/*
* Copyright © 2010 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>
*
*/
/** @file register_allocate.c
*
* Graph-coloring register allocator.
*
* The basic idea of graph coloring is to make a node in a graph for
* every thing that needs a register (color) number assigned, and make
* edges in the graph between nodes that interfere (can't be allocated
* to the same register at the same time).
*
* During the "simplify" process, any any node with fewer edges than
* there are registers means that that edge can get assigned a
* register regardless of what its neighbors choose, so that node is
* pushed on a stack and removed (with its edges) from the graph.
* That likely causes other nodes to become trivially colorable as well.
*
* Then during the "select" process, nodes are popped off of that
* stack, their edges restored, and assigned a color different from
* their neighbors. Because they were pushed on the stack only when
* they were trivially colorable, any color chosen won't interfere
* with the registers to be popped later.
*
* The downside to most graph coloring is that real hardware often has
* limitations, like registers that need to be allocated to a node in
* pairs, or aligned on some boundary. This implementation follows
* the paper "Retargetable Graph-Coloring Register Allocation for
* Irregular Architectures" by Johan Runeson and Sven-Olof Nyström.
*
* In this system, there are register classes each containing various
* registers, and registers may interfere with other registers. For
* example, one might have a class of base registers, and a class of
* aligned register pairs that would each interfere with their pair of
* the base registers. Each node has a register class it needs to be
* assigned to. Define p(B) to be the size of register class B, and
* q(B,C) to be the number of registers in B that the worst choice
* register in C could conflict with. Then, this system replaces the
* basic graph coloring test of "fewer edges from this node than there
* are registers" with "For this node of class B, the sum of q(B,C)
* for each neighbor node of class C is less than pB".
*
* A nice feature of the pq test is that q(B,C) can be computed once
* up front and stored in a 2-dimensional array, so that the cost of
* coloring a node is constant with the number of registers. We do
* this during ra_set_finalize().
*/
#include <stdbool.h>
#include "ralloc.h"
#include "main/imports.h"
#include "main/macros.h"
#include "main/mtypes.h"
#include "util/bitset.h"
#include "register_allocate.h"
#define NO_REG ~0U
struct ra_reg {
BITSET_WORD *conflicts;
unsigned int *conflict_list;
unsigned int conflict_list_size;
unsigned int num_conflicts;
};
struct ra_regs {
struct ra_reg *regs;
unsigned int count;
struct ra_class **classes;
unsigned int class_count;
bool round_robin;
};
struct ra_class {
/**
* Bitset indicating which registers belong to this class.
*
* (If bit N is set, then register N belongs to this class.)
*/
BITSET_WORD *regs;
/**
* p(B) in Runeson/Nyström paper.
*
* This is "how many regs are in the set."
*/
unsigned int p;
/**
* q(B,C) (indexed by C, B is this register class) in
* Runeson/Nyström paper. This is "how many registers of B could
* the worst choice register from C conflict with".
*/
unsigned int *q;
};
struct ra_node {
/** @{
*
* List of which nodes this node interferes with. This should be
* symmetric with the other node.
*/
BITSET_WORD *adjacency;
unsigned int *adjacency_list;
unsigned int adjacency_list_size;
unsigned int adjacency_count;
/** @} */
unsigned int class;
/* Register, if assigned, or NO_REG. */
unsigned int reg;
/**
* Set when the node is in the trivially colorable stack. When
* set, the adjacency to this node is ignored, to implement the
* "remove the edge from the graph" in simplification without
* having to actually modify the adjacency_list.
*/
bool in_stack;
/**
* The q total, as defined in the Runeson/Nyström paper, for all the
* interfering nodes not in the stack.
*/
unsigned int q_total;
/* For an implementation that needs register spilling, this is the
* approximate cost of spilling this node.
*/
float spill_cost;
};
struct ra_graph {
struct ra_regs *regs;
/**
* the variables that need register allocation.
*/
struct ra_node *nodes;
unsigned int count; /**< count of nodes. */
unsigned int *stack;
unsigned int stack_count;
/**
* Tracks the start of the set of optimistically-colored registers in the
* stack.
*/
unsigned int stack_optimistic_start;
};
/**
* Creates a set of registers for the allocator.
*
* mem_ctx is a ralloc context for the allocator. The reg set may be freed
* using ralloc_free().
