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mesa/src/intel/compiler/brw_cfg.cpp

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/*
* Copyright © 2012 Intel Corporation
*
* 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:
* Eric Anholt <eric@anholt.net>
*
*/
#include "brw_cfg.h"
#include "util/u_dynarray.h"
#include "brw_fs.h"
/** @file brw_cfg.cpp
*
* Walks the shader instructions generated and creates a set of basic
* blocks with successor/predecessor edges connecting them.
*/
using namespace brw;
static bblock_t *
pop_stack(exec_list *list)
{
bblock_link *link = (bblock_link *)list->get_tail();
bblock_t *block = link->block;
link->link.remove();
return block;
}
static exec_node *
link(void *mem_ctx, bblock_t *block, enum bblock_link_kind kind)
{
bblock_link *l = new(mem_ctx) bblock_link(block, kind);
return &l->link;
}
void
push_stack(exec_list *list, void *mem_ctx, bblock_t *block)
{
/* The kind of the link is immaterial, but we need to provide one since
* this is (ab)using the edge data structure in order to implement a stack.
*/
list->push_tail(link(mem_ctx, block, bblock_link_logical));
}
bblock_t::bblock_t(cfg_t *cfg) :
cfg(cfg), start_ip(0), end_ip(0), end_ip_delta(0), num(0)
{
instructions.make_empty();
parents.make_empty();
children.make_empty();
}
void
bblock_t::add_successor(void *mem_ctx, bblock_t *successor,
enum bblock_link_kind kind)
{
successor->parents.push_tail(::link(mem_ctx, this, kind));
children.push_tail(::link(mem_ctx, successor, kind));
}
bool
bblock_t::is_predecessor_of(const bblock_t *block,
enum bblock_link_kind kind) const
{
foreach_list_typed_safe (bblock_link, parent, link, &block->parents) {
if (parent->block == this && parent->kind <= kind) {
return true;
}
}
return false;
}
bool
bblock_t::is_successor_of(const bblock_t *block,
enum bblock_link_kind kind) const
{
foreach_list_typed_safe (bblock_link, child, link, &block->children) {
if (child->block == this && child->kind <= kind) {
return true;
}
}
return false;
}
static bool
ends_block(const fs_inst *inst)
{
enum opcode op = inst->opcode;
return op == BRW_OPCODE_IF ||
op == BRW_OPCODE_ELSE ||
op == BRW_OPCODE_CONTINUE ||
op == BRW_OPCODE_BREAK ||
op == BRW_OPCODE_DO ||
op == BRW_OPCODE_WHILE;
}
static bool
starts_block(const fs_inst *inst)
{
enum opcode op = inst->opcode;
return op == BRW_OPCODE_DO ||
op == BRW_OPCODE_ENDIF;
}
bool
bblock_t::can_combine_with(const bblock_t *that) const
{
if ((const bblock_t *)this->link.next != that)
return false;
if (ends_block(this->end()) ||
starts_block(that->start()))
return false;
return true;
}
void
bblock_t::combine_with(bblock_t *that)
{
assert(this->can_combine_with(that));
foreach_list_typed (bblock_link, link, link, &that->parents) {
assert(link->block == this);
}
this->end_ip = that->end_ip;
this->instructions.append_list(&that->instructions);
this->cfg->remove_block(that);
}
void
bblock_t::dump(FILE *file) const
{
const fs_visitor *s = this->cfg->s;
int ip = this->start_ip;
foreach_inst_in_block(fs_inst, inst, this) {
fprintf(file, "%5d: ", ip);
s->dump_instruction(inst, file);
ip++;
}
}
void
bblock_t::unlink_list(exec_list *list)
{
assert(list == &parents || list == &children);
const bool remove_parent = list == &children;
foreach_list_typed_safe(bblock_link, link, link, list) {
/* Also break the links from the other block back to this block. */
