/* * 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; }