libgcc2 __fixsfdi

[Alan Modra <amodra@bigpond.net.au>]

gcc's __fixsfdi function returns extremely surprising results for
floats that overflow the range of DWtype.  The integer value
wraps rather than limiting to min/max values.  I know that return
values for the overflow case are unspecified, even in current
draft ieee-754, but "each of the operations shall be performed as if
it first produced an intermediate result correct to infinite precision
and with unbounded range, and then coerced this intermediate result to
fit in the destination's format" could be taken as support for this
patch.  After all, the nearest values that the intermediate result can
be coerced to in case of overflow, are the min and max integers.  Also
see http://754r.ucbtest.org/proposals/integer/integer.html where this
behaviour is specified.

If the following is reasonable, I'll do the same for __fixtfdi,
__fixxfdi and __fixdfdi, and submit a proper patch with changelog.

Index: gcc/libgcc2.c
===================================================================
RCS file: /cvs/gcc/gcc/gcc/libgcc2.c,v
retrieving revision 1.149
diff -u -p -r1.149 libgcc2.c
--- gcc/libgcc2.c	8 Sep 2002 12:47:26 -0000	1.149
+++ gcc/libgcc2.c	4 Oct 2002 14:25:34 -0000
@@ -994,12 +994,27 @@ __fixunssfDI (SFtype original_a)
 #endif
 
 #ifdef L_fixsfdi
+#define DWORD_SIZE (sizeof (DWtype) * BITS_PER_UNIT)
+#define MAXDWtype ((((DWtype) 1) << (DWORD_SIZE-1))-1)
+
 DWtype
 __fixsfdi (SFtype a)
 {
+  DWtype tmp;
+
   if (a < 0)
-    return - __fixunssfDI (-a);
-  return __fixunssfDI (a);
+    {
+      tmp = __fixunssfDI (-a);
+      tmp = -tmp;
+      if (tmp > 0)
+	tmp = -MAXDWtype - 1;
+      return tmp;
+    }
+
+  tmp = __fixunssfDI (a);
+  if (tmp < 0)
+    tmp = MAXDWtype;
+  return tmp;
 }
 #endif
 

-- 
Alan Modra
IBM OzLabs - Linux Technology Centre