exec_list *sub_list = remove_parent ? &link->block->parents : &link->block->children;
foreach_list_typed_safe(bblock_link, sub_link, link, sub_list) {
if (sub_link->block == this) {
sub_link->link.remove();
ralloc_free(sub_link);
}
}
link->link.remove();
ralloc_free(link);
}
}
cfg_t::cfg_t(const fs_visitor *s, exec_list *instructions) :
s(s)
{
mem_ctx = ralloc_context(NULL);
block_list.make_empty();
blocks = NULL;
num_blocks = 0;
bblock_t *cur = NULL;
int ip = 0;
bblock_t *entry = new_block();
bblock_t *cur_if = NULL; /**< BB ending with IF. */
bblock_t *cur_else = NULL; /**< BB ending with ELSE. */
bblock_t *cur_do = NULL; /**< BB starting with DO. */
bblock_t *cur_while = NULL; /**< BB immediately following WHILE. */
exec_list if_stack, else_stack, do_stack, while_stack;
bblock_t *next;
set_next_block(&cur, entry, ip);
foreach_in_list_safe(fs_inst, inst, instructions) {
/* set_next_block wants the post-incremented ip */
ip++;
inst->exec_node::remove();
switch (inst->opcode) {
case BRW_OPCODE_IF:
cur->instructions.push_tail(inst);
/* Push our information onto a stack so we can recover from
* nested ifs.
*/
push_stack(&if_stack, mem_ctx, cur_if);
push_stack(&else_stack, mem_ctx, cur_else);
cur_if = cur;
cur_else = NULL;
/* Set up our immediately following block, full of "then"
* instructions.
*/
next = new_block();
cur_if->add_successor(mem_ctx, next, bblock_link_logical);
set_next_block(&cur, next, ip);
break;
case BRW_OPCODE_ELSE:
cur->instructions.push_tail(inst);
cur_else = cur;
next = new_block();
assert(cur_if != NULL);
cur_if->add_successor(mem_ctx, next, bblock_link_logical);
cur_else->add_successor(mem_ctx, next, bblock_link_physical);
set_next_block(&cur, next, ip);
break;
case BRW_OPCODE_ENDIF: {
bblock_t *cur_endif;
if (cur->instructions.is_empty()) {
/* New block was just created; use it. */
cur_endif = cur;
} else {
cur_endif = new_block();
cur->add_successor(mem_ctx, cur_endif, bblock_link_logical);
set_next_block(&cur, cur_endif, ip - 1);
}
cur->instructions.push_tail(inst);
if (cur_else) {
cur_else->add_successor(mem_ctx, cur_endif, bblock_link_logical);
} else {
assert(cur_if != NULL);
cur_if->add_successor(mem_ctx, cur_endif, bblock_link_logical);
}
assert(cur_if->end()->opcode == BRW_OPCODE_IF);
assert(!cur_else || cur_else->end()->opcode == BRW_OPCODE_ELSE);
/* Pop the stack so we're in the previous if/else/endif */
cur_if = pop_stack(&if_stack);
cur_else = pop_stack(&else_stack);
break;
}
case BRW_OPCODE_DO:
/* Push our information onto a stack so we can recover from
* nested loops.
*/
push_stack(&do_stack, mem_ctx, cur_do);
push_stack(&while_stack, mem_ctx, cur_while);
/* Set up the block just after the while. Don't know when exactly
* it will start, yet.
*/
cur_while = new_block();
if (cur->instructions.is_empty()) {
/* New block was just created; use it. */
cur_do = cur;
} else {
cur_do = new_block();
cur->add_successor(mem_ctx, cur_do, bblock_link_logical);
set_next_block(&cur, cur_do, ip - 1);
}
cur->instructions.push_tail(inst);
/* Represent divergent execution of the loop as a pair of alternative
* edges coming out of the DO instruction: For any physical iteration
* of the loop a given logical thread can either start off enabled
* (which is represented as the "next" successor), or disabled (if it
* has reached a non-uniform exit of the loop during a previous
* iteration, which is represented as the "cur_while" successor).
