1. got 29 (Mpscale) more bits of precision

out of floating constant multiply
2. added rounding code to "const fix=float"
to allow up to 29 (Mpscale) bits of
slop and still get an exact fixed constant.

fixes #931

R=rsc
CC=golang-dev
https://golang.org/cl/1692055

src/cmd/gc/mparith1.c

@@ -156,10 +156,11 @@ mpmovefixflt(Mpflt *a, Mpint *b)
 
 // convert (truncate) b to a.
 // return -1 (but still convert) if b was non-integer.
-int
-mpmovefltfix(Mpint *a, Mpflt *b)
+static int
+mpexactfltfix(Mpint *a, Mpflt *b)
 {
 	Mpflt f;
+
 	*a = b->val;
 	mpshiftfix(a, b->exp);
 	if(b->exp < 0) {
@@ -172,6 +173,35 @@ mpmovefltfix(Mpint *a, Mpflt *b)
 	return 0;
 }
 
+int
+mpmovefltfix(Mpint *a, Mpflt *b)
+{
+	Mpflt f;
+	int i;
+
+	if(mpexactfltfix(a, b) == 0)
+		return 0;
+
+	// try rounding down a little
+	f = *b;
+	f.val.a[0] = 0;
+	if(mpexactfltfix(a, &f) == 0)
+		return 0;
+
+	// try rounding up a little
+	for(i=1; i<Mpprec; i++) {
+		f.val.a[i]++;
+		if(f.val.a[i] != Mpbase)
+			break;
+		f.val.a[i] = 0;
+	}
+	mpnorm(&f);
+	if(mpexactfltfix(a, &f) == 0)
+		return 0;
+
+	return -1;
+}
+
 void
 mpmovefixfix(Mpint *a, Mpint *b)
 {

src/cmd/gc/mparith2.c

@@ -319,7 +319,11 @@ mpmulfract(Mpint *a, Mpint *b)
 	s.neg = 0;
 	mpmovecfix(&q, 0);
 
-	for(i=0; i<Mpprec; i++) {
+	x = *--a1;
+	if(x != 0)
+		yyerror("mpmulfract not normal");
+
+	for(i=0; i<Mpprec-1; i++) {
 		x = *--a1;
 		if(x == 0) {
 			mprshw(&s);

src/cmd/gc/mparith3.c

@@ -109,7 +109,7 @@ mpmulfltflt(Mpflt *a, Mpflt *b)
 	}
 
 	mpmulfract(&a->val, &b->val);
-	a->exp = (a->exp + b->exp) + Mpscale*Mpprec - 1;
+	a->exp = (a->exp + b->exp) + Mpscale*Mpprec - Mpscale - 1;
 
 	mpnorm(a);
 	if(Mpdebug)