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vc4: Convert vc4_opt_dead_code to work in the presence of control flow.

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With control flow, we can't be sure that we'll see the uses of a variable
before its def as we walk backwards.  Given that NIR is eliminating our
long chains of dead code, a simple solution for now seems fine.

This slightly changes the order of some optimizations, and so an opt_vpm
happens before opt_dce, causing 3 dead MOVs to be turned into dead FMAXes
in Minecraft:

instructions in affected programs:     52 -> 54 (3.85%)
This commit is contained in:
Eric Anholt 2016-07-08 16:23:38 -07:00
parent 4e797bd98f
commit ee69cfd11d
1 changed files with 29 additions and 18 deletions
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47
src/gallium/drivers/vc4/vc4_opt_dead_code.c
View file

@ -24,11 +24,15 @@
/**
* @file vc4_opt_dead_code.c
*
* This is a simmple dead code eliminator for QIR with no control flow.
* This is a simple dead code eliminator for SSA values in QIR.
*
* It walks from the bottom of the instruction list, removing instructions
* with a destination that is never used, and marking the sources of non-dead
* instructions as used.
* It walks all the instructions finding what temps are used, then walks again
* to remove instructions writing unused temps.
*
* This is an inefficient implementation if you have long chains of
* instructions where the entire chain is dead, but we expect those to have
* been eliminated at the NIR level, and here we're just cleaning up small
* problems produced by NIR->QIR.
*/
#include "vc4_qir.h"
@ -83,15 +87,27 @@ qir_opt_dead_code(struct vc4_compile *c)
bool progress = false;
bool *used = calloc(c->num_temps, sizeof(bool));
list_for_each_entry_safe_rev(struct qinst, inst,
&c->cur_block->instructions,
link) {
if ((inst->dst.file == QFILE_NULL ||
(inst->dst.file == QFILE_TEMP &&
!used[inst->dst.index])) &&
!inst->sf &&
!qir_has_side_effects(c, inst) &&
!has_nonremovable_reads(c, inst)) {
qir_for_each_inst_inorder(inst, c) {
for (int i = 0; i < qir_get_op_nsrc(inst->op); i++) {
if (inst->src[i].file == QFILE_TEMP)
used[inst->src[i].index] = true;
}
}
qir_for_each_block(block, c) {
qir_for_each_inst_safe(inst, block) {
if (inst->dst.file != QFILE_NULL &&
!(inst->dst.file == QFILE_TEMP &&
!used[inst->dst.index])) {
continue;
}
if (inst->sf ||
qir_has_side_effects(c, inst) ||
has_nonremovable_reads(c, inst)) {
continue;
}
for (int i = 0; i < qir_get_op_nsrc(inst->op); i++) {
if (inst->src[i].file != QFILE_VPM)
continue;
@ -108,11 +124,6 @@ qir_opt_dead_code(struct vc4_compile *c)
progress = true;
continue;
}
for (int i = 0; i < qir_get_op_nsrc(inst->op); i++) {
if (inst->src[i].file == QFILE_TEMP)
used[inst->src[i].index] = true;
}
}
free(used);