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25 * @file vc4_opt_dead_code.c
27 * This is a simple dead code eliminator for SSA values in QIR.
29 * It walks all the instructions finding what temps are used, then walks again
30 * to remove instructions writing unused temps.
32 * This is an inefficient implementation if you have long chains of
33 * instructions where the entire chain is dead, but we expect those to have
34 * been eliminated at the NIR level, and here we're just cleaning up small
35 * problems produced by NIR->QIR.
43 dce(struct vc4_compile
*c
, struct qinst
*inst
)
46 fprintf(stderr
, "Removing: ");
47 qir_dump_inst(c
, inst
);
48 fprintf(stderr
, "\n");
51 qir_remove_instruction(c
, inst
);
55 has_nonremovable_reads(struct vc4_compile
*c
, struct qinst
*inst
)
57 for (int i
= 0; i
< qir_get_nsrc(inst
); i
++) {
58 if (inst
->src
[i
].file
== QFILE_VPM
) {
59 uint32_t attr
= inst
->src
[i
].index
/ 4;
60 uint32_t offset
= (inst
->src
[i
].index
% 4) * 4;
62 if (c
->vattr_sizes
[attr
] != offset
+ 4)
65 /* Can't get rid of the last VPM read, or the
66 * simulator (at least) throws an error.
68 uint32_t total_size
= 0;
69 for (uint32_t i
= 0; i
< ARRAY_SIZE(c
->vattr_sizes
); i
++)
70 total_size
+= c
->vattr_sizes
[i
];
75 if (inst
->src
[i
].file
== QFILE_VARY
&&
76 c
->input_slots
[inst
->src
[i
].index
].slot
== 0xff) {
85 qir_opt_dead_code(struct vc4_compile
*c
)
87 bool progress
= false;
88 bool *used
= calloc(c
->num_temps
, sizeof(bool));
90 qir_for_each_inst_inorder(inst
, c
) {
91 for (int i
= 0; i
< qir_get_nsrc(inst
); i
++) {
92 if (inst
->src
[i
].file
== QFILE_TEMP
)
93 used
[inst
->src
[i
].index
] = true;
97 qir_for_each_block(block
, c
) {
98 qir_for_each_inst_safe(inst
, block
) {
99 if (inst
->dst
.file
!= QFILE_NULL
&&
100 !(inst
->dst
.file
== QFILE_TEMP
&&
101 !used
[inst
->dst
.index
])) {
105 if (qir_has_side_effects(c
, inst
))
109 has_nonremovable_reads(c
, inst
)) {
110 /* If we can't remove the instruction, but we
111 * don't need its destination value, just
112 * remove the destination. The register
113 * allocator would trivially color it and it
114 * wouldn't cause any register pressure, but
115 * it's nicer to read the QIR code without
116 * unused destination regs.
118 if (inst
->dst
.file
== QFILE_TEMP
) {
121 "Removing dst from: ");
122 qir_dump_inst(c
, inst
);
123 fprintf(stderr
, "\n");
125 c
->defs
[inst
->dst
.index
] = NULL
;
126 inst
->dst
.file
= QFILE_NULL
;
132 for (int i
= 0; i
< qir_get_nsrc(inst
); i
++) {
133 if (inst
->src
[i
].file
!= QFILE_VPM
)
135 uint32_t attr
= inst
->src
[i
].index
/ 4;
136 uint32_t offset
= (inst
->src
[i
].index
% 4) * 4;
138 if (c
->vattr_sizes
[attr
] == offset
+ 4) {
140 c
->vattr_sizes
[attr
] -= 4;