*
* The disabled edge will be taken by the logical thread anytime we
* arrive at the DO instruction through a back-edge coming from a
* conditional exit of the loop where divergent control flow started.
*
* This guarantees that there is a control-flow path from any
* divergence point of the loop into the convergence point
* (immediately past the WHILE instruction) such that it overlaps the
* whole IP region of divergent control flow (potentially the whole
* loop) *and* doesn't imply the execution of any instructions part
* of the loop (since the corresponding execution mask bit will be
* disabled for a diverging thread).
*
* This way we make sure that any variables that are live throughout
* the region of divergence for an inactive logical thread are also
* considered to interfere with any other variables assigned by
* active logical threads within the same physical region of the
* program, since otherwise we would risk cross-channel data
* corruption.
*/
next = new_block();
cur->add_successor(mem_ctx, next, bblock_link_logical);
cur->add_successor(mem_ctx, cur_while, bblock_link_physical);
set_next_block(&cur, next, ip);
break;
case BRW_OPCODE_CONTINUE:
cur->instructions.push_tail(inst);
/* A conditional CONTINUE may start a region of divergent control
* flow until the start of the next loop iteration (*not* until the
* end of the loop which is why the successor is not the top-level
* divergence point at cur_do). The live interval of any variable
* extending through a CONTINUE edge is guaranteed to overlap the
* whole region of divergent execution, because any variable live-out
* at the CONTINUE instruction will also be live-in at the top of the
* loop, and therefore also live-out at the bottom-most point of the
* loop which is reachable from the top (since a control flow path
* exists from a definition of the variable through this CONTINUE
* instruction, the top of the loop, the (reachable) bottom of the
* loop, the top of the loop again, into a use of the variable).
*/
assert(cur_do != NULL);
cur->add_successor(mem_ctx, cur_do->next(), bblock_link_logical);
next = new_block();
if (inst->predicate)
cur->add_successor(mem_ctx, next, bblock_link_logical);
else
cur->add_successor(mem_ctx, next, bblock_link_physical);
set_next_block(&cur, next, ip);
break;
case BRW_OPCODE_BREAK:
cur->instructions.push_tail(inst);
/* A conditional BREAK instruction may start a region of divergent
* control flow until the end of the loop if the condition is
* non-uniform, in which case the loop will execute additional
* iterations with the present channel disabled. We model this as a
* control flow path from the divergence point to the convergence
* point that overlaps the whole IP range of the loop and skips over
* the execution of any other instructions part of the loop.
*
* See the DO case for additional explanation.
*/
assert(cur_do != NULL);
cur->add_successor(mem_ctx, cur_do, bblock_link_physical);
cur->add_successor(mem_ctx, cur_while, bblock_link_logical);
next = new_block();
if (inst->predicate)
cur->add_successor(mem_ctx, next, bblock_link_logical);
else
cur->add_successor(mem_ctx, next, bblock_link_physical);
set_next_block(&cur, next, ip);
break;
case BRW_OPCODE_WHILE:
cur->instructions.push_tail(inst);
assert(cur_do != NULL && cur_while != NULL);
/* A conditional WHILE instruction may start a region of divergent
* control flow until the end of the loop, just like the BREAK
* instruction. See the BREAK case for more details. OTOH an
* unconditional WHILE instruction is non-divergent (just like an
* unconditional CONTINUE), and will necessarily lead to the
* execution of an additional iteration of the loop for all enabled
* channels, so we may skip over the divergence point at the top of
* the loop to keep the CFG as unambiguous as possible.
