scrub assembly output

Notably, this adds a new pass to get rid of
jumps on jumps.

all.h

@@ -576,6 +576,7 @@ int dom(Blk *, Blk *);
 void fillfron(Fn *);
 void loopiter(Fn *, void (*)(Blk *, Blk *));
 void fillloop(Fn *);
+void simpljmp(Fn *);
 
 /* mem.c */
 void memopt(Fn *);

cfg.c

@@ -278,3 +278,47 @@ fillloop(Fn *fn)
 		b->loop = 1;
 	loopiter(fn, multloop);
 }
+
+static void
+uffind(Blk **pb, Blk **uf, Fn *fn)
+{
+	Blk **pb1;
+
+	pb1 = &uf[(*pb)->id];
+	if (*pb1 != *pb)
+		uffind(pb1, uf, fn);
+	*pb = *pb1;
+}
+
+/* requires rpo and no phis, breaks cfg */
+void
+simpljmp(Fn *fn)
+{
+
+	Blk **uf; /* union-find */
+	Blk *b;
+	uint n;
+	int c;
+
+	uf = alloc(fn->nblk * sizeof uf[0]);
+	for (n=0; n<fn->nblk; n++)
+		uf[n] = fn->rpo[n];
+	for (b=fn->start; b; b=b->link) {
+		assert(!b->phi);
+		if (b->nins == 0)
+		if (b->jmp.type == Jjmp)
+			uf[b->id] = b->s1;
+	}
+	for (b=fn->start; b; b=b->link) {
+		if (b->s1)
+			uffind(&b->s1, uf, fn);
+		if (b->s2)
+			uffind(&b->s2, uf, fn);
+		c = b->jmp.type - Jxjc;
+		if (0 <= c && c <= NXICmp)
+		if (b->s1 == b->s2) {
+			b->jmp.type = Jjmp;
+			b->s2 = 0;
+		}
+	}
+}

emit.c

@@ -505,7 +505,7 @@ emitfn(Fn *fn, FILE *f)
 	static int id0;
 	Blk *b, *s;
 	Ins *i, itmp;
-	int *r, c, fs, o, n;
+	int *r, c, fs, o, n, lbl;
 
 	fprintf(f, ".text\n");
 	if (fn->export)
@@ -532,11 +532,12 @@ emitfn(Fn *fn, FILE *f)
 			emitf("pushq %L0", &itmp, fn, f);
 		}
 
-	for (b=fn->start; b; b=b->link) {
-		fprintf(f, "%sbb%d:\n", locprefix, id0+b->id);
-		fprintf(f, "/* @%s */\n", b->name);
+	for (lbl=0, b=fn->start; b; b=b->link) {
+		if (lbl || b->npred > 1)
+			fprintf(f, "%sbb%d:\n", locprefix, id0+b->id);
 		for (i=b->ins; i!=&b->ins[b->nins]; i++)
 			emitins(*i, fn, f);
+		lbl = 1;
 		switch (b->jmp.type) {
 		case Jret0:
 			for (r=&rclob[NRClob]; r>rclob;)
@@ -554,6 +555,8 @@ emitfn(Fn *fn, FILE *f)
 			if (b->s1 != b->link)
 				fprintf(f, "\tjmp %sbb%d\n",
 					locprefix, id0+b->s1->id);
+			else
+				lbl = 0;
 			break;
 		default:
 			c = b->jmp.type - Jxjc;

main.c

@@ -72,6 +72,9 @@ func(Fn *fn)
 	spill(fn);
 	rega(fn);
 	fillrpo(fn);
+	simpljmp(fn);
+	fillpreds(fn);
+	fillrpo(fn);
 	assert(fn->rpo[0] == fn->start);
 	for (n=0;; n++)
 		if (n == fn->nblk-1) {