Mesa (glsl-to-tgsi): glsl_to_tgsi: add a better, more advanced dead code elimination pass
Bryan Cain
bryanc at kemper.freedesktop.org
Sat Jun 25 01:48:16 UTC 2011
Module: Mesa
Branch: glsl-to-tgsi
Commit: de3a9fbd852b86a9a7818abf9ad7ca2e32e26ba2
URL: http://cgit.freedesktop.org/mesa/mesa/commit/?id=de3a9fbd852b86a9a7818abf9ad7ca2e32e26ba2
Author: Bryan Cain <bryancain3 at gmail.com>
Date: Fri Jun 24 18:45:04 2011 -0500
glsl_to_tgsi: add a better, more advanced dead code elimination pass
---
src/mesa/state_tracker/st_glsl_to_tgsi.cpp | 140 ++++++++++++++++++++++++++++
1 files changed, 140 insertions(+), 0 deletions(-)
diff --git a/src/mesa/state_tracker/st_glsl_to_tgsi.cpp b/src/mesa/state_tracker/st_glsl_to_tgsi.cpp
index 721ba28..d47364f 100644
--- a/src/mesa/state_tracker/st_glsl_to_tgsi.cpp
+++ b/src/mesa/state_tracker/st_glsl_to_tgsi.cpp
@@ -208,6 +208,7 @@ public:
int sampler; /**< sampler index */
int tex_target; /**< One of TEXTURE_*_INDEX */
GLboolean tex_shadow;
+ int dead_mask; /**< Used in dead code elimination */
class function_entry *function; /* Set on TGSI_OPCODE_CAL or TGSI_OPCODE_BGNSUB */
};
@@ -384,6 +385,7 @@ public:
void copy_propagate(void);
void eliminate_dead_code(void);
+ int eliminate_dead_code_advanced(void);
void merge_registers(void);
void renumber_registers(void);
@@ -480,6 +482,7 @@ glsl_to_tgsi_visitor::emit(ir_instruction *ir, unsigned op,
inst->src[1] = src1;
inst->src[2] = src2;
inst->ir = ir;
+ inst->dead_mask = 0;
inst->function = NULL;
@@ -3257,6 +3260,142 @@ glsl_to_tgsi_visitor::eliminate_dead_code(void)
}
}
+/*
+ * On a basic block basis, tracks available PROGRAM_TEMPORARY registers for dead
+ * code elimination. This is less primitive than eliminate_dead_code(), as it
+ * is per-channel and can detect consecutive writes without a read between them
+ * as dead code. However, there is some dead code that can be eliminated by
+ * eliminate_dead_code() but not this function - for example, this function
+ * cannot eliminate an instruction writing to a register that is never read and
+ * is the only instruction writing to that register.
+ *
+ * The glsl_to_tgsi_visitor lazily produces code assuming that this pass
+ * will occur.
+ */
+int
+glsl_to_tgsi_visitor::eliminate_dead_code_advanced(void)
+{
+ glsl_to_tgsi_instruction **writes = rzalloc_array(mem_ctx,
+ glsl_to_tgsi_instruction *,
+ this->next_temp * 4);
+ int *write_level = rzalloc_array(mem_ctx, int, this->next_temp * 4);
+ int level = 0;
+ int removed = 0;
+
+ foreach_iter(exec_list_iterator, iter, this->instructions) {
+ glsl_to_tgsi_instruction *inst = (glsl_to_tgsi_instruction *)iter.get();
+
+ assert(inst->dst.file != PROGRAM_TEMPORARY
+ || inst->dst.index < this->next_temp);
+
+ switch (inst->op) {
+ case TGSI_OPCODE_BGNLOOP:
+ case TGSI_OPCODE_ENDLOOP:
+ /* End of a basic block, clear the write array entirely.
+ * FIXME: This keeps us from killing dead code when the writes are
+ * on either side of a loop, even when the register isn't touched
+ * inside the loop.
