to make it easy to use normal libraries (such as libdraw, libsec, and libmp)
with the kernel, which uses extern register, don't stray into the external
register set when allocating values to registers.

Reference: /n/sources/patch/applied/6c-extreg
Date: Thu Sep 13 16:22:58 CES 2012
Signed-off-by: charles.forsyth@gmail.com

--- /sys/src/cmd/6c/reg.c	Thu Sep 13 16:22:05 2012
+++ /sys/src/cmd/6c/reg.c	Thu Sep 13 16:22:03 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 Sep 13 16:22:10 2012
+++ /sys/src/cmd/6c/txt.c	Thu Sep 13 16:22:07 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;