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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 licm.cc
/// @brief Phase A Loop Invariant Code Motion (LICM).
///
/// See docs/licm.md for architectural notes.
#include "rcc/all.h"
#include <vector>
namespace rcc {
namespace {
struct LoopInfo {
Blk* header;
std::vector<Blk*> body; // includes header
Blk* preheader; // nullptr if no unique non-back-edge predecessor
};
Ins make_nop() {
Ins i = {};
i.op = Onop;
return i;
}
// Loop Identification.
void build_loops(Fn* fn, std::vector<LoopInfo>& loops, std::vector<int>& blk_loop) {
loops.clear();
blk_loop.assign(static_cast<size_t>(fn->nblk), -1);
for (Blk* b = fn->start; b; b = b->link)
b->visit = ~0u;
auto refine_inner = [&](uint bid, int lidx) {
int cur = blk_loop[bid];
if (cur < 0 || loops[static_cast<size_t>(cur)].header->id <
loops[static_cast<size_t>(lidx)].header->id)
blk_loop[bid] = lidx;
};
for (uint i = 0; i < fn->nblk; i++) {
Blk* hd = fn->rpo[i];
bool has_back = false;
for (uint k = 0; k < hd->npred; k++) {
if (hd->pred[k]->id >= i) {
has_back = true;
break;
}
}
if (!has_back)
continue;
int lidx = static_cast<int>(loops.size());
loops.push_back({hd, {hd}, nullptr});
refine_inner(hd->id, lidx);
// DFS body collect.
std::vector<Blk*> stack;
for (uint k = 0; k < hd->npred; k++)
if (hd->pred[k]->id >= i)
stack.push_back(hd->pred[k]);
while (!stack.empty()) {
Blk* n = stack.back();
stack.pop_back();
if (n->id < hd->id || n->visit == hd->id)
continue;
n->visit = hd->id;
loops[static_cast<size_t>(lidx)].body.push_back(n);
refine_inner(n->id, lidx);
for (uint k = 0; k < n->npred; k++)
stack.push_back(n->pred[k]);
}
}
// Find preheader.
for (auto& L : loops) {
Blk* phdr = nullptr;
bool unique = true;
for (uint k = 0; k < L.header->npred; k++) {
Blk* p = L.header->pred[k];
bool in_body = false;
for (Blk* b : L.body)
if (b == p) {
in_body = true;
break;
}
if (in_body)
continue; // back-edge
if (phdr) {
unique = false;
break;
}
phdr = p;
}
L.preheader = unique ? phdr : nullptr;
}
}
// Invariance
bool blk_in(Blk* x, const std::vector<Blk*>& body) {
for (Blk* b : body)
if (b == x)
return true;
return false;
}
bool is_invariant(Fn* fn, Ref r, const LoopInfo& L) {
if (rtype(r) == RCon)
return true;
if (rtype(r) != RTmp)
return false;
Tmp* t = &fn->tmp[r.val];
if (t->bid == ~0u)
return true; // Undefined (treat as param/global-like) - equivalent to "outside loop"
return !blk_in(fn->rpo[t->bid], L.body);
}
// Check safety.
bool clobbered_in_loop(Fn* fn, Ins* load_ins, Ref addr, int sz, const LoopInfo& L) {
for (Blk* b : L.body) {
for (Ins* i = b->ins; i < &b->ins[b->nins]; i++) {
if (i == load_ins)
continue;
if (i->op == Ocall)
return true; // Conservative: call is treated as writing all escapable memory
if (isstore(i->op)) {
int ssz = storesz(i);
int delta;
if (alias(addr, 0, sz, i->arg[1], ssz, &delta, fn) != NoAlias)
return true;
}
}
}
return false;
}
} // namespace
void licm(Ctx* ctx, Fn* fn) {
fillloop(fn);
// Phase A hoisted load.
int total_hoisted = 0;
int iter = 0;
for (; iter < 16; iter++) {
std::vector<LoopInfo> loops;
std::vector<int> blk_loop;
build_loops(fn, loops, blk_loop);
if (loops.empty())
break;
int round_hoisted = 0;
for (Blk* b = fn->start; b; b = b->link) {
if (b->id >= blk_loop.size())
continue;
int lidx = blk_loop[b->id];
if (lidx < 0)
continue;
LoopInfo& L = loops[static_cast<size_t>(lidx)];
if (!L.preheader)
continue;
for (Ins* i = b->ins; i < &b->ins[b->nins]; i++) {
if (!isload(i->op))
continue;
if (!is_invariant(fn, i->arg[0], L))
continue;
int sz = loadsz(i);
if (clobbered_in_loop(fn, i, i->arg[0], sz, L))
continue;
// Hit: move to preheader.
addins(ctx, &L.preheader->ins, &L.preheader->nins, i);
*i = make_nop();
round_hoisted++;
}
}
total_hoisted += round_hoisted;
if (round_hoisted == 0)
break;
// Reset loop state.
filluse(ctx, fn);
}
if (debug['L']) {
FILE* out = ctx->dbg_buf ? ctx->dbg_buf : stderr;
fprintf(out, "\n> LICM hoisted %d load(s) in %s (in %d round%s)\n", total_hoisted, fn->name,
iter, iter == 1 ? "" : "s");
}
}
} // namespace rcc
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