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24 /** @file brw_fs_combine_constants.cpp
26 * This file contains the opt_combine_constants() pass that runs after the
27 * regular optimization loop. It passes over the instruction list and
28 * selectively promotes immediate values to registers by emitting a mov(1)
31 * This is useful on Gen 7 particularly, because a few instructions can be
32 * coissued (i.e., issued in the same cycle as another thread on the same EU
33 * issues an instruction) under some circumstances, one of which is that they
34 * cannot use immediate values.
38 #include "brw_fs_live_variables.h"
43 /* Returns whether an instruction could co-issue if its immediate source were
44 * replaced with a GRF source.
47 could_coissue(const struct brw_device_info
*devinfo
, const fs_inst
*inst
)
49 if (devinfo
->gen
!= 7)
52 switch (inst
->opcode
) {
64 * Returns true for instructions that don't support immediate sources.
67 must_promote_imm(const struct brw_device_info
*devinfo
, const fs_inst
*inst
)
69 switch (inst
->opcode
) {
70 case SHADER_OPCODE_POW
:
71 return devinfo
->gen
< 8;
80 /** A box for putting fs_regs in a linked list. */
82 DECLARE_RALLOC_CXX_OPERATORS(reg_link
)
84 reg_link(fs_reg
*reg
) : reg(reg
) {}
86 struct exec_node link
;
90 static struct exec_node
*
91 link(void *mem_ctx
, fs_reg
*reg
)
93 reg_link
*l
= new(mem_ctx
) reg_link(reg
);
98 * Information about an immediate value.
101 /** The common ancestor of all blocks using this immediate value. */
105 * The instruction generating the immediate value, if all uses are contained
106 * within a single basic block. Otherwise, NULL.
111 * A list of fs_regs that refer to this immediate. If we promote it, we'll
112 * have to patch these up to refer to the new GRF.
116 /** The immediate value. We currently only handle floats. */
120 * The GRF register and subregister number where we've decided to store the
123 uint8_t subreg_offset
;
126 /** The number of coissuable instructions using this immediate. */
127 uint16_t uses_by_coissue
;
130 * Whether this constant is used by an instruction that can't handle an
131 * immediate source (and already has to be promoted to a GRF).
135 uint16_t first_use_ip
;
136 uint16_t last_use_ip
;
139 /** The working set of information about immediates. */
147 find_imm(struct table
*table
, float val
)
149 assert(signbit(val
) == 0);
151 for (int i
= 0; i
< table
->len
; i
++) {
152 if (table
->imm
[i
].val
== val
) {
153 return &table
->imm
[i
];
160 new_imm(struct table
*table
, void *mem_ctx
)
162 if (table
->len
== table
->size
) {
164 table
->imm
= reralloc(mem_ctx
, table
->imm
, struct imm
, table
->size
);
166 return &table
->imm
[table
->len
++];
170 * Comparator used for sorting an array of imm structures.
172 * We sort by basic block number, then last use IP, then first use IP (least
173 * to greatest). This sorting causes immediates live in the same area to be
174 * allocated to the same register in the hopes that all values will be dead
175 * about the same time and the register can be reused.
178 compare(const void *_a
, const void *_b
)
180 const struct imm
*a
= (const struct imm
*)_a
,
181 *b
= (const struct imm
*)_b
;
183 int block_diff
= a
->block
->num
- b
->block
->num
;
187 int end_diff
= a
->last_use_ip
- b
->last_use_ip
;
191 return a
->first_use_ip
- b
->first_use_ip
;
195 fs_visitor::opt_combine_constants()
197 void *const_ctx
= ralloc_context(NULL
);
202 table
.imm
= ralloc_array(const_ctx
, struct imm
, table
.size
);
204 cfg
->calculate_idom();
207 /* Make a pass through all instructions and count the number of times each
208 * constant is used by coissueable instructions or instructions that cannot
209 * take immediate arguments.
