel->src[entry_src].src.ssa->parent_instr),
prev_block));
exec_list_push_tail(&phi->srcs, &phi_src->node);
phi_src = ralloc(phi, nir_phi_src);
phi_src->pred = continue_block;
phi_src->src =
nir_src_for_ssa(ssa_for_phi_from_block(nir_instr_as_phi(bcsel->src[continue_src].src.ssa->parent_instr),
continue_block));
exec_list_push_tail(&phi->srcs, &phi_src->node);
nir_ssa_dest_init(&phi->instr,
&phi->dest,
nir_dest_num_components(bcsel->dest.dest),
nir_dest_bit_size(bcsel->dest.dest),
NULL);
b->cursor = nir_after_phis(header_block);
nir_builder_instr_insert(b, &phi->instr);
/* Modify all readers of the bcsel instruction to read the result of
* the phi.
*/
nir_foreach_use_safe(use_src, &bcsel->dest.dest.ssa) {
nir_instr_rewrite_src(use_src->parent_instr,
use_src,
nir_src_for_ssa(&phi->dest.ssa));
}
nir_foreach_if_use_safe(use_src, &bcsel->dest.dest.ssa) {
nir_if_rewrite_condition(use_src->parent_if,
nir_src_for_ssa(&phi->dest.ssa));
}
/* Since the original bcsel instruction no longer has any readers,
* just remove it.
*/
nir_instr_remove_v(&bcsel->instr);
ralloc_free(bcsel);
progress = true;
}
return progress;
}
static bool
is_block_empty(nir_block *block)
{
@@ -1109,6 +1328,7 @@ opt_if_cf_list(nir_builder *b, struct exec_list *cf_list)
case nir_cf_node_loop: {
nir_loop *loop = nir_cf_node_as_loop(cf_node);
progress |= opt_if_cf_list(b, &loop->body);
progress |= opt_simplify_bcsel_of_phi(b, loop);
progress |= opt_peel_loop_initial_if(loop);
progress |= opt_if_loop_last_continue(loop);
break;