powerpc: Fix logbl on power7 [BZ# 21280]

 1. Fix the results for negative subnormals by ignoring the signal when
    normalizing the value.
 2. Fix the output when the high part is a power of 2 and the low part
    is a nonzero number with opposite sign.  This fix is based on commit
    380bd0fd2418f8988217de950f8b8ff18af0cb2b.

After applying this patch, logbl() tests pass cleanly on POWER >= 7.

Tested on powerpc, powerpc64 and powerpc64le

	[BZ #21280]
	* sysdeps/powerpc/power7/fpu/s_logbl.c (__logbl): Ignore the
	signal of subnormals and adjust the exponent of power of 2 down
	when low part has opposite sign.

(cherry picked from commit c064f6a613844181f411aabb2662384a6aefb69e)

2 files changed, 25 insertions, 6 deletions

diff --git a/ChangeLog b/ChangeLog
index ff0853d3d2..b70096ae6d 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,10 @@
+2017-04-28  Tulio Magno Quites Machado Filho  <tuliom@linux.vnet.ibm.com>
+
+	[BZ #21280]
+	* sysdeps/powerpc/power7/fpu/s_logbl.c (__logbl): Ignore the
+	signal of subnormals and adjust the exponent of power of 2 down
+	when low part has opposite sign.
+
 2017-07-26  H.J. Lu  <hongjiu.lu@intel.com>
 
 	[BZ #21666]
diff --git a/sysdeps/powerpc/power7/fpu/s_logbl.c b/sysdeps/powerpc/power7/fpu/s_logbl.c
index f7ecbd105a..3ae383a831 100644
--- a/sysdeps/powerpc/power7/fpu/s_logbl.c
+++ b/sysdeps/powerpc/power7/fpu/s_logbl.c
@@ -35,14 +35,16 @@ static const union {
 long double
 __logbl (long double x)
 {
-  double xh;
+  double xh, xl;
   double ret;
+  int64_t hx;
 
   if (__builtin_expect (x == 0.0L, 0))
     /* Raise FE_DIVBYZERO and return -HUGE_VAL[LF].  */
     return -1.0L / __builtin_fabsl (x);
 
-  xh = ldbl_high (x);
+  ldbl_unpack (x, &xh, &xl);
+  EXTRACT_WORDS64 (hx, xh);
   /* ret = x & 0x7ff0000000000000;  */
   asm (
     "xxland %x0,%x1,%x2\n"
@@ -58,10 +60,20 @@ __logbl (long double x)
     {
       /* POSIX specifies that denormal number is treated as
          though it were normalized.  */
-      int64_t hx;
-
-      EXTRACT_WORDS64 (hx, xh);
-      return (long double) (-1023 - (__builtin_clzll (hx) - 12));
+      return (long double) (- (__builtin_clzll (hx & 0x7fffffffffffffffLL) \
+			       - 12) - 1023);
+    }
+  else if ((hx & 0x000fffffffffffffLL) == 0)
+    {
+      /* If the high part is a power of 2, and the low part is nonzero
+	 with the opposite sign, the low part affects the
+	 exponent.  */
+      int64_t lx, rhx;
+      EXTRACT_WORDS64 (lx, xl);
+      rhx = (hx & 0x7ff0000000000000LL) >> 52;
+      if ((hx ^ lx) < 0 && (lx & 0x7fffffffffffffffLL) != 0)
+	rhx--;
+      return (long double) (rhx - 1023);
     }
   /* Test to avoid logb_downward (0.0) == -0.0.  */
   return ret == -0.0 ? 0.0 : ret;