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27 /** @file brw_fs_sel_peephole.cpp
29 * This file contains the opt_peephole_sel() optimization pass that replaces
30 * MOV instructions to the same destination in the "then" and "else" bodies of
31 * an if statement with SEL instructions.
34 /* Four MOVs seems to be pretty typical, so I picked the next power of two in
35 * the hopes that it would handle almost anything possible in a single
38 #define MAX_MOVS 8 /**< The maximum number of MOVs to attempt to match. */
41 * Scans forwards from an IF counting consecutive MOV instructions in the
42 * "then" and "else" blocks of the if statement.
44 * A pointer to the bblock_t following the IF is passed as the <then_block>
45 * argument. The function stores pointers to the MOV instructions in the
46 * <then_mov> and <else_mov> arrays.
48 * \return the minimum number of MOVs found in the two branches or zero if
53 * then_mov[0] = MOV g4, ...
54 * then_mov[1] = MOV g5, ...
55 * then_mov[2] = MOV g6, ...
57 * else_mov[0] = MOV g4, ...
58 * else_mov[1] = MOV g5, ...
59 * else_mov[2] = MOV g7, ...
64 count_movs_from_if(fs_inst
*then_mov
[MAX_MOVS
], fs_inst
*else_mov
[MAX_MOVS
],
65 bblock_t
*then_block
, bblock_t
*else_block
)
68 foreach_inst_in_block(fs_inst
, inst
, then_block
) {
69 if (then_movs
== MAX_MOVS
|| inst
->opcode
!= BRW_OPCODE_MOV
)
72 then_mov
[then_movs
] = inst
;
77 foreach_inst_in_block(fs_inst
, inst
, else_block
) {
78 if (else_movs
== MAX_MOVS
|| inst
->opcode
!= BRW_OPCODE_MOV
)
81 else_mov
[else_movs
] = inst
;
85 return MIN2(then_movs
, else_movs
);
89 * Try to replace IF/MOV+/ELSE/MOV+/ENDIF with SEL.
91 * Many GLSL shaders contain the following pattern:
93 * x = condition ? foo : bar
97 * if (...) a.xyzw = foo.xyzw;
98 * else a.xyzw = bar.xyzw;
100 * The compiler emits an ir_if tree for this, since each subexpression might be
101 * a complex tree that could have side-effects or short-circuit logic.
103 * However, the common case is to simply select one of two constants or
104 * variable values---which is exactly what SEL is for. In this case, the
105 * assembly looks like:
115 * where each pair of MOVs to a common destination and can be easily translated
118 * (+f0) SEL dst src0 src1
120 * If src0 is an immediate value, we promote it to a temporary GRF.
123 fs_visitor::opt_peephole_sel()
125 bool progress
= false;
127 foreach_block (block
, cfg
) {
128 /* IF instructions, by definition, can only be found at the ends of
131 fs_inst
*if_inst
= (fs_inst
*)block
->end();
132 if (if_inst
->opcode
!= BRW_OPCODE_IF
)
135 fs_inst
*else_mov
[MAX_MOVS
] = { NULL
};
136 fs_inst
*then_mov
[MAX_MOVS
] = { NULL
};
138 bblock_t
*then_block
= block
->next();
139 bblock_t
*else_block
= NULL
;
140 foreach_list_typed(bblock_link
, child
, link
, &block
->children
) {
141 if (child
->block
!= then_block
) {
142 if (child
->block
->prev()->end()->opcode
== BRW_OPCODE_ELSE
) {
143 else_block
= child
->block
;
148 if (else_block
== NULL
)
151 int movs
= count_movs_from_if(then_mov
, else_mov
, then_block
, else_block
);
156 fs_inst
*sel_inst
[MAX_MOVS
] = { NULL
};
157 fs_inst
*mov_imm_inst
[MAX_MOVS
] = { NULL
};
159 enum brw_predicate predicate
;
160 bool predicate_inverse
;
161 if (devinfo
->gen
== 6 && if_inst
->conditional_mod
) {
162 /* For Sandybridge with IF with embedded comparison */
163 predicate
= BRW_PREDICATE_NORMAL
;
164 predicate_inverse
= false;
166 /* Separate CMP and IF instructions */
167 predicate
= if_inst
->predicate
;
168 predicate_inverse
= if_inst
->predicate_inverse
;
171 /* Generate SEL instructions for pairs of MOVs to a common destination. */
172 for (int i
= 0; i
< movs
; i
++) {
173 if (!then_mov
[i
] || !else_mov
[i
])
176 /* Check that the MOVs are the right form. */
177 if (!then_mov
[i
]->dst
.equals(else_mov
[i
]->dst
) ||
178 then_mov
[i
]->is_partial_write() ||
179 else_mov
[i
]->is_partial_write() ||
180 then_mov
[i
]->conditional_mod
!= BRW_CONDITIONAL_NONE
||
181 else_mov
[i
]->conditional_mod
!= BRW_CONDITIONAL_NONE
) {
186 /* Check that source types for mov operations match. */
187 if (then_mov
[i
]->src
[0].type
!= else_mov
[i
]->src
[0].type
) {
192 if (then_mov
[i
]->src
[0].equals(else_mov
[i
]->src
[0])) {
193 sel_inst
[i
] = MOV(then_mov
[i
]->dst
, then_mov
[i
]->src
[0]);
195 /* Only the last source register can be a constant, so if the MOV
196 * in the "then" clause uses a constant, we need to put it in a
199 fs_reg
src0(then_mov
[i
]->src
[0]);
200 if (src0
.file
== IMM
) {
201 src0
= vgrf(glsl_type::float_type
);
202 src0
.type
= then_mov
[i
]->src
[0].type
;
203 mov_imm_inst
[i
] = MOV(src0
, then_mov
[i
]->src
[0]);
206 sel_inst
[i
] = SEL(then_mov
[i
]->dst
, src0
, else_mov
[i
]->src
[0]);
207 sel_inst
[i
]->predicate
= predicate
;
208 sel_inst
[i
]->predicate_inverse
= predicate_inverse
;
215 /* Emit a CMP if our IF used the embedded comparison */
216 if (devinfo
->gen
== 6 && if_inst
->conditional_mod
) {
217 fs_inst
*cmp_inst
= CMP(reg_null_d
, if_inst
->src
[0], if_inst
->src
[1],
218 if_inst
->conditional_mod
);
219 if_inst
->insert_before(block
, cmp_inst
);
222 for (int i
= 0; i
< movs
; i
++) {
224 if_inst
->insert_before(block
, mov_imm_inst
[i
]);
225 if_inst
->insert_before(block
, sel_inst
[i
]);
227 then_mov
[i
]->remove(then_block
);
228 else_mov
[i
]->remove(else_block
);
235 invalidate_live_intervals();