scc

commit a2e934bedcd03584e32e7d0c2e66eb23eb6faa50
parent f2a433b0a0c6d336bf4f0e49be6cd6270d3bedfc
Author: Roberto E. Vargas Caballero <k0ga@shike2.com>
Date:   Sat, 28 Dec 2024 12:57:03 +0100

cc2: Add jump optimizations

After all the optimizationas are performed we can find useless
jumps or labels not used. We can do these optimizations only
at the end because they make invalid the cfg and it has to be
rebuilt again.

Diffstat:
M
src/cmd/scc-cc/cc2/cfg.c
 | 
77
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++----

1 file changed, 73 insertions(+), 4 deletions(-)
diff --git a/src/cmd/scc-cc/cc2/cfg.c b/src/cmd/scc-cc/cc2/cfg.c
@@ -223,13 +223,82 @@ buildcfg(void)
 	PRCFG("cfg");
 }
 
+static Node *
+optlabels(Range *rp, Node *np)
+{
+	if (np->op == ONOP && np->label->refcnt == 0) {
+		cfg.dirty = 1;
+		return NULL;
+	}
+	return np;
+}
+
+static Node *
+optjmps(Range *rp, Node *np)
+{
+	Range r;
+	Symbol *label;
+	Node *p, *stmt, *last;
+
+	switch (np->op) {
+	case OBRANCH:
+		label = np->u.sym;
+		stmt = label->u.stmt;
+		goto chain_jumps;
+	case OJMP:
+		label = np->u.sym;
+		stmt = label->u.stmt;
+
+		/* Avoid jump over a set of NOPs */
+		r = range(np, rp->end);
+		for (p = nextstmt(&r, SETCUR); p; p = nextstmt(&r, SETCUR)) {
+			if (p == stmt) {
+				cfg.dirty = 1;
+				label->refcnt--;
+				return NULL;
+			}
+			if (p->op != ONOP)
+				break;
+		}
+
+	chain_jumps:
+		/* avoid chain of jumps to jumps */
+		r = range(stmt, rp->end);
+		for (p = stmt; p && p->op == ONOP; p = nextstmt(&r, SETCUR))
+			;
+		if (p && p != np && p->op == OJMP) {
+			cfg.dirty = 1;
+			label->refcnt--;
+			label = p->u.sym;
+			stmt = label->u.stmt;
+			np->u.sym = label;
+			goto chain_jumps;
+		}
+	}
+}
+
 void
 gencfg(void)
 {
-	PRTREE("before_cfgopt");
-	buildcfg();
-	trimcfg();
-	PRTREE("after_cfgopt");
+	PRTREE("before_gencfg");
+
+	cfg.dirty = 1;
+	while (cfg.dirty) {
+		DBG("new cfg");
+		cleancfg();
+		buildcfg();
+		trimcfg();
+
+		/*
+		 * Jump optimizations only can happen after trimming the cfg,
+		 * because any change in the jumps make invalid the cfg
+		 * automatically
+		 */
+		apply(fbody(), optjmps);
+		apply(fbody(), optlabels);
+	}
+
+	PRTREE("after_gencfg");
 }
 
 static Node *