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vm: slightly faster +, - and idiv

--- a/src/flisp.h

+++ b/src/flisp.h

@@ -109,7 +109,7 @@

 #define iscons(x) (tag(x) == TAG_CONS)

 #define issymbol(x) (tag(x) == TAG_SYM)

 #define isfixnum(x) (((x)&3) == TAG_NUM)

-#define bothfixnums(x, y) ((((x)|(y)) & 3) == TAG_NUM)

+#define bothfixnums(x, y) (isfixnum(x) && isfixnum(y))

 #define isvector(x) (tag(x) == TAG_VECTOR)

 #define iscvalue(x) (tag(x) == TAG_CVALUE)

 #define iscprim(x) (tag(x) == TAG_CPRIM)

--- a/src/vm.inc

+++ b/src/vm.inc

@@ -205,6 +205,7 @@

 }

 OP(OP_ADD2) {

+	fixnum_t a, b, q;

 	value_t v;

 LABEL(do_add2):

 	*ipd = (uintptr_t)ip;

@@ -214,15 +215,17 @@

 		*ipd = (uintptr_t)ip;

 		v = sp[-1];

 		int64_t i64;

-		sp[-1] = isfixnum(v) ? fixnum_neg(v) : fl_neg(v);

+		b = isfixnum(v) ? fixnum_neg(v) : fl_neg(v);

+	}else{

+		b = sp[-1];

 	}

-	value_t a, b, q;

 	a = sp[-2];

-	b = sp[-1];

-	if(bothfixnums(a, b) && !sadd_overflow(numval(a), numval(b), &q) && fits_fixnum(q))

-		v = fixnum(q);

-	else

+	if(bothfixnums(a, b) && !sadd_overflow(a, b, &q))

+		v = q;

+	else{

+		sp[-1] = b;

 		v = fl_add_any(sp-2, 2);

+	}

 	sp--;

 	sp[-1] = v;

 	NEXT_OP;

@@ -770,15 +773,15 @@

 OP(OP_IDIV) {

 	value_t a = sp[-2];

 	value_t b = sp[-1];

+	if(fl_unlikely(b == 0)){

+		*ipd = (uintptr_t)ip;

+		FL(sp) = sp;

+		divide_by_0_error();

+	}

 	value_t v;

-	if(bothfixnums(a, b)){

-		if(b == 0){

-			*ipd = (uintptr_t)ip;

-			FL(sp) = sp;

-			divide_by_0_error();

-		}

-		v = fixnum(numval(a) / numval(b));

-	}else{

+	if(bothfixnums(a, b))

+		v = fixnum((fixnum_t)a / (fixnum_t)b);

+	else{

 		*ipd = (uintptr_t)ip;

 		v = fl_idiv2(a, b);

 	}