Mesa (master): nir: simplify phi handling in divergence analysis
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Wed May 13 19:14:52 UTC 2020
Module: Mesa
Branch: master
Commit: 1b881f3d8e4c367049aeb376602525559ef09ed5
URL: http://cgit.freedesktop.org/mesa/mesa/commit/?id=1b881f3d8e4c367049aeb376602525559ef09ed5
Author: Daniel Schürmann <daniel at schuermann.dev>
Date: Wed Feb 5 18:36:34 2020 +0100
nir: simplify phi handling in divergence analysis
This patch adds some control flow information to the
state to keep track whether a loop contains divergent
continue or break statements to not having to
recalculate this property for every phi.
Reviewed-by: Jason Ekstrand <jason at jlekstrand.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4062>
---
src/compiler/nir/nir_divergence_analysis.c | 229 +++++++++++++++--------------
1 file changed, 116 insertions(+), 113 deletions(-)
diff --git a/src/compiler/nir/nir_divergence_analysis.c b/src/compiler/nir/nir_divergence_analysis.c
index 1e3ead1e114..b2f34b0dfb6 100644
--- a/src/compiler/nir/nir_divergence_analysis.c
+++ b/src/compiler/nir/nir_divergence_analysis.c
@@ -39,6 +39,18 @@
struct divergence_state {
const nir_divergence_options options;
const gl_shader_stage stage;
+
+ /** current control flow state */
+ /* True if some loop-active invocations might take a different control-flow path.
+ * A divergent break does not cause subsequent control-flow to be considered
+ * divergent because those invocations are no longer active in the loop.
+ * For a divergent if, both sides are considered divergent flow because
+ * the other side is still loop-active. */
+ bool divergent_loop_cf;
+ /* True if a divergent continue happened since the loop header */
+ bool divergent_loop_continue;
+ /* True if a divergent break happened since the loop header */
+ bool divergent_loop_break;
};
static bool
@@ -545,6 +557,28 @@ visit_deref(nir_deref_instr *deref, struct divergence_state *state)
return is_divergent;
}
+static bool
+visit_jump(nir_jump_instr *jump, struct divergence_state *state)
+{
+ switch (jump->type) {
+ case nir_jump_continue:
+ if (state->divergent_loop_continue)
+ return false;
+ if (state->divergent_loop_cf)
+ state->divergent_loop_continue = true;
+ return state->divergent_loop_continue;
+ case nir_jump_break:
+ if (state->divergent_loop_break)
+ return false;
+ if (state->divergent_loop_cf)
+ state->divergent_loop_break = true;
+ return state->divergent_loop_break;
+ case nir_jump_return:
+ unreachable("NIR divergence analysis: Unsupported return instruction.");
+ }
+ return false;
+}
+
static bool
visit_block(nir_block *block, struct divergence_state *state)
{
@@ -570,11 +604,12 @@ visit_block(nir_block *block, struct divergence_state *state)
case nir_instr_type_deref:
has_changed |= visit_deref(nir_instr_as_deref(instr), state);
break;
+ case nir_instr_type_jump:
+ has_changed |= visit_jump(nir_instr_as_jump(instr), state);
+ break;
/* phis are handled when processing the branches */
case nir_instr_type_phi:
break;
- case nir_instr_type_jump:
- break;
case nir_instr_type_call:
case nir_instr_type_parallel_copy:
unreachable("NIR divergence analysis: Unsupported instruction type.");
@@ -612,6 +647,7 @@ visit_if_merge_phi(nir_phi_instr *phi, bool if_cond_divergent)
phi->dest.ssa.divergent = true;
return true;
}
+
return false;
}
@@ -622,64 +658,35 @@ visit_if_merge_phi(nir_phi_instr *phi, bool if_cond_divergent)
* is divergent or a divergent loop continue condition
* is associated with a different ssa-def. */
static bool
-visit_loop_header_phi(nir_phi_instr *phi, nir_loop *loop)
+visit_loop_header_phi(nir_phi_instr *phi, nir_block *preheader, bool divergent_continue)
{
if (phi->dest.ssa.divergent)
return false;
- nir_cf_node *prev = nir_cf_node_prev(&loop->cf_node);
nir_ssa_def* same = NULL;
- bool all_same = true;
-
- /* first, check if all loop-carried values are from the same ssa-def */
