Fix external register problems and synchronize a little bit with Plan 9.

Reference: /n/patches.lsub.org/patch/compext
Date: Thu Sep 13 23:54:30 CES 2012
Signed-off-by: paurea@lsub.org

--- /sys/src/cmd/6c/reg.c	Thu Apr 12 12:25:19 2012
+++ /sys/src/cmd/6c/reg.c	Thu Sep 13 23:20:43 2012
@@ -50,6 +50,8 @@
 	lastr = R;
 	nvar = 0;
 	regbits = RtoB(D_SP) | RtoB(D_AX) | RtoB(D_X0);
+	if(REGEXT)
+		regbits |= RtoB(REGEXT) | RtoB(REGEXT-1);
 	for(z=0; z<BITS; z++) {
 		externs.b[z] = 0;
 		params.b[z] = 0;
--- /sys/src/cmd/6c/txt.c	Thu Apr 12 12:25:19 2012
+++ /sys/src/cmd/6c/txt.c	Thu Sep 13 23:20:55 2012
@@ -1,5 +1,7 @@
 #include "gc.h"
 
+static	int	resvreg[nelem(reg)];
+
 void
 ginit(void)
 {
@@ -94,6 +96,7 @@
 	if(0)
 		com64init();
 
+	memset(reg, 0, sizeof(reg));
 	for(i=0; i<nelem(reg); i++) {
 		reg[i] = 1;
 		if(i >= D_AX && i <= D_R15 && i != D_SP)
@@ -101,6 +104,10 @@
 		if(i >= D_X0 && i <= D_X7)
 			reg[i] = 0;
 	}
+	/* keep two external registers */
+	reg[REGEXT] = 1;
+	reg[REGEXT-1] = 1;
+	memmove(resvreg, reg, sizeof(resvreg));
 }
 
 void
@@ -111,10 +118,10 @@
 
 	reg[D_SP]--;
 	for(i=D_AX; i<=D_R15; i++)
-		if(reg[i])
+		if(reg[i] && !resvreg[i])
 			diag(Z, "reg %R left allocated", i);
 	for(i=D_X0; i<=D_X7; i++)
-		if(reg[i])
+		if(reg[i] && !resvreg[i])
 			diag(Z, "reg %R left allocated", i);
 	while(mnstring)
 		outstring("", 1L);
@@ -179,7 +186,7 @@
 
 	n = 0;
 	for(i=D_AX; i<=D_R15; i++)
-		if(reg[i] == 0)
+		if(reg[i] == 0 && !resvreg[i])
 			n++;
 	return n;
 }
@@ -337,7 +344,7 @@
 				goto out;
 		}
 		for(i=D_AX; i<=D_R15; i++)
-			if(reg[i] == 0)
+			if(reg[i] == 0 && !resvreg[i])
 				goto out;
 		diag(tn, "out of fixed registers");
 		goto err;
@@ -350,7 +357,7 @@
 				goto out;
 		}
 		for(i=D_X0; i<=D_X7; i++)
-			if(reg[i] == 0)
+			if(reg[i] == 0 && !resvreg[i])
 				goto out;
 		diag(tn, "out of float registers");
 		goto out;
--- /sys/src/cmd/cc/pswt.c	Thu Apr 12 12:25:26 2012
+++ /sys/src/cmd/cc/pswt.c	Thu Sep 13 23:44:05 2012
@@ -35,20 +35,14 @@
 		isv |= c->isv;
 		nc++;
 	}
-	if(typev[n->type->etype])
-		isv = 1;
-	else if(isv){
+	if(isv && !typev[n->type->etype])
 		warn(n, "32-bit switch expression with 64-bit case constant");
-		isv = 0;
-	}
 
 	iq = alloc(nc*sizeof(C1));
 	q = iq;
 	for(c = cases; c->link != C; c = c->link) {
 		if(c->def)
 			continue;
-		if(c->isv && !isv)
-			continue;	/* can never match */
 		q->label = c->label;
 		if(isv)
 			q->val = c->val;
@@ -118,7 +112,7 @@
 	}
 	patch(hsb, pc);
 if(0){for(int k=0; k<nh; k++)print("k*=%d h=%#llux lab=%ld\n", k, (vlong)iqh[k].val,  iqh[k].label);}
-	swit1(iqh, nh, def, vr[1]);
+	swit1(iq, nc, def, n);
 }
 
 void