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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 divopt.cc
/// @brief Constant integer division/modulo strength reduction: rewrite `x / c`, `x % c`
/// into mul + shift + sign correction sequence when c is a compile-time constant.
///
/// See docs/divopt.md for architectural notes.
#include "rcc/all.h"
#include <cstdint>
namespace rcc {
namespace {
struct Magic {
int32_t M;
int s;
bool need_add;
};
Magic compute_magic_s32(uint32_t ad) {
const uint32_t t = 0x80000000u; // 2^31
uint32_t anc = t - 1 - t % ad;
uint32_t p = 31;
uint32_t q1 = t / anc, r1 = t - q1 * anc;
uint32_t q2 = t / ad, r2 = t - q2 * ad;
uint32_t delta;
do {
++p;
q1 *= 2;
r1 *= 2;
if (r1 >= anc) {
++q1;
r1 -= anc;
}
q2 *= 2;
r2 *= 2;
if (r2 >= ad) {
++q2;
r2 -= ad;
}
delta = ad - r2;
} while (q1 < delta || (q1 == delta && r1 == 0));
Magic m;
uint32_t M_u = q2 + 1;
m.M = static_cast<int32_t>(M_u);
m.s = static_cast<int>(p - 32);
m.need_add = (M_u >= 0x80000000u);
return m;
}
void emit_pow2_div_into(Ctx* ctx, Fn* fn, Ref dst, Ref x, int k) {
Ref sign = newtmp(ctx, "divopt", Kw, fn);
Ref mask = newtmp(ctx, "divopt", Kw, fn);
Ref tmp = newtmp(ctx, "divopt", Kw, fn);
emit(ctx, Osar, Kw, dst, tmp, getcon(ctx, k, fn));
emit(ctx, Oadd, Kw, tmp, x, mask);
emit(ctx, Oshr, Kw, mask, sign, getcon(ctx, 32 - k, fn));
emit(ctx, Osar, Kw, sign, x, getcon(ctx, 31, fn));
}
void emit_magic_div_into(Ctx* ctx, Fn* fn, Ref dst, Ref x, int32_t d_abs, bool d_neg) {
Magic m = compute_magic_s32(static_cast<uint32_t>(d_abs));
int64_t M64 = static_cast<int64_t>(m.M); // Sign extend i32 to i64
Ref xl = newtmp(ctx, "divopt", Kl, fn);
Ref prod = newtmp(ctx, "divopt", Kl, fn);
Ref hi = newtmp(ctx, "divopt", Kl, fn);
Ref hi2 = m.need_add ? newtmp(ctx, "divopt", Kl, fn) : hi;
Ref q1 = (m.s > 0) ? newtmp(ctx, "divopt", Kl, fn) : hi2;
Ref sg = newtmp(ctx, "divopt", Kl, fn);
Ref qpre = d_neg ? newtmp(ctx, "divopt", Kw, fn) : dst;
// Reverse emit:
if (d_neg)
emit(ctx, Oneg, Kw, dst, qpre, Ref::none());
emit(ctx, Osub, Kw, qpre, q1, sg);
emit(ctx, Osar, Kl, sg, xl, getcon(ctx, 63, fn));
if (m.s > 0)
emit(ctx, Osar, Kl, q1, hi2, getcon(ctx, m.s, fn));
if (m.need_add)
emit(ctx, Oadd, Kl, hi2, hi, xl);
emit(ctx, Osar, Kl, hi, prod, getcon(ctx, 32, fn));
emit(ctx, Omul, Kl, prod, xl, getcon(ctx, M64, fn));
emit(ctx, Oextsw, Kl, xl, x, Ref::none());
}
bool lower_div(Ctx* ctx, Fn* fn, Ins* i, Blk* b, bool* rebuilt) {
if (i->op != Odiv && i->op != Orem)
return false;
if (i->cls != Kw)
return false;
if (rtype(i->arg[1]) != RCon)
return false;
Con* c = &fn->con[i->arg[1].val];
if (c->type != Con::CBits)
return false;
int32_t d = static_cast<int32_t>(c->bits.i);
if (d == 0 || d == static_cast<int32_t>(0x80000000))
return false; // Skip d=0 (leave to hardware) and d=INT_MIN (|d| overflows i32)
bool is_rem = (i->op == Orem);
Ref dst = i->to;
Ref x = i->arg[0];
// First hit: move unprocessed tail to end of insb (same as simpl.cc)
if (!*rebuilt) {
ctx->curi = ctx->insb + NIns;
ulong tail = static_cast<ulong>((b->ins + b->nins) - (i + 1));
ctx->curi -= tail;
icpy(ctx->curi, i + 1, tail);
*rebuilt = true;
}
// d = +-1 degenerate: rem -> 0; div(1)=x; div(-1)=-x.
if (d == 1 || d == -1) {
if (is_rem)
emit(ctx, Ocopy, Kw, dst, getcon(ctx, 0, fn), Ref::none());
else if (d == 1)
emit(ctx, Ocopy, Kw, dst, x, Ref::none());
else
emit(ctx, Oneg, Kw, dst, x, Ref::none());
return true;
}
// For rem: emit tail r = x - q*d first (reverse order), then div sequence.
Ref q_dst = is_rem ? newtmp(ctx, "divopt", Kw, fn) : dst;
if (is_rem) {
Ref qd = newtmp(ctx, "divopt", Kw, fn);
emit(ctx, Osub, Kw, dst, x, qd);
emit(ctx, Omul, Kw, qd, q_dst, getcon(ctx, static_cast<int64_t>(d), fn));
}
int32_t ad = (d < 0) ? -d : d;
if ((ad & (ad - 1)) == 0) {
// |d| is power of 2, k >= 1
int k = __builtin_ctz(static_cast<uint32_t>(ad));
if (d < 0) {
Ref q_pos = newtmp(ctx, "divopt", Kw, fn);
emit(ctx, Oneg, Kw, q_dst, q_pos, Ref::none());
emit_pow2_div_into(ctx, fn, q_pos, x, k);
} else {
emit_pow2_div_into(ctx, fn, q_dst, x, k);
}
} else {
emit_magic_div_into(ctx, fn, q_dst, x, ad, d < 0);
}
return true;
}
} // namespace
void divopt(Ctx* ctx, Fn* fn) {
for (Blk* b = fn->start; b; b = b->link) {
bool rebuilt = false;
for (Ins* i = b->ins + b->nins; i != b->ins;) {
--i;
if (lower_div(ctx, fn, i, b, &rebuilt))
continue;
if (rebuilt)
emiti(ctx, *i);
}
if (rebuilt)
idup(ctx, b, ctx->curi, static_cast<ulong>(ctx->insb + NIns - ctx->curi));
}
}
} // namespace rcc
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