scc

commit 310831073070f33d03d475cab91fdba47d179272
parent 780603b37374b368f65f10c26d7622a243781519
Author: Roberto E. Vargas Caballero <k0ga@shike2.com>
Date:   Wed, 25 Dec 2024 19:45:06 +0100

cc1: Optimize constant branches

This is a trivial optimization that can lead later to
dead code removal.

Diffstat:
M
src/cmd/scc-cc/cc1/stmt.c
 | 
27
+++++++++++++++++++--------

1 file changed, 19 insertions(+), 8 deletions(-)
diff --git a/src/cmd/scc-cc/cc1/stmt.c b/src/cmd/scc-cc/cc1/stmt.c
@@ -13,6 +13,21 @@ Symbol *curfun;
 static void stmt(Symbol *lbreak, Symbol *lcont, Switch *lswitch);
 
 static void
+branch(Symbol *label, Node *np)
+{
+	if (!np) {
+		emit(OJUMP, label);
+	} else if ((np->flags & NCONST) == 0) {
+		emit(OBRANCH, label);
+		emit(OEXPR, np);
+	} else {
+		if (np->sym->u.i != 0)
+			emit(OJUMP, label);
+		freetree(np);
+	}
+}
+
+static void
 label(void)
 {
 	Symbol *sym;
@@ -85,8 +100,7 @@ While(Symbol *lbreak, Symbol *lcont, Switch *lswitch)
 	emit(OLABEL, begin);
 	stmt(lbreak, lcont, lswitch);
 	emit(OLABEL, lcont);
-	emit(OBRANCH, begin);
-	emit(OEXPR, np);
+	branch(begin, np);
 	emit(OELOOP, NULL);
 
 	emit(OLABEL, lbreak);
@@ -133,8 +147,7 @@ For(Symbol *lbreak, Symbol *lcont, Switch *lswitch)
 	emit(OLABEL, lcont);
 	emit(OEXPR, einc);
 	emit(OLABEL, cond);
-	emit((econd) ? OBRANCH : OJUMP, begin);
-	emit(OEXPR, econd);
+	branch(begin, econd);
 	emit(OELOOP, NULL);
 
 	emit(OLABEL, lbreak);
@@ -163,8 +176,7 @@ Dowhile(Symbol *lbreak, Symbol *lcont, Switch *lswitch)
 	expect(';');
 
 	emit(OLABEL, lcont);
-	emit(OBRANCH, begin);
-	emit(OEXPR, np);
+	branch(begin, np);
 	emit(OELOOP, NULL);
 
 	emit(OLABEL, lbreak);
@@ -311,8 +323,7 @@ If(Symbol *lbreak, Symbol *lcont, Switch *lswitch)
 	lelse = newlabel();
 	expect(IF);
 	np = condition(NEGATE);
-	emit(OBRANCH, lelse);
-	emit(OEXPR, np);
+	branch(lelse, np);
 	stmt(lbreak, lcont, lswitch);
 	if (accept(ELSE)) {
 		end = newlabel();