Fix pow missing underflows (bug 18825).

Similar to various other bugs in this area, pow functions can fail to
raise the underflow exception when the result is tiny and inexact but
one or more low bits of the intermediate result that is scaled down
(or, in the i386 case, converted from a wider evaluation format) are
zero.  This patch forces the exception in a similar way to previous
fixes, thereby concluding the fixes for known