211 foreach_block_and_inst(block
, fs_inst
, inst
, cfg
) {
214 if (!could_coissue(devinfo
, inst
) && !must_promote_imm(devinfo
, inst
))
217 for (int i
= 0; i
< inst
->sources
; i
++) {
218 if (inst
->src
[i
].file
!= IMM
||
219 inst
->src
[i
].type
!= BRW_REGISTER_TYPE_F
)
222 float val
= fabsf(inst
->src
[i
].fixed_hw_reg
.dw1
.f
);
223 struct imm
*imm
= find_imm(&table
, val
);
226 bblock_t
*intersection
= cfg_t::intersect(block
, imm
->block
);
227 if (intersection
!= imm
->block
)
229 imm
->block
= intersection
;
230 imm
->uses
->push_tail(link(const_ctx
, &inst
->src
[i
]));
231 imm
->uses_by_coissue
+= could_coissue(devinfo
, inst
);
232 imm
->must_promote
= imm
->must_promote
|| must_promote_imm(devinfo
, inst
);
233 imm
->last_use_ip
= ip
;
235 imm
= new_imm(&table
, const_ctx
);
238 imm
->uses
= new(const_ctx
) exec_list();
239 imm
->uses
->push_tail(link(const_ctx
, &inst
->src
[i
]));
241 imm
->uses_by_coissue
= could_coissue(devinfo
, inst
);
242 imm
->must_promote
= must_promote_imm(devinfo
, inst
);
243 imm
->first_use_ip
= ip
;
244 imm
->last_use_ip
= ip
;
249 /* Remove constants from the table that don't have enough uses to make them
250 * profitable to store in a register.
252 for (int i
= 0; i
< table
.len
;) {
253 struct imm
*imm
= &table
.imm
[i
];
255 if (!imm
->must_promote
&& imm
->uses_by_coissue
< 4) {
256 table
.imm
[i
] = table
.imm
[table
.len
- 1];
262 if (table
.len
== 0) {
263 ralloc_free(const_ctx
);
266 if (cfg
->num_blocks
!= 1)
267 qsort(table
.imm
, table
.len
, sizeof(struct imm
), compare
);
270 /* Insert MOVs to load the constant values into GRFs. */
271 fs_reg
reg(GRF
, alloc
.allocate(dispatch_width
/ 8));
273 for (int i
= 0; i
< table
.len
; i
++) {
274 struct imm
*imm
= &table
.imm
[i
];
275 /* Insert it either before the instruction that generated the immediate
276 * or after the last non-control flow instruction of the common ancestor.
278 exec_node
*n
= (imm
->inst
? imm
->inst
:
279 imm
->block
->last_non_control_flow_inst()->next
);
280 const fs_builder ibld
= bld
.at(imm
->block
, n
).exec_all();
282 ibld
.MOV(reg
, fs_reg(imm
->val
));
284 imm
->subreg_offset
= reg
.subreg_offset
;
286 reg
.subreg_offset
+= sizeof(float);
287 if ((unsigned)reg
.subreg_offset
== dispatch_width
* sizeof(float)) {
288 reg
.reg
= alloc
.allocate(dispatch_width
/ 8);
289 reg
.subreg_offset
= 0;
292 promoted_constants
= table
.len
;
294 /* Rewrite the immediate sources to refer to the new GRFs. */
295 for (int i
= 0; i
< table
.len
; i
++) {
296 foreach_list_typed(reg_link
, link
, link
, table
.imm
[i
].uses
) {
297 fs_reg
*reg
= link
->reg
;
299 reg
->reg
= table
.imm
[i
].reg
;
300 reg
->subreg_offset
= table
.imm
[i
].subreg_offset
;
302 reg
->negate
= signbit(reg
->fixed_hw_reg
.dw1
.f
) !=
303 signbit(table
.imm
[i
].val
);
304 assert(fabsf(reg
->fixed_hw_reg
.dw1
.f
) == table
.imm
[i
].val
);
308 ralloc_free(const_ctx
);
309 invalidate_live_intervals();