/** @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 <ralloc.h>
#include "main/mtypes.h"
#include "register_allocate.h"
+#define NO_REG ~0
+
struct ra_reg {
GLboolean *conflicts;
unsigned int *conflict_list;
GLboolean *regs;
/**
- * p_B in Runeson/Nyström paper.
+ * p(B) in Runeson/Nyström paper.
*
* This is "how many regs are in the set."
*/
unsigned int p;
/**
- * q_B,C in Runeson/Nyström paper.
+ * 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.
+ */
GLboolean *adjacency;
unsigned int *adjacency_list;
- unsigned int class;
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.
+ */
GLboolean in_stack;
+
+ /* For an implementation that needs register spilling, this is the
+ * approximate cost of spilling this node.
+ */
float spill_cost;
};
unsigned int stack_count;
};
+/**
+ * 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(unsigned int count)
+ra_alloc_reg_set(void *mem_ctx, unsigned int count)
{
unsigned int i;
struct ra_regs *regs;
- regs = rzalloc(NULL, struct ra_regs);
+ regs = rzalloc(mem_ctx, struct ra_regs);
regs->count = count;
regs->regs = rzalloc_array(regs, struct ra_reg, count);
}
}
+/**
+ * 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)
+{
+ 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)
{
/**
* 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)
+ra_set_finalize(struct ra_regs *regs, unsigned int **q_values)
{
unsigned int b, c;
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.
*/
g->nodes[i].adjacency_list = ralloc_array(g, unsigned int, count);
g->nodes[i].adjacency_count = 0;
ra_add_node_adjacency(g, i, i);
- g->nodes[i].reg = ~0;
+ g->nodes[i].reg = NO_REG;
}
return g;
progress = GL_FALSE;
for (i = g->count - 1; i >= 0; i--) {
- if (g->nodes[i].in_stack)
+ if (g->nodes[i].in_stack || g->nodes[i].reg != NO_REG)
continue;
if (pq_test(g, i)) {
unsigned int i;
for (i = 0; i < g->count; i++) {
- if (g->nodes[i].in_stack)
+ if (g->nodes[i].in_stack || g->nodes[i].reg != NO_REG)
continue;
g->stack[g->stack_count] = i;
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 = GL_FALSE;
+}
+
static float
ra_get_spill_benefit(struct ra_graph *g, unsigned int n)
{
if (cost <= 0.0)
continue;
+ /* Only consider registers for spilling if they are still in the
+ * interference graph (those on the stack have already been proven to be
+ * allocatable without spilling).
+ */
+ if (g->nodes[n].in_stack)
+ continue;
+
benefit = ra_get_spill_benefit(g, n);
if (benefit / cost > best_benefit) {
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