reserve two registers for extern register use, and ensure that
neither they nor any other special register can be allocated
to temporaries or variables

also use temporary registers cyclically even though `strongarm does register forwarding'
because it will forward without that, and not all arms are strongarms.

Reference: /n/sources/patch/applied/5c-extreg
Date: Wed Jul  4 17:53:21 CES 2007
Signed-off-by: forsyth@terzarima.net

--- /sys/src/cmd/5c/txt.c	Wed Jul  4 17:50:04 2007
+++ /sys/src/cmd/5c/txt.c	Wed Jul  4 17:50:00 2007
@@ -1,5 +1,7 @@
 #include "gc.h"
 
+static	char	resvreg[nelem(reg)];
+
 void
 ginit(void)
 {
@@ -80,6 +82,16 @@
 	com64init();
 
 	memset(reg, 0, sizeof(reg));
+	/* don't allocate */
+	reg[REGTMP] = 1;
+	reg[REGSB] = 1;
+	reg[REGSP] = 1;
+	reg[REGLINK] = 1;
+	reg[REGPC] = 1;
+	/* keep two external registers */
+	reg[REGEXT] = 1;
+	reg[REGEXT-1] = 1;
+	memmove(resvreg, reg, sizeof(reg));
 }
 
 void
@@ -89,10 +101,10 @@
 	Sym *s;
 
 	for(i=0; i<NREG; i++)
-		if(reg[i])
+		if(reg[i] && !resvreg[i])
 			diag(Z, "reg %d left allocated", i);
 	for(i=NREG; i<NREG+NFREG; i++)
-		if(reg[i])
+		if(reg[i] && !resvreg[i])
 			diag(Z, "freg %d left allocated", i-NREG);
 	while(mnstring)
 		outstring("", 1L);
@@ -283,7 +295,7 @@
 		for(i=REGRET+1; i<NREG; i++) {
 			if(j >= NREG)
 				j = REGRET+1;
-			if(reg[j] == 0) {
+			if(reg[j] == 0 && resvreg[j] == 0) {
 				i = j;
 				goto out;
 			}
@@ -319,7 +331,7 @@
 	return;
 out:
 	reg[i]++;
-/* 	lasti++;	*** StrongARM does register forwarding */	
+	lasti++;
 	if(lasti >= 5)
 		lasti = 0;
 	nodreg(n, tn, i);
@@ -1184,9 +1196,12 @@
 	long o;
 
 	if(typechlp[t->etype]) {
-		if(exregoffset <= REGEXT-4)
+		if(exregoffset <= REGEXT-2)
 			return 0;
 		o = exregoffset;
+		if(reg[o] && !resvreg[o])
+			return 0;
+		resvreg[o] = reg[o] = 1;
 		exregoffset--;
 		return o;
 	}
@@ -1194,6 +1209,9 @@
 		if(exfregoffset <= NFREG-1)
 			return 0;
 		o = exfregoffset + NREG;
+		if(reg[o] && !resvreg[o])
+			return 0;
+		resvreg[o] = reg[o] = 1;
 		exfregoffset--;
 		return o;
 	}