+ */
+ memset(writes, 0, sizeof(*writes) * this->next_temp * 4);
+ break;
+
+ case TGSI_OPCODE_IF:
+ ++level;
+ break;
+
+ case TGSI_OPCODE_ENDIF:
+ --level;
+ break;
+
+ case TGSI_OPCODE_ELSE:
+ /* Clear all channels written inside the preceding if block from the
+ * write array, but leave those that were not touched.
+ *
+ * FIXME: This destroys opportunities to remove dead code inside of
+ * IF blocks that are followed by an ELSE block.
+ */
+ for (int r = 0; r < this->next_temp; r++) {
+ for (int c = 0; c < 4; c++) {
+ if (!writes[4 * r + c])
+ continue;
+
+ if (write_level[4 * r + c] >= level)
+ writes[4 * r + c] = NULL;
+ }
+ }
+ break;
+
+ default:
+ /* Continuing the block, clear any channels from the write array that
+ * are read by this instruction.
+ */
+ for (int i = 0; i < 4; i++) {
+ if (inst->src[i].file == PROGRAM_TEMPORARY && inst->src[i].reladdr){
+ /* Any temporary might be read, so no dead code elimination
+ * across this instruction.
+ */
+ memset(writes, 0, sizeof(*writes) * this->next_temp * 4);
+ } else if (inst->src[i].file == PROGRAM_TEMPORARY) {
+ /* Clear where it's used as src. */
+ int src_chans = 1 << GET_SWZ(inst->src[i].swizzle, 0);
+ src_chans |= 1 << GET_SWZ(inst->src[i].swizzle, 1);
+ src_chans |= 1 << GET_SWZ(inst->src[i].swizzle, 2);
+ src_chans |= 1 << GET_SWZ(inst->src[i].swizzle, 3);
+
+ for (int c = 0; c < 4; c++) {
+ if (src_chans & (1 << c)) {
+ writes[4 * inst->src[i].index + c] = NULL;
+ }
+ }
+ }
+ }
+ break;
+ }
+
+ /* If this instruction writes to a temporary, add it to the write array.
+ * If there is already an instruction in the write array for one or more
+ * of the channels, flag that channel write as dead.
+ */
+ if (inst->dst.file == PROGRAM_TEMPORARY &&
+ !inst->dst.reladdr &&
+ !inst->saturate) {
+ for (int c = 0; c < 4; c++) {
+ if (inst->dst.writemask & (1 << c)) {
+ if (writes[4 * inst->dst.index + c]) {
+ if (write_level[4 * inst->dst.index + c] < level)
+ continue;
+ else
+ writes[4 * inst->dst.index + c]->dead_mask |= (1 << c);
+ }
+ writes[4 * inst->dst.index + c] = inst;
+ write_level[4 * inst->dst.index + c] = level;
+ }
+ }
+ }
+ }
+
+ /* Now actually remove the instructions that are completely dead and update
+ * the writemask of other instructions with dead channels.
+ */
+ foreach_iter(exec_list_iterator, iter, this->instructions) {
+ glsl_to_tgsi_instruction *inst = (glsl_to_tgsi_instruction *)iter.get();
+
+ if (!inst->dead_mask || !inst->dst.writemask)
+ continue;
+ else if (inst->dead_mask == inst->dst.writemask) {
+ iter.remove();
+ delete inst;
+ removed++;
+ } else
+ inst->dst.writemask &= ~(inst->dead_mask);
+ }
+
+ ralloc_free(write_level);
+ ralloc_free(writes);
+
+ return removed;
+}
+
/* Merges temporary registers together where possible to reduce the number of
* registers needed to run a program.
*
@@ -4269,6 +4408,7 @@ get_mesa_program(struct gl_context *ctx,
*/
if (!v->indirect_addr_temps) {
v->copy_propagate();
+ while (v->eliminate_dead_code_advanced());
v->eliminate_dead_code();
v->merge_registers();
v->renumber_registers();
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