nir_foreach_phi_src(src, phi) {
/* if any source value is divergent, the resulting value is divergent */
if (src->src.ssa->divergent) {
phi->dest.ssa.divergent = true;
return true;
}
- /* skip the loop preheader */
- if (src->pred == nir_cf_node_as_block(prev))
+ /* if this loop is uniform, we're done here */
+ if (!divergent_continue)
continue;
- if (src->src.ssa->parent_instr->type == nir_instr_type_ssa_undef)
- continue;
- if (!same)
- same = src->src.ssa;
- else if (same != src->src.ssa)
- all_same = false;
- }
-
- /* if all loop-carried values are the same, the resulting value is uniform */
- if (all_same)
- return false;
-
- /* check if the loop-carried values come from different ssa-defs
- * and the corresponding condition is divergent. */
- nir_foreach_phi_src(src, phi) {
/* skip the loop preheader */
- if (src->pred == nir_cf_node_as_block(prev))
- continue;
-
- /* skip the unconditional back-edge */
- if (src->pred == nir_loop_last_block(loop))
+ if (src->pred == preheader)
continue;
-
- /* if the value is undef, we don't need to check the condition */
+ /* skip undef values */
if (src->src.ssa->parent_instr->type == nir_instr_type_ssa_undef)
continue;
- nir_cf_node *current = src->pred->cf_node.parent;
- /* check recursively the conditions if any is divergent */
- while (current->type != nir_cf_node_loop) {
- assert (current->type == nir_cf_node_if);
- nir_if *if_node = nir_cf_node_as_if(current);
- if (if_node->condition.ssa->divergent) {
- phi->dest.ssa.divergent = true;
- return true;
- }
- current = current->parent;
+ /* check if all loop-carried values are from the same ssa-def */
+ if (!same)
+ same = src->src.ssa;
+ else if (same != src->src.ssa) {
+ phi->dest.ssa.divergent = true;
+ return true;
}
- assert(current == &loop->cf_node);
}
return false;
@@ -692,82 +699,60 @@ visit_loop_header_phi(nir_phi_instr *phi, nir_loop *loop)
* not loop-invariant.
* (note: there should be no phi for loop-invariant variables.) */
static bool
-visit_loop_exit_phi(nir_phi_instr *phi, nir_loop *loop)
+visit_loop_exit_phi(nir_phi_instr *phi, bool divergent_break)
{
if (phi->dest.ssa.divergent)
return false;
- /* Check if any loop exit condition is divergent:
- * That is any break happens under divergent condition or
- * a break is preceeded by a divergent continue
- */
+ if (divergent_break) {
+ phi->dest.ssa.divergent = true;
+ return true;
+ }
+
+ /* if any source value is divergent, the resulting value is divergent */
nir_foreach_phi_src(src, phi) {
- /* if any source value is divergent, the resulting value is divergent */
if (src->src.ssa->divergent) {
phi->dest.ssa.divergent = true;
return true;
}
-
- nir_cf_node *current = src->pred->cf_node.parent;
-
- /* check recursively the conditions if any is divergent */
- while (current->type != nir_cf_node_loop) {
- assert(current->type == nir_cf_node_if);
- nir_if *if_node = nir_cf_node_as_if(current);
- if (if_node->condition.ssa->divergent) {
- phi->dest.ssa.divergent = true;
- return true;
- }
- current = current->parent;
- }
-
- /* check if any divergent continue happened before the break */
- nir_foreach_block_in_cf_node(block, &loop->cf_node) {
- if (block == src->pred)
- break;
- if (!nir_block_ends_in_jump(block))
- continue;
-
- nir_jump_instr *jump = nir_instr_as_jump(nir_block_last_instr(block));
- if (jump->type != nir_jump_continue)
- continue;
-
- current = block->cf_node.parent;
- bool is_divergent = false;
- while (current != &loop->cf_node) {
- /* the continue belongs to an inner loop */
- if (current->type == nir_cf_node_loop) {
- is_divergent = false;
- break;
- }
- assert(current->type == nir_cf_node_if);
- nir_if *if_node = nir_cf_node_as_if(current);
- is_divergent |= if_node->condition.ssa->divergent;
- current = current->parent;
- }
-
- if (is_divergent) {
- phi->dest.ssa.divergent = true;
- return true;
- }
- }
}
+
return false;
