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/*
* Copyright 2021 Alyssa Rosenzweig
* Copyright 2019-2020 Collabora, Ltd.
* SPDX-License-Identifier: MIT
*/
#include "util/sparse_bitset.h"
#include "agx_compiler.h"
/* Liveness analysis is a backwards-may dataflow analysis pass. Within a block,
* we compute live_out from live_in. The intrablock pass is linear-time. It
* returns whether progress was made. */
/* live_in[s] = GEN[s] + (live_out[s] - KILL[s]) */
void
agx_liveness_ins_update(struct u_sparse_bitset *live, agx_instr *I)
{
agx_foreach_ssa_dest(I, d)
u_sparse_bitset_clear(live, I->dest[d].value);
agx_foreach_ssa_src(I, s) {
/* If the source is not live after this instruction, but becomes live
* at this instruction, this is the use that kills the source
*/
I->src[s].kill = !u_sparse_bitset_test(live, I->src[s].value);
u_sparse_bitset_set(live, I->src[s].value);
}
}
/* Globally, liveness analysis uses a fixed-point algorithm based on a
* worklist. We initialize a work list with the exit block. We iterate the work
* list to compute live_in from live_out for each block on the work list,
* adding the predecessors of the block to the work list if we made progress.
*/
void
agx_compute_liveness(agx_context *ctx)
{
u_worklist worklist;
u_worklist_init(&worklist, ctx->num_blocks, NULL);
/* Free any previous liveness, and allocate */
agx_foreach_block(ctx, block) {
u_sparse_bitset_free(&block->live_in);
u_sparse_bitset_free(&block->live_out);
u_sparse_bitset_init(&block->live_in, ctx->alloc, block);
u_sparse_bitset_init(&block->live_out, ctx->alloc, block);
agx_worklist_push_head(&worklist, block);
}
/* Iterate the work list */
while (!u_worklist_is_empty(&worklist)) {
/* Pop in reverse order since liveness is a backwards pass */
agx_block *blk = agx_worklist_pop_head(&worklist);
/* Update its liveness information */
u_sparse_bitset_dup(&blk->live_in, &blk->live_out);
agx_foreach_instr_in_block_rev(blk, I) {
if (I->op != AGX_OPCODE_PHI)
agx_liveness_ins_update(&blk->live_in, I);
}
/* Propagate the live in of the successor (blk) to the live out of
* predecessors.
*
* Phi nodes are logically on the control flow edge and act in parallel.
* To handle when propagating, we kill writes from phis and make live the
* corresponding sources.
*/
agx_foreach_predecessor(blk, pred) {
struct u_sparse_bitset live;
u_sparse_bitset_dup(&live, &blk->live_in);
/* Kill write */
agx_foreach_phi_in_block(blk, phi) {
assert(phi->dest[0].type == AGX_INDEX_NORMAL);
u_sparse_bitset_clear(&live, phi->dest[0].value);
}
/* Make live the corresponding source */
agx_foreach_phi_in_block(blk, phi) {
agx_index operand = phi->src[agx_predecessor_index(blk, *pred)];
if (operand.type == AGX_INDEX_NORMAL) {
u_sparse_bitset_set(&live, operand.value);
phi->src[agx_predecessor_index(blk, *pred)].kill = false;
}
}
if (u_sparse_bitset_merge(&(*pred)->live_out, &live))
agx_worklist_push_tail(&worklist, *pred);
}
}
u_worklist_fini(&worklist);
}
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