From e8365dd0a2cb3050b62e9cb263e1a9a09481d76f Mon Sep 17 00:00:00 2001
From: Quentin Carbonneaux <quentin@c9x.me>
Date: Tue, 13 Jan 2026 18:15:01 +0100
Subject: new simplcfg pass

Useful for ifopt to match more
often. Empty blocks are fused
and conditional jumps on empty
blocks with the same successor
(and no phis in the successor)
are collapsed.
---
 cfg.c | 121 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 121 insertions(+)

(limited to 'cfg.c')

diff --git a/cfg.c b/cfg.c
index 8d31f18..d3860df 100644
--- a/cfg.c
+++ b/cfg.c
@@ -441,3 +441,124 @@ ifgraph(Blk *ifb, Blk **pthenb, Blk **pelseb, Blk **pjoinb)
 	*pjoinb = s1->s1;
 	return 1;
 }
+
+typedef struct Jmp Jmp;
+
+struct Jmp {
+	int type;
+	Ref arg;
+	Blk *s1, *s2;
+};
+
+static int
+jmpeq(Jmp *a, Jmp *b)
+{
+	return a->type == b->type && req(a->arg, b->arg)
+		&& a->s1 == b->s1 && a->s2 == b->s2;
+}
+
+static int
+jmpnophi(Jmp *j)
+{
+	if (j->s1 && j->s1->phi)
+		return 0;
+	if (j->s2 && j->s2->phi)
+		return 0;
+	return 1;
+}
+
+/* require cfg rpo, breaks use */
+void
+simplcfg(Fn *fn)
+{
+	Ins cpy, *i;
+	Blk *b, *bb, **pb;
+	Jmp *jmp, *j, *jj;
+	Phi *p;
+	int *empty, done;
+	uint n;
+
+	if (debug['C']) {
+		fprintf(stderr, "\n> Before CFG simplification:\n");
+		printfn(fn, stderr);
+	}
+
+	cpy = (Ins){.op = Ocopy};
+	for (b=fn->start; b; b=b->link)
+		if (b->npred == 1) {
+			bb = b->pred[0];
+			for (p=b->phi; p; p=p->link) {
+				cpy.cls = p->cls;
+				cpy.to = p->to;
+				cpy.arg[0] = phiarg(p, bb);
+				addins(&bb->ins, &bb->nins, &cpy);
+			}
+			b->phi = 0;
+		}
+
+	jmp = emalloc(fn->nblk * sizeof jmp[0]);
+	empty = emalloc(fn->nblk * sizeof empty[0]);
+	for (b=fn->start; b; b=b->link) {
+		jmp[b->id].type = b->jmp.type;
+		jmp[b->id].arg = b->jmp.arg;
+		jmp[b->id].s1 = b->s1;
+		jmp[b->id].s2 = b->s2;
+		empty[b->id] = !b->phi;
+		for (i=b->ins; i<&b->ins[b->nins]; i++)
+			if (i->op != Onop || i->op != Odbgloc) {
+				empty[b->id] = 0;
+				break;
+			}
+	}
+
+	do {
+		done = 1;
+		for (b=fn->start; b; b=b->link) {
+			if (b->id == -1u)
+				continue;
+			j = &jmp[b->id];
+			if (j->type == Jjmp && j->s1->npred == 1) {
+				assert(!j->s1->phi);
+				addbins(&b->ins, &b->nins, j->s1);
+				empty[b->id] &= empty[j->s1->id];
+				jj = &jmp[j->s1->id];
+				pb = (Blk*[]){jj->s1, jj->s2, 0};
+				for (; (bb=*pb); pb++)
+					for (p=bb->phi; p; p=p->link) {
+						n = phiargn(p, j->s1);
+						p->blk[n] = b;
+					}
+				j->s1->id = -1u;
+				*j = *jj;
+				done = 0;
+			}
+			else if (j->type == Jjnz
+			&& empty[j->s1->id] && empty[j->s2->id]
+			&& jmpeq(&jmp[j->s1->id], &jmp[j->s2->id])
+			&& jmpnophi(&jmp[j->s1->id])) {
+				*j = jmp[j->s1->id];
+				done = 0;
+			}
+		}
+	} while (!done);
+
+	for (b=fn->start; b; b=b->link)
+		if (b->id != -1u) {
+			j = &jmp[b->id];
+			b->jmp.type = j->type;
+			b->jmp.arg = j->arg;
+			b->s1 = j->s1;
+			b->s2 = j->s2;
+			assert(!j->s1 || j->s1->id != -1u);
+			assert(!j->s2 || j->s2->id != -1u);
+		}
+
+	fillcfg(fn);
+	free(empty);
+	free(jmp);
+
+	if (debug['C']) {
+		fprintf(stderr, "\n> After CFG simplification:\n");
+		printfn(fn, stderr);
+	}
+}
-- 
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