}
static bool
visit_if(nir_if *if_stmt, struct divergence_state *state)
{
- bool progress = visit_cf_list(&if_stmt->then_list, state) |
- visit_cf_list(&if_stmt->else_list, state);
+ bool progress = false;
+
+ struct divergence_state then_state = *state;
+ then_state.divergent_loop_cf |= if_stmt->condition.ssa->divergent;
+ progress |= visit_cf_list(&if_stmt->then_list, &then_state);
+
+ struct divergence_state else_state = *state;
+ else_state.divergent_loop_cf |= if_stmt->condition.ssa->divergent;
+ progress |= visit_cf_list(&if_stmt->else_list, &else_state);
/* handle phis after the IF */
nir_foreach_instr(instr, nir_cf_node_cf_tree_next(&if_stmt->cf_node)) {
if (instr->type != nir_instr_type_phi)
break;
- progress |= visit_if_merge_phi(nir_instr_as_phi(instr), if_stmt->condition.ssa->divergent);
+
+ progress |= visit_if_merge_phi(nir_instr_as_phi(instr),
+ if_stmt->condition.ssa->divergent);
}
+ /* join loop divergence information from both branch legs */
+ state->divergent_loop_continue |= then_state.divergent_loop_continue ||
+ else_state.divergent_loop_continue;
+ state->divergent_loop_break |= then_state.divergent_loop_break ||
+ else_state.divergent_loop_break;
+
+ /* A divergent continue makes succeeding loop CF divergent:
+ * not all loop-active invocations participate in the remaining loop-body
+ * which means that a following break might be taken by some invocations, only */
+ state->divergent_loop_cf |= state->divergent_loop_continue;
+
return progress;
}
@@ -775,36 +760,51 @@ static bool
visit_loop(nir_loop *loop, struct divergence_state *state)
{
bool progress = false;
+ nir_block *loop_header = nir_loop_first_block(loop);
+ nir_block *loop_preheader = nir_block_cf_tree_prev(loop_header);
/* handle loop header phis first */
- nir_foreach_instr(instr, nir_loop_first_block(loop)) {
+ nir_foreach_instr(instr, loop_header) {
if (instr->type != nir_instr_type_phi)
break;
- progress |= visit_loop_header_phi(nir_instr_as_phi(instr), loop);
+
+ progress |= visit_loop_header_phi(nir_instr_as_phi(instr),
+ loop_preheader, false);
+
}
- bool repeat = true;
- while (repeat) {
- /* process loop body */
- repeat = visit_cf_list(&loop->body, state);
-
- if (repeat) {
- repeat = false;
- /* revisit loop header phis to see if something has changed */
- nir_foreach_instr(instr, nir_loop_first_block(loop)) {
- if (instr->type != nir_instr_type_phi)
- break;
- repeat |= visit_loop_header_phi(nir_instr_as_phi(instr), loop);
- }
- progress = true;
+ /* setup loop state */
+ struct divergence_state loop_state = *state;
+ loop_state.divergent_loop_cf = false;
+ loop_state.divergent_loop_continue = false;
+ loop_state.divergent_loop_break = false;
+
+ /* process loop body until no further changes are made */
+ bool repeat;
+ do {
+ progress |= visit_cf_list(&loop->body, &loop_state);
+ repeat = false;
+
+ /* revisit loop header phis to see if something has changed */
+ nir_foreach_instr(instr, loop_header) {
+ if (instr->type != nir_instr_type_phi)
+ break;
+
+ repeat |= visit_loop_header_phi(nir_instr_as_phi(instr),
+ loop_preheader,
+ loop_state.divergent_loop_continue);
}
- }
+
+ loop_state.divergent_loop_cf = false;
+ } while (repeat);
/* handle phis after the loop */
nir_foreach_instr(instr, nir_cf_node_cf_tree_next(&loop->cf_node)) {
if (instr->type != nir_instr_type_phi)
break;
- progress |= visit_loop_exit_phi(nir_instr_as_phi(instr), loop);
+
+ progress |= visit_loop_exit_phi(nir_instr_as_phi(instr),
+ loop_state.divergent_loop_break);
}
return progress;
@@ -855,6 +855,9 @@ nir_divergence_analysis(nir_shader *shader, nir_divergence_options options)
struct divergence_state state = {
.options = options,
.stage = shader->info.stage,
+ .divergent_loop_cf = false,
+ .divergent_loop_continue = false,
+ .divergent_loop_break = false,
};
visit_cf_list(&impl->body, &state);
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