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/* Source code for RCC-Legacy.
Copyright (C) 2026 Richard Qin
This file is part of RCC.
RCC is free software; you can redistribute it and/or modify it under
the terms of the GNU General Public License as published by the Free
Software Foundation; either version 3, or (at your option) any later
version.
RCC is distributed in the hope that it will be useful, but WITHOUT ANY
WARRANTY; without even the implied warranty of MERCHANTABILITY or
FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
for more details.
You should have received a copy of the GNU General Public License
along with RCC; see the file LICENSE. If not see
<http://www.gnu.org/licenses/>. */
/// @file dead_loop.cc
/// @brief Dead Loop Elimination: removal of entire loops that have no side-effects, are provably
/// terminating, and have no live-outs.
///
/// See docs/dead_loop.md for architectural notes.
#include "rcc/all.h"
#include "rcc/loopinfo.h"
#include <unordered_set>
#include <vector>
namespace rcc {
namespace {
bool has_side_effect_op(uint op) {
if (isstore(op))
return true;
if (isalloc(op))
return true;
if (ispar(op))
return true;
if (isarg(op))
return true;
if (op == Ocall)
return true;
if (op == Oblit0 || op == Oblit1)
return true;
if (op == Ovastart || op == Ovaarg)
return true;
if (op == Odbgloc)
return true;
if (op == Osel0 || op == Osel1)
return true;
return false;
}
bool defined_outside(Fn* fn, const Loop& L, Ref r) {
if (rtype(r) == RCon)
return true;
if (rtype(r) != RTmp)
return false;
if (r.val >= static_cast<uint>(fn->ntmp))
return false;
uint bid = fn->tmp[r.val].bid;
return L.body_bid.find(bid) == L.body_bid.end();
}
Ins* find_cmp_in(Blk* b, Ref cond) {
for (uint i = 0; i < b->nins; i++)
if (req(b->ins[i].to, cond))
return &b->ins[i];
return nullptr;
}
int find_biv_idx(const std::vector<Biv>& bivs, Ref r) {
for (size_t k = 0; k < bivs.size(); k++)
if (req(bivs[k].to, r))
return static_cast<int>(k);
return -1;
}
void collect_body_defs(const Loop& L, std::vector<Ref>& defs) {
for (Blk* b : L.body) {
for (Phi* p = b->phi; p; p = p->link)
if (rtype(p->to) == RTmp)
defs.push_back(p->to);
for (uint k = 0; k < b->nins; k++)
if (rtype(b->ins[k].to) == RTmp)
defs.push_back(b->ins[k].to);
}
}
bool all_uses_inside(Fn* fn, const Loop& L, Ref tmp_ref) {
if (rtype(tmp_ref) != RTmp)
return true;
if (tmp_ref.val >= static_cast<uint>(fn->ntmp))
return true;
Tmp* t = &fn->tmp[tmp_ref.val];
for (uint u = 0; u < t->nuse; u++) {
const Use& uu = t->use[u];
if (L.body_bid.find(uu.bid) == L.body_bid.end())
return false;
}
return true;
}
bool delete_loop_if_dead(Fn* fn, const Loop& L) {
// Topology: unique exit_block + unique exiting_block
Blk* exit_block = nullptr;
Blk* exiting_block = nullptr;
for (Blk* b : L.body) {
Blk* succs[2] = {b->s1, b->s2};
for (Blk* s : succs) {
if (!s)
continue;
if (L.body_bid.find(s->id) != L.body_bid.end())
continue; // Internal edge
// Exit loop edge
if (!exit_block) {
exit_block = s;
exiting_block = b;
} else if (exit_block != s || exiting_block != b) {
return false; // Multiple exits or multiple exiting blocks
}
}
}
if (!exit_block)
return false; // No exit -- infinite loop
// Termination: at least one constant BIV + header Jjnz cmp(BIV, invariant)
std::vector<Biv> bivs;
detect_bivs(fn, L, bivs);
if (bivs.empty())
return false;
if (L.header->jmp.type != Jjnz)
return false;
Ref cond = L.header->jmp.arg;
if (rtype(cond) != RTmp)
return false;
Ins* cmp = find_cmp_in(L.header, cond);
if (!cmp)
return false;
int bi = find_biv_idx(bivs, cmp->arg[0]);
Ref other = cmp->arg[1];
if (bi < 0) {
bi = find_biv_idx(bivs, cmp->arg[1]);
other = cmp->arg[0];
}
if (bi < 0)
return false;
if (!defined_outside(fn, L, other))
return false;
if (exiting_block != L.header)
return false;
// Side-effect scan
for (Blk* b : L.body) {
for (uint k = 0; k < b->nins; k++) {
if (has_side_effect_op(b->ins[k].op))
return false;
}
}
// Live-out scan
for (Phi* p = exit_block->phi; p; p = p->link) {
for (uint k = 0; k < p->narg; k++) {
if (p->blk[k] != exiting_block)
continue;
if (!defined_outside(fn, L, p->arg[k]))
return false;
}
}
std::vector<Ref> defs;
collect_body_defs(L, defs);
for (Ref r : defs) {
if (!all_uses_inside(fn, L, r))
return false;
}
for (Phi* p = exit_block->phi; p; p = p->link) {
for (uint k = 0; k < p->narg; k++)
if (p->blk[k] == L.preheader)
return false;
}
// Rewrite
// preheader's jmp outgoing edge: the one pointing to header is changed to point to exit_block;
if (L.preheader->s1 == L.header)
L.preheader->s1 = exit_block;
if (L.preheader->s2 == L.header)
L.preheader->s2 = exit_block;
for (Phi* p = exit_block->phi; p; p = p->link) {
for (uint k = 0; k < p->narg; k++)
if (p->blk[k] == exiting_block)
p->blk[k] = L.preheader;
}
return true;
}
} // namespace
void dead_loop(Ctx* ctx, Fn* fn) {
fillpreds(ctx, fn);
fillcfg(ctx, fn);
filluse(ctx, fn);
std::vector<Loop> loops;
find_loops(fn, loops);
if (loops.empty())
return;
int deleted = 0;
for (const Loop& L : loops) {
if (delete_loop_if_dead(fn, L))
deleted++;
}
if (debug['L']) {
FILE* out = ctx->dbg_buf ? ctx->dbg_buf : stderr;
fprintf(out, "\n> dead_loop %s: deleted %d of %zu loop(s)\n", fn->name, deleted,
loops.size());
}
}
} // namespace rcc
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