*/
if (inst->predicate) {
cur->add_successor(mem_ctx, cur_do, bblock_link_logical);
} else {
cur->add_successor(mem_ctx, cur_do->next(), bblock_link_logical);
}
set_next_block(&cur, cur_while, ip);
/* Pop the stack so we're in the previous loop */
cur_do = pop_stack(&do_stack);
cur_while = pop_stack(&while_stack);
break;
default:
cur->instructions.push_tail(inst);
break;
}
}
cur->end_ip = ip - 1;
make_block_array();
}
cfg_t::~cfg_t()
{
ralloc_free(mem_ctx);
}
void
cfg_t::remove_block(bblock_t *block)
{
foreach_list_typed_safe (bblock_link, predecessor, link, &block->parents) {
/* cfg_t::validate checks that predecessor and successor lists are well
* formed, so it is known that the loop here would find exactly one
* block. Set old_link_kind to silence "variable used but not set"
* warnings.
*/
bblock_link_kind old_link_kind = bblock_link_logical;
/* Remove block from all of its predecessors' successor lists. */
foreach_list_typed_safe (bblock_link, successor, link,
&predecessor->block->children) {
if (block == successor->block) {
old_link_kind = successor->kind;
successor->link.remove();
ralloc_free(successor);
break;
}
}
/* Add removed-block's successors to its predecessors' successor lists. */
foreach_list_typed (bblock_link, successor, link, &block->children) {
bool need_to_link = true;
bblock_link_kind new_link_kind = MAX2(old_link_kind, successor->kind);
foreach_list_typed_safe (bblock_link, child, link, &predecessor->block->children) {
/* There is already a link between the two blocks. If the links
* are the same kind or the link is logical, do nothing. If the
* existing link is physical and the proposed new link is logical,
* promote the existing link to logical.
*
* This is accomplished by taking the minimum of the existing link
* kind and the proposed link kind.
*/
if (child->block == successor->block) {
child->kind = MIN2(child->kind, new_link_kind);
need_to_link = false;
break;
}
}
if (need_to_link) {
predecessor->block->children.push_tail(link(mem_ctx,
successor->block,
new_link_kind));
}
}
}
foreach_list_typed_safe (bblock_link, successor, link, &block->children) {
/* cfg_t::validate checks that predecessor and successor lists are well
* formed, so it is known that the loop here would find exactly one
* block. Set old_link_kind to silence "variable used but not set"
* warnings.
*/
bblock_link_kind old_link_kind = bblock_link_logical;
/* Remove block from all of its childrens' parents lists. */
foreach_list_typed_safe (bblock_link, predecessor, link,
&successor->block->parents) {
if (block == predecessor->block) {
old_link_kind = predecessor->kind;
predecessor->link.remove();
ralloc_free(predecessor);
}
}
/* Add removed-block's predecessors to its successors' predecessor lists. */
foreach_list_typed (bblock_link, predecessor, link, &block->parents) {
bool need_to_link = true;
bblock_link_kind new_link_kind = MAX2(old_link_kind, predecessor->kind);
foreach_list_typed_safe (bblock_link, parent, link, &successor->block->parents) {
/* There is already a link between the two blocks. If the links
* are the same kind or the link is logical, do nothing. If the
* existing link is physical and the proposed new link is logical,
* promote the existing link to logical.
*
* This is accomplished by taking the minimum of the existing link
* kind and the proposed link kind.
