From 89996e1fb1b3834746340570c2d1b48ef9fc1b9b Mon Sep 17 00:00:00 2001
From: Ken Thompson <ken@golang.org>
Date: Tue, 2 Dec 2008 17:03:47 -0800
Subject: [PATCH] truncating of float constants when used in float64 or float32
 contexts

R=r
OCL=20297
CL=20297
---
 src/cmd/gc/const.c    | 53 +++++++++++++++++++++++++++++--------------
 src/cmd/gc/mparith3.c | 15 ++++++++----
 2 files changed, 46 insertions(+), 22 deletions(-)

diff --git a/src/cmd/gc/const.c b/src/cmd/gc/const.c
index 3a3fef191d2..ad8607794e4 100644
--- a/src/cmd/gc/const.c
+++ b/src/cmd/gc/const.c
@@ -5,6 +5,34 @@
 #include	"go.h"
 #define	TUP(x,y)	(((x)<<16)|(y))
 
+void
+truncfltlit(Mpflt *fv, Type *t)
+{
+	double d;
+	float f;
+
+	if(t == T)
+		return;
+
+	// convert large precision literal floating
+	// into limited precision (float64 or float32)
+	// botch -- this assumes that compiler fp
+	//    has same precision as runtime fp
+	switch(t->etype) {
+	case TFLOAT64:
+		d = mpgetflt(fv);
+		mpmovecflt(fv, d);
+		break;
+
+	case TFLOAT32:
+		d = mpgetflt(fv);
+		f = d;
+		d = f;
+		mpmovecflt(fv, d);
+		break;
+	}
+}
+
 void
 convlit(Node *n, Type *t)
 {
@@ -90,15 +118,18 @@ convlit(Node *n, Type *t)
 		if(isfloat[et]) {
 			// int to float
 			Mpint *xv;
+			Mpflt *fv;
 
 			xv = n->val.u.xval;
 			if(mpcmpfixflt(xv, minfltval[et]) < 0)
 				goto bad2;
 			if(mpcmpfixflt(xv, maxfltval[et]) > 0)
 				goto bad2;
-			n->val.u.fval = mal(sizeof(*n->val.u.fval));
-			mpmovefixflt(n->val.u.fval, xv);
+			fv = mal(sizeof(*n->val.u.fval));
+			n->val.u.fval = fv;
+			mpmovefixflt(fv, xv);
 			n->val.ctype = CTFLT;
+			truncfltlit(fv, t);
 			break;
 		}
 		goto bad1;
@@ -126,8 +157,6 @@ convlit(Node *n, Type *t)
 		}
 		if(isfloat[et]) {
 			// float to float
-			double d;
-			float f;
 			Mpflt *fv;
 
 			fv = n->val.u.fval;
@@ -135,24 +164,13 @@ convlit(Node *n, Type *t)
 				goto bad2;
 			if(mpcmpfltflt(fv, maxfltval[et]) > 0)
 				goto bad2;
-//			switch(et) {
-//			case TFLOAT64:
-//				d = mpgetflt(fv);
-//				mpmovecflt(fv, d);
-//				break;
-//
-//			case TFLOAT32:
-//				d = mpgetflt(fv);
-//				f = d;
-//				d = f;
-//				mpmovecflt(fv, d);
-//				break;
-//			}
+			truncfltlit(fv, t);
 			break;
 		}
 		goto bad1;
 	}
 	n->type = t;
+	
 	return;
 
 bad1:
@@ -442,6 +460,7 @@ ret:
 	} else
 	if(wl == Wlitfloat) {
 		n->val.u.fval = fval;
+		truncfltlit(fval, n->type);
 	}
 }
 
diff --git a/src/cmd/gc/mparith3.c b/src/cmd/gc/mparith3.c
index 1bf39c9fb43..7098ba68b24 100644
--- a/src/cmd/gc/mparith3.c
+++ b/src/cmd/gc/mparith3.c
@@ -176,11 +176,15 @@ mpgetflt(Mpflt *a)
 		mpnorm(a);
 	}
 
-	while((a->val.a[Mpnorm-1] & (1L<<(Mpscale-1))) == 0) {
+	while((a->val.a[Mpnorm-1] & Mpsign) == 0) {
 		mpshiftfix(&a->val, 1);
 		a->exp -= 1;
 	}
 
+	// the magic numbers (64, 63, 53, 10) are
+	// IEEE specific. this should be done machine
+	// independently or in the 6g half of the compiler
+
 	// pick up the mantissa in a uvlong
 	s = 63;
 	v = 0;
@@ -191,13 +195,14 @@ mpgetflt(Mpflt *a)
 	if(s > 0)
 		v = (v<<s) | (a->val.a[i]>>(Mpscale-s));
 
-	// should do this in multi precision
 	// 63 bits of mantissa being rounded to 53
+	// should do this in multi precision
 	if((v&0x3ffULL) != 0x200ULL || (v&0x400) != 0)
-		v += 0x200ULL;		// round
-	v &= ~0x3ffULL;
+		v += 0x200ULL;		// round toward even
+
+	v >>= 10;
 	f = (double)(v);
-	f = ldexp(f, Mpnorm*Mpscale + a->exp - 63);
+	f = ldexp(f, Mpnorm*Mpscale + a->exp - 53);
 
 	if(a->val.neg)
 		f = -f;