From 95b93c28e395e4ea88ad831e87736c7ef75eb734 Mon Sep 17 00:00:00 2001
From: Ken Thompson <ken@golang.org>
Date: Sat, 17 Jul 2010 16:32:40 -0700
Subject: [PATCH] 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 | 34 ++++++++++++++++++++++++++++++++--
 src/cmd/gc/mparith2.c |  6 +++++-
 src/cmd/gc/mparith3.c |  2 +-
 3 files changed, 38 insertions(+), 4 deletions(-)

diff --git a/src/cmd/gc/mparith1.c b/src/cmd/gc/mparith1.c
index 8110e77b98..14226d6a9a 100644
--- a/src/cmd/gc/mparith1.c
+++ b/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)
 {
diff --git a/src/cmd/gc/mparith2.c b/src/cmd/gc/mparith2.c
index b025917fa8..67375adb17 100644
--- a/src/cmd/gc/mparith2.c
+++ 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);
diff --git a/src/cmd/gc/mparith3.c b/src/cmd/gc/mparith3.c
index b9cd4ea847..5ee8b0308f 100644
--- a/src/cmd/gc/mparith3.c
+++ b/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)
-- 
2.48.1