*/
if (parent->block == predecessor->block) {
parent->kind = MIN2(parent->kind, new_link_kind);
need_to_link = false;
break;
}
}
if (need_to_link) {
successor->block->parents.push_tail(link(mem_ctx,
predecessor->block,
new_link_kind));
}
}
}
block->link.remove();
for (int b = block->num; b < this->num_blocks - 1; b++) {
this->blocks[b] = this->blocks[b + 1];
this->blocks[b]->num = b;
}
this->blocks[this->num_blocks - 1]->num = this->num_blocks - 2;
this->num_blocks--;
}
bblock_t *
cfg_t::new_block()
{
bblock_t *block = new(mem_ctx) bblock_t(this);
return block;
}
void
cfg_t::set_next_block(bblock_t **cur, bblock_t *block, int ip)
{
if (*cur) {
(*cur)->end_ip = ip - 1;
}
block->start_ip = ip;
block->num = num_blocks++;
block_list.push_tail(&block->link);
*cur = block;
}
void
cfg_t::make_block_array()
{
blocks = ralloc_array(mem_ctx, bblock_t *, num_blocks);
int i = 0;
foreach_block (block, this) {
blocks[i++] = block;
}
assert(i == num_blocks);
}
namespace {
struct link_desc {
char kind;
int num;
};
int
compare_link_desc(const void *a, const void *b)
{
const link_desc *la = (const link_desc *)a;
const link_desc *lb = (const link_desc *)b;
return la->num < lb->num ? -1 :
la->num > lb->num ? +1 :
la->kind < lb->kind ? -1 :
la->kind > lb->kind ? +1 :
0;
}
void
sort_links(util_dynarray *scratch, exec_list *list)
{
util_dynarray_clear(scratch);
foreach_list_typed(bblock_link, link, link, list) {
link_desc l;
l.kind = link->kind == bblock_link_logical ? '-' : '~';
l.num = link->block->num;
util_dynarray_append(scratch, link_desc, l);
}
qsort(scratch->data, util_dynarray_num_elements(scratch, link_desc),
sizeof(link_desc), compare_link_desc);
}
} /* namespace */
void
cfg_t::dump(FILE *file)
{
const idom_tree *idom = (s ? &s->idom_analysis.require() : NULL);
/* Temporary storage to sort the lists of blocks. This normalizes the
* output, making it possible to use it for certain tests.
*/
util_dynarray scratch;
util_dynarray_init(&scratch, NULL);
foreach_block (block, this) {
if (idom && idom->parent(block))
fprintf(file, "START B%d IDOM(B%d)", block->num,
idom->parent(block)->num);
else
fprintf(file, "START B%d IDOM(none)", block->num);
sort_links(&scratch, &block->parents);
util_dynarray_foreach(&scratch, link_desc, l)
fprintf(file, " <%cB%d", l->kind, l->num);
fprintf(file, "\n");
if (s != NULL)
block->dump(file);
fprintf(file, "END B%d", block->num);
sort_links(&scratch, &block->children);
util_dynarray_foreach(&scratch, link_desc, l)
fprintf(file, " %c>B%d", l->kind, l->num);
fprintf(file, "\n");
}
util_dynarray_fini(&scratch);
}
/* Calculates the immediate dominator of each block, according to "A Simple,
* Fast Dominance Algorithm" by Keith D. Cooper, Timothy J. Harvey, and Ken
* Kennedy.
*
* The authors claim that for control flow graphs of sizes normally encountered
* (less than 1000 nodes) that this algorithm is significantly faster than
* others like Lengauer-Tarjan.
*/
idom_tree::idom_tree(const fs_visitor *s) :
num_parents(s->cfg->num_blocks),
parents(new bblock_t *[num_parents]())
{
bool changed;
parents[0] = s->cfg->blocks[0];
do {
changed = false;
foreach_block(block, s->cfg) {
if (block->num == 0)
continue;
bblock_t *new_idom = NULL;
foreach_list_typed(bblock_link, parent_link, link, &block->parents) {
if (parent(parent_link->block)) {
new_idom = (new_idom ? intersect(new_idom, parent_link->block) :
parent_link->block);
}
}
if (parent(block) != new_idom) {
parents[block->num] = new_idom;
changed = true;
}
}
} while (changed);
}
idom_tree::~idom_tree()
{
delete[] parents;
}
bblock_t *
idom_tree::intersect(bblock_t *b1, bblock_t *b2) const
{
/* Note, the comparisons here are the opposite of what the paper says
* because we index blocks from beginning -> end (i.e. reverse post-order)
* instead of post-order like they assume.
