Mesa (staging/19.0): nir: Convert a bcsel with only phi node sources to a phi node
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Mon Feb 11 18:02:07 UTC 2019
Module: Mesa
Branch: staging/19.0
Commit: fcdf80f3a15df255fcd398cf0a93b1cb437aa268
URL: http://cgit.freedesktop.org/mesa/mesa/commit/?id=fcdf80f3a15df255fcd398cf0a93b1cb437aa268
Author: Ian Romanick <ian.d.romanick at intel.com>
Date: Thu Jan 17 17:53:40 2019 -0800
nir: Convert a bcsel with only phi node sources to a phi node
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: 8fb8ebfbb05 ("intel/compiler: More peephole select")
Reviewed-by: Timothy Arceri <tarceri at itsqueeze.com>
(cherry picked from commit b031c643491a92a5574c7a4bd659df33f2d89bb6)
---
src/compiler/nir/nir_opt_if.c | 220 ++++++++++++++++++++++++++++++++++++++++++
1 file changed, 220 insertions(+)
diff --git a/src/compiler/nir/nir_opt_if.c b/src/compiler/nir/nir_opt_if.c
index be0a2d55d7..9afb901be1 100644
--- a/src/compiler/nir/nir_opt_if.c
+++ b/src/compiler/nir/nir_opt_if.c
@@ -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;
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