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/*
* Copyright (C) 2018-2019 Alyssa Rosenzweig <alyssa@rosenzweig.io>
*
* Permission is hereby granted, free of charge, to any person obtaining a
* copy of this software and associated documentation files (the "Software"),
* to deal in the Software without restriction, including without limitation
* the rights to use, copy, modify, merge, publish, distribute, sublicense,
* and/or sell copies of the Software, and to permit persons to whom the
* Software is furnished to do so, subject to the following conditions:
*
* The above copyright notice and this permission notice (including the next
* paragraph) shall be included in all copies or substantial portions of the
* Software.
*
* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
* FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
* THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
* LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
* OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
* SOFTWARE.
*/
/* mir_is_live_after performs liveness analysis on the MIR, used primarily
* as part of register allocation. TODO: Algorithmic improvements for
* compiler performance (this is the worst algorithm possible -- see
* backlog with Connor on IRC) */
#include "compiler.h"
/* Determine if a variable is live in the successors of a block */
static bool
is_live_after_successors(compiler_context *ctx, midgard_block *bl, int src)
{
for (unsigned i = 0; i < bl->nr_successors; ++i) {
midgard_block *succ = bl->successors[i];
/* If we already visited, the value we're seeking
* isn't down this path (or we would have short
* circuited */
if (succ->visited) continue;
/* Otherwise (it's visited *now*), check the block */
succ->visited = true;
/* Within this block, check if it's overwritten first */
unsigned overwritten_mask = 0;
mir_foreach_instr_in_block(succ, ins) {
/* Did we read any components that we haven't overwritten yet? */
if (mir_mask_of_read_components(ins, src) & ~overwritten_mask)
return true;
/* If written-before-use, we're gone */
if (ins->ssa_args.dest == src)
overwritten_mask |= ins->mask;
}
/* ...and also, check *its* successors */
if (is_live_after_successors(ctx, succ, src))
return true;
}
/* Welp. We're really not live. */
return false;
}
bool
mir_is_live_after(compiler_context *ctx, midgard_block *block, midgard_instruction *start, int src)
{
/* Check the rest of the block for liveness */
mir_foreach_instr_in_block_from(block, ins, mir_next_op(start)) {
if (mir_has_arg(ins, src))
return true;
}
/* Check the rest of the blocks for liveness recursively */
bool succ = is_live_after_successors(ctx, block, src);
mir_foreach_block(ctx, block) {
block->visited = false;
}
return succ;
}
/* Just a quick check -- is it written more than once? (I.e. are we definitely
* not SSA?) */
bool
mir_has_multiple_writes(compiler_context *ctx, int dest)
{
unsigned write_count = 0;
mir_foreach_instr_global(ctx, ins) {
if (ins->ssa_args.dest == dest)
write_count++;
}
return write_count > 1;
}
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