*/
while (b1->num != b2->num) {
while (b1->num > b2->num)
b1 = parent(b1);
while (b2->num > b1->num)
b2 = parent(b2);
}
assert(b1);
return b1;
}
void
idom_tree::dump() const
{
printf("digraph DominanceTree {\n");
for (unsigned i = 0; i < num_parents; i++)
printf("\t%d -> %d\n", parents[i]->num, i);
printf("}\n");
}
void
cfg_t::dump_cfg()
{
printf("digraph CFG {\n");
for (int b = 0; b < num_blocks; b++) {
bblock_t *block = this->blocks[b];
foreach_list_typed_safe (bblock_link, child, link, &block->children) {
printf("\t%d -> %d\n", b, child->block->num);
}
}
printf("}\n");
}
void
fs_visitor::calculate_cfg()
{
if (this->cfg)
return;
cfg = new(mem_ctx) cfg_t(this, &this->instructions);
}
#define cfgv_assert(assertion) \
{ \
if (!(assertion)) { \
fprintf(stderr, "ASSERT: CFG validation in %s failed!\n", stage_abbrev); \
fprintf(stderr, "%s:%d: '%s' failed\n", __FILE__, __LINE__, #assertion); \
abort(); \
} \
}
#ifndef NDEBUG
void
cfg_t::validate(const char *stage_abbrev)
{
foreach_block(block, this) {
foreach_list_typed(bblock_link, successor, link, &block->children) {
/* Each successor of a block must have one predecessor link back to
* the block.
*/
bool successor_links_back_to_predecessor = false;
bblock_t *succ_block = successor->block;
foreach_list_typed(bblock_link, predecessor, link, &succ_block->parents) {
if (predecessor->block == block) {
cfgv_assert(!successor_links_back_to_predecessor);
cfgv_assert(successor->kind == predecessor->kind);
successor_links_back_to_predecessor = true;
}
}
cfgv_assert(successor_links_back_to_predecessor);
/* Each successor block must appear only once in the list of
* successors.
*/
foreach_list_typed_from(bblock_link, later_successor, link,
&block->children, successor->link.next) {
cfgv_assert(successor->block != later_successor->block);
}
}
foreach_list_typed(bblock_link, predecessor, link, &block->parents) {
/* Each predecessor of a block must have one successor link back to
* the block.
*/
bool predecessor_links_back_to_successor = false;
bblock_t *pred_block = predecessor->block;
foreach_list_typed(bblock_link, successor, link, &pred_block->children) {
if (successor->block == block) {
cfgv_assert(!predecessor_links_back_to_successor);
cfgv_assert(successor->kind == predecessor->kind);
predecessor_links_back_to_successor = true;
}
}
cfgv_assert(predecessor_links_back_to_successor);
/* Each precessor block must appear only once in the list of
* precessors.
*/
foreach_list_typed_from(bblock_link, later_precessor, link,
&block->parents, predecessor->link.next) {
cfgv_assert(predecessor->block != later_precessor->block);
}
}
fs_inst *first_inst = block->start();
if (first_inst->opcode == BRW_OPCODE_DO) {
/* DO instructions both begin and end a block, so the DO instruction
* must be the only instruction in the block.
*/
cfgv_assert(exec_list_is_singular(&block->instructions));
/* A block starting with DO should have exactly two successors. One
* is a physical link to the block starting after the WHILE
* instruction. The other is a logical link to the block starting the
* body of the loop.
*/
bblock_t *physical_block = nullptr;
bblock_t *logical_block = nullptr;
foreach_list_typed(bblock_link, child, link, &block->children) {
if (child->kind == bblock_link_physical) {
cfgv_assert(physical_block == nullptr);
physical_block = child->block;
} else {
cfgv_assert(logical_block == nullptr);
logical_block = child->block;
}
}
cfgv_assert(logical_block != nullptr);
cfgv_assert(physical_block != nullptr);
}
}
}
#endif
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