*/
struct ra_regs *
ra_alloc_reg_set(void *mem_ctx, unsigned int count)
{
unsigned int i;
struct ra_regs *regs;
regs = rzalloc(mem_ctx, struct ra_regs);
regs->count = count;
regs->regs = rzalloc_array(regs, struct ra_reg, count);
for (i = 0; i < count; i++) {
regs->regs[i].conflicts = rzalloc_array(regs->regs, BITSET_WORD,
BITSET_WORDS(count));
BITSET_SET(regs->regs[i].conflicts, i);
regs->regs[i].conflict_list = ralloc_array(regs->regs, unsigned int, 4);
regs->regs[i].conflict_list_size = 4;
regs->regs[i].conflict_list[0] = i;
regs->regs[i].num_conflicts = 1;
}
return regs;
}
/**
* The register allocator by default prefers to allocate low register numbers,
* since it was written for hardware (gen4/5 Intel) that is limited in its
* multithreadedness by the number of registers used in a given shader.
*
* However, for hardware without that restriction, densely packed register
* allocation can put serious constraints on instruction scheduling. This
* function tells the allocator to rotate around the registers if possible as
* it allocates the nodes.
*/
void
ra_set_allocate_round_robin(struct ra_regs *regs)
{
regs->round_robin = true;
}
static void
ra_add_conflict_list(struct ra_regs *regs, unsigned int r1, unsigned int r2)
{
struct ra_reg *reg1 = ®s->regs[r1];
if (reg1->conflict_list_size == reg1->num_conflicts) {
reg1->conflict_list_size *= 2;
reg1->conflict_list = reralloc(regs->regs, reg1->conflict_list,
unsigned int, reg1->conflict_list_size);
}
reg1->conflict_list[reg1->num_conflicts++] = r2;
BITSET_SET(reg1->conflicts, r2);
}
void
ra_add_reg_conflict(struct ra_regs *regs, unsigned int r1, unsigned int r2)
{
if (!BITSET_TEST(regs->regs[r1].conflicts, r2)) {
ra_add_conflict_list(regs, r1, r2);
ra_add_conflict_list(regs, r2, r1);
}
}
/**
* Adds a conflict between base_reg and reg, and also between reg and
* anything that base_reg conflicts with.
*
* This can simplify code for setting up multiple register classes
* which are aggregates of some base hardware registers, compared to
* explicitly using ra_add_reg_conflict.
*/
void
ra_add_transitive_reg_conflict(struct ra_regs *regs,
unsigned int base_reg, unsigned int reg)
{
unsigned int i;
ra_add_reg_conflict(regs, reg, base_reg);
for (i = 0; i < regs->regs[base_reg].num_conflicts; i++) {
ra_add_reg_conflict(regs, reg, regs->regs[base_reg].conflict_list[i]);
}
}
unsigned int
ra_alloc_reg_class(struct ra_regs *regs)
{
struct ra_class *class;
regs->classes = reralloc(regs->regs, regs->classes, struct ra_class *,
regs->class_count + 1);
class = rzalloc(regs, struct ra_class);
regs->classes[regs->class_count] = class;
class->regs = rzalloc_array(class, BITSET_WORD, BITSET_WORDS(regs->count));
return regs->class_count++;
}
void
ra_class_add_reg(struct ra_regs *regs, unsigned int c, unsigned int r)
{
struct ra_class *class = regs->classes[c];
BITSET_SET(class->regs, r);
class->p++;
}
/**
* Returns true if the register belongs to the given class.
*/
static bool
reg_belongs_to_class(unsigned int r, struct ra_class *c)
{
return BITSET_TEST(c->regs, r);
}
/**
* Must be called after all conflicts and register classes have been
* set up and before the register set is used for allocation.
* To avoid costly q value computation, use the q_values paramater
* to pass precomputed q values to this function.
*/
void
ra_set_finalize(struct ra_regs *regs, unsigned int **q_values)
{
unsigned int b, c;
for (b = 0; b < regs->class_count; b++) {
regs->classes[b]->q = ralloc_array(regs, unsigned int, regs->class_count);
}
if (q_values) {
for (b = 0; b < regs->class_count; b++) {
for (c = 0; c < regs->class_count; c++) {
regs->classes[b]->q[c] = q_values[b][c];
}
}
return;
}
/* Compute, for each class B and C, how many regs of B an
* allocation to C could conflict with.
