This patch workarounds a GCC bug on PowerPC 32-bit, converting 0 from
int to float might return -0.0. Upstream consider the bug fixed, as it
doesn't affect GCC built for PPC970/Power4 or later. It is clearly 
something we don't want in Debian.

2012-07-22  Aurelien Jarno  <aurelien@aurel32.net>

	* sysdeps/ieee754/dbl-64/s_logb.c (__logb): avoid logb_downward (0.0)
	return -0.0.
	* sysdeps/ieee754/flt-32/s_logbf.c (__logbf): Likewise.
	* sysdeps/ieee754/ldbl-128ibm/s_logbl.c (__logbl): Likewise.

diff --git a/sysdeps/ieee754/dbl-64/s_logb.c b/sysdeps/ieee754/dbl-64/s_logb.c
index 17aa94b..d7fd59d 100644
--- a/sysdeps/ieee754/dbl-64/s_logb.c
+++ b/sysdeps/ieee754/dbl-64/s_logb.c
@@ -23,6 +23,7 @@ double
 __logb (double x)
 {
   int32_t lx, ix, rix;
+  double ret;
 
   EXTRACT_WORDS (ix, lx, x);
   ix &= 0x7fffffff;             /* high |x| */
@@ -41,7 +42,9 @@ __logb (double x)
 	ma = __builtin_clz (ix);
       rix -= ma - 12;
     }
-  return (double) (rix - 1023);
+  ret = (double) (rix - 1023);
+  /* The test is to avoid logb_downward (0.0) == -0.0.  */
+  return ret == -0.0 ? 0.0 : ret;
 }
 weak_alias (__logb, logb)
 #ifdef NO_LONG_DOUBLE
diff --git a/sysdeps/ieee754/flt-32/s_logbf.c b/sysdeps/ieee754/flt-32/s_logbf.c
index e2b3aaa..1dce251 100644
--- a/sysdeps/ieee754/flt-32/s_logbf.c
+++ b/sysdeps/ieee754/flt-32/s_logbf.c
@@ -20,6 +20,7 @@ float
 __logbf (float x)
 {
   int32_t ix, rix;
+  float ret;
 
   GET_FLOAT_WORD (ix, x);
   ix &= 0x7fffffff;		/* high |x| */
@@ -33,6 +34,8 @@ __logbf (float x)
          though it were normalized.  */
       rix -= __builtin_clz (ix) - 9;
     }
-  return (float) (rix - 127);
+  ret = (float) (rix - 127);
+  /* The test is to avoid logb_downward (0.0) == -0.0.  */
+  return ret == -0.0 ? 0.0 : ret;
 }
 weak_alias (__logbf, logbf)
diff --git a/sysdeps/ieee754/ldbl-128ibm/s_logbl.c b/sysdeps/ieee754/ldbl-128ibm/s_logbl.c
index 92ce2c1..db030a7 100644
--- a/sysdeps/ieee754/ldbl-128ibm/s_logbl.c
+++ b/sysdeps/ieee754/ldbl-128ibm/s_logbl.c
@@ -27,6 +27,7 @@ long double
 {
   int64_t hx, rhx;
   double xhi;
+  long double ret;

   xhi = ldbl_high (x);
   EXTRACT_WORDS64 (hx, xhi);
@@ -40,7 +41,9 @@ __logbl (long double x)
          though it were normalized.  */
       rhx -= __builtin_clzll (hx) - 12;
     }
-  return (long double) (rhx - 1023);
+  ret = (long double) (rhx - 1023);
+  /* The test is to avoid logb_downward (0.0) == -0.0.  */
+  return ret == -0.0 ? 0.0 : ret;
 }
 #ifndef __logbl
 long_double_symbol (libm, __logbl, logbl);