From 2b1f6c1e2cc22eef6d1a7cdc1c852fddf534fcc7 Mon Sep 17 00:00:00 2001
From: Richard-Qin-X <richard.qin.001@gmail.com>
Date: Mon, 8 Jun 2026 17:06:35 +0800
Subject: [legacy/mid] Add control-flow optimization: if-conversion

---
 rcc-legacy/src/mid/ifopt.cc | 127 ++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 127 insertions(+)
 create mode 100644 rcc-legacy/src/mid/ifopt.cc

(limited to 'rcc-legacy/src')

diff --git a/rcc-legacy/src/mid/ifopt.cc b/rcc-legacy/src/mid/ifopt.cc
new file mode 100644
index 0000000..c82fb4c
--- /dev/null
+++ b/rcc-legacy/src/mid/ifopt.cc
@@ -0,0 +1,127 @@
+/* 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 ifopt.cc
+/// @brief If-conversion: collapses if-then[-else] diamond CFG into straight-line block + sel
+/// instructions.
+///
+/// See docs/ifopt.md for architectural notes.
+
+#include "rcc/all.h"
+
+namespace rcc {
+
+namespace {
+
+enum {
+    MaxIns = 2,
+    MaxPhis = 2,
+};
+
+bool okbranch(Blk* b) {
+    int n = 0;
+    for (Ins* i = b->ins; i < &b->ins[b->nins]; ++i) {
+        if (i->op == Odbgloc)
+            continue;
+        if (pinned(i))
+            return false;
+        if (i->op != Onop)
+            ++n;
+    }
+    return n <= MaxIns;
+}
+
+bool okjoin(Blk* b) {
+    int n = 0;
+    for (Phi* p = b->phi; p; p = p->link) {
+        if (KBASE(p->cls) != 0)
+            return false;
+        ++n;
+    }
+    return n <= MaxPhis;
+}
+
+bool okgraph(Blk* ifb, Blk* thenb, Blk* elseb, Blk* joinb) {
+    assert(thenb != elseb);
+    return joinb->npred == 2 && okjoin(joinb) && (thenb == ifb || okbranch(thenb)) &&
+           (elseb == ifb || okbranch(elseb));
+}
+
+void convert(Ctx* ctx, Blk* ifb, Blk* thenb, Blk* elseb, Blk* joinb) {
+    Ins* ins = static_cast<Ins*>(vnew(ctx, 0, sizeof ins[0], PHeap));
+    uint nins = 0;
+
+    addbins(ctx, &ins, &nins, ifb);
+    if (thenb != ifb)
+        addbins(ctx, &ins, &nins, thenb);
+    if (elseb != ifb)
+        addbins(ctx, &ins, &nins, elseb);
+
+    assert(joinb->npred == 2);
+    Ins sel = {};
+    if (joinb->phi) {
+        sel.op = Osel0;
+        sel.cls = static_cast<uint>(Kw);
+        sel.arg[0] = ifb->jmp.arg;
+        addins(ctx, &ins, &nins, &sel);
+    }
+
+    sel = {};
+    sel.op = Osel1;
+    for (Phi* p = joinb->phi; p; p = p->link) {
+        sel.to = p->to;
+        sel.cls = static_cast<uint>(p->cls);
+        sel.arg[0] = phiarg(p, thenb);
+        sel.arg[1] = phiarg(p, elseb);
+        addins(ctx, &ins, &nins, &sel);
+    }
+
+    idup(ctx, ifb, ins, nins);
+    ifb->jmp.type = Jjmp;
+    ifb->jmp.arg = Ref::none();
+    ifb->s1 = joinb;
+    ifb->s2 = nullptr;
+    joinb->npred = 1;
+    joinb->pred[0] = ifb;
+    joinb->phi = nullptr;
+    vfree(ins);
+}
+
+}  // namespace
+
+void ifconvert(Ctx* ctx, Fn* fn) {
+    if (debug['K'])
+        fputs("\n> If-conversion:\n", ctx->dbg_buf);
+
+    Blk *thenb, *elseb, *joinb;
+    for (Blk* ifb = fn->start; ifb; ifb = ifb->link)
+        if (ifgraph(ifb, &thenb, &elseb, &joinb) && okgraph(ifb, thenb, elseb, joinb)) {
+            if (debug['K'])
+                fprintf(ctx->dbg_buf, "    @%s -> @%s, @%s -> @%s\n", ifb->name, thenb->name,
+                        elseb->name, joinb->name);
+            convert(ctx, ifb, thenb, elseb, joinb);
+        }
+
+    if (debug['K']) {
+        fprintf(ctx->dbg_buf, "\n> After if-conversion:\n");
+        printfn(fn, ctx->dbg_buf);
+    }
+}
+
+}  // namespace rcc
-- 
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