*/
for (b = 0; b < regs->class_count; b++) {
for (c = 0; c < regs->class_count; c++) {
unsigned int rc;
int max_conflicts = 0;
for (rc = 0; rc < regs->count; rc++) {
int conflicts = 0;
unsigned int i;
if (!reg_belongs_to_class(rc, regs->classes[c]))
continue;
for (i = 0; i < regs->regs[rc].num_conflicts; i++) {
unsigned int rb = regs->regs[rc].conflict_list[i];
if (reg_belongs_to_class(rb, regs->classes[b]))
conflicts++;
}
max_conflicts = MAX2(max_conflicts, conflicts);
}
regs->classes[b]->q[c] = max_conflicts;
}
}
}
static void
ra_add_node_adjacency(struct ra_graph *g, unsigned int n1, unsigned int n2)
{
BITSET_SET(g->nodes[n1].adjacency, n2);
if (n1 != n2) {
int n1_class = g->nodes[n1].class;
int n2_class = g->nodes[n2].class;
g->nodes[n1].q_total += g->regs->classes[n1_class]->q[n2_class];
}
if (g->nodes[n1].adjacency_count >=
g->nodes[n1].adjacency_list_size) {
g->nodes[n1].adjacency_list_size *= 2;
g->nodes[n1].adjacency_list = reralloc(g, g->nodes[n1].adjacency_list,
unsigned int,
g->nodes[n1].adjacency_list_size);
}
g->nodes[n1].adjacency_list[g->nodes[n1].adjacency_count] = n2;
g->nodes[n1].adjacency_count++;
}
struct ra_graph *
ra_alloc_interference_graph(struct ra_regs *regs, unsigned int count)
{
struct ra_graph *g;
unsigned int i;
g = rzalloc(NULL, struct ra_graph);
g->regs = regs;
g->nodes = rzalloc_array(g, struct ra_node, count);
g->count = count;
g->stack = rzalloc_array(g, unsigned int, count);
for (i = 0; i < count; i++) {
int bitset_count = BITSET_WORDS(count);
g->nodes[i].adjacency = rzalloc_array(g, BITSET_WORD, bitset_count);
g->nodes[i].adjacency_list_size = 4;
g->nodes[i].adjacency_list =
ralloc_array(g, unsigned int, g->nodes[i].adjacency_list_size);
g->nodes[i].adjacency_count = 0;
g->nodes[i].q_total = 0;
ra_add_node_adjacency(g, i, i);
g->nodes[i].reg = NO_REG;
}
return g;
}
void
ra_set_node_class(struct ra_graph *g,
unsigned int n, unsigned int class)
{
g->nodes[n].class = class;
}
void
ra_add_node_interference(struct ra_graph *g,
unsigned int n1, unsigned int n2)
{
if (!BITSET_TEST(g->nodes[n1].adjacency, n2)) {
ra_add_node_adjacency(g, n1, n2);
ra_add_node_adjacency(g, n2, n1);
}
}
static bool
pq_test(struct ra_graph *g, unsigned int n)
{
int n_class = g->nodes[n].class;
return g->nodes[n].q_total < g->regs->classes[n_class]->p;
}
static void
decrement_q(struct ra_graph *g, unsigned int n)
{
unsigned int i;
int n_class = g->nodes[n].class;
for (i = 0; i < g->nodes[n].adjacency_count; i++) {
unsigned int n2 = g->nodes[n].adjacency_list[i];
unsigned int n2_class = g->nodes[n2].class;
if (n != n2 && !g->nodes[n2].in_stack) {
assert(g
->nodes
[n2
].
q_total >= g
->regs
->classes
[n2_class
]->q
[n_class
]); g->nodes[n2].q_total -= g->regs->classes[n2_class]->q[n_class];
}
}
}
/**
* Simplifies the interference graph by pushing all
* trivially-colorable nodes into a stack of nodes to be colored,
* removing them from the graph, and rinsing and repeating.
*
* If we encounter a case where we can't push any nodes on the stack, then
* we optimistically choose a node and push it on the stack. We heuristically
* push the node with the lowest total q value, since it has the fewest
* neighbors and therefore is most likely to be allocated.
*/
static void
ra_simplify(struct ra_graph *g)
{
bool progress = true;
unsigned int stack_optimistic_start = UINT_MAX;
int i;
while (progress) {
unsigned int best_optimistic_node = ~0;
unsigned int lowest_q_total = ~0;
progress = false;
for (i = g->count - 1; i >= 0; i--) {
if (g->nodes[i].in_stack || g->nodes[i].reg != NO_REG)
continue;
if (pq_test(g, i)) {
decrement_q(g, i);
g->stack[g->stack_count] = i;
g->stack_count++;
g->nodes[i].in_stack = true;
progress = true;
} else {
unsigned int new_q_total = g->nodes[i].q_total;
if (new_q_total < lowest_q_total) {
best_optimistic_node = i;
lowest_q_total = new_q_total;
}
}
}
if (!progress && best_optimistic_node != ~0U) {
if (stack_optimistic_start == UINT_MAX)
stack_optimistic_start = g->stack_count;
decrement_q(g, best_optimistic_node);
g->stack[g->stack_count] = best_optimistic_node;
g->stack_count++;
g->nodes[best_optimistic_node].in_stack = true;
progress = true;
}
}
g->stack_optimistic_start = stack_optimistic_start;
}
/**
* Pops nodes from the stack back into the graph, coloring them with
* registers as they go.
*
* If all nodes were trivially colorable, then this must succeed. If
* not (optimistic coloring), then it may return false;
*/
static bool
ra_select(struct ra_graph *g)
{
int start_search_reg = 0;
while (g->stack_count != 0) {
unsigned int i;
unsigned int ri;
unsigned int r = -1;
int n = g->stack[g->stack_count - 1];
struct ra_class *c = g->regs->classes[g->nodes[n].class];
/* Find the lowest-numbered reg which is not used by a member
* of the graph adjacent to us.
*/
for (ri = 0; ri < g->regs->count; ri++) {
r = (start_search_reg + ri) % g->regs->count;
if (!reg_belongs_to_class(r, c))
continue;
/* Check if any of our neighbors conflict with this register choice. */
for (i = 0; i < g->nodes[n].adjacency_count; i++) {
unsigned int n2 = g->nodes[n].adjacency_list[i];
if (!g->nodes[n2].in_stack &&
BITSET_TEST(g->regs->regs[r].conflicts, g->nodes[n2].reg)) {
break;
}
}
if (i == g->nodes[n].adjacency_count)
break;
}
/* set this to false even if we return here so that
* ra_get_best_spill_node() considers this node later.
*/
g->nodes[n].in_stack = false;
if (ri == g->regs->count)
return false;
g->nodes[n].reg = r;
g->stack_count--;
/* Rotate the starting point except for any nodes above the lowest
* optimistically colorable node. The likelihood that we will succeed
* at allocating optimistically colorable nodes is highly dependent on
* the way that the previous nodes popped off the stack are laid out.
* The round-robin strategy increases the fragmentation of the register
* file and decreases the number of nearby nodes assigned to the same
* color, what increases the likelihood of spilling with respect to the
* dense packing strategy.
*/
if (g->regs->round_robin &&
g->stack_count - 1 <= g->stack_optimistic_start)
start_search_reg = r + 1;
}
return true;
}
bool
ra_allocate(struct ra_graph *g)
{
ra_simplify(g);
return ra_select(g);
}
unsigned int
ra_get_node_reg(struct ra_graph *g, unsigned int n)
{
return g->nodes[n].reg;
}
/**
* Forces a node to a specific register. This can be used to avoid
* creating a register class containing one node when handling data
* that must live in a fixed location and is known to not conflict
* with other forced register assignment (as is common with shader
* input data). These nodes do not end up in the stack during
* ra_simplify(), and thus at ra_select() time it is as if they were
* the first popped off the stack and assigned their fixed locations.
* Nodes that use this function do not need to be assigned a register
* class.
*
* Must be called before ra_simplify().
*/
void
ra_set_node_reg(struct ra_graph *g, unsigned int n, unsigned int reg)
{
g->nodes[n].reg = reg;
g->nodes[n].in_stack = false;
}
static float
ra_get_spill_benefit(struct ra_graph *g, unsigned int n)
{
unsigned int j;
float benefit = 0;
int n_class = g->nodes[n].class;
/* Define the benefit of eliminating an interference between n, n2
* through spilling as q(C, B) / p(C). This is similar to the
* "count number of edges" approach of traditional graph coloring,
* but takes classes into account.
*/
for (j = 0; j < g->nodes[n].adjacency_count; j++) {
unsigned int n2 = g->nodes[n].adjacency_list[j];
if (n != n2) {
unsigned int n2_class = g->nodes[n2].class;
benefit += ((float)g->regs->classes[n_class]->q[n2_class] /
g->regs->classes[n_class]->p);
}
}
return benefit;
}
/**
* Returns a node number to be spilled according to the cost/benefit using
* the pq test, or -1 if there are no spillable nodes.
*/
int
ra_get_best_spill_node(struct ra_graph *g)
{
unsigned int best_node = -1;
float best_benefit = 0.0;
unsigned int n;
/* Consider any nodes that we colored successfully or the node we failed to
* color for spilling. When we failed to color a node in ra_select(), we
* only considered these nodes, so spilling any other ones would not result
* in us making progress.
*/
for (n = 0; n < g->count; n++) {
float cost = g->nodes[n].spill_cost;
float benefit;
if (cost <= 0.0)
continue;
if (g->nodes[n].in_stack)
continue;
benefit = ra_get_spill_benefit(g, n);
if (benefit / cost > best_benefit) {
best_benefit = benefit / cost;
best_node = n;
}
}
return best_node;
}
/**
* Only nodes with a spill cost set (cost != 0.0) will be considered
* for register spilling.
*/
void
ra_set_node_spill_cost(struct ra_graph *g, unsigned int n, float cost)
{
g->nodes[n].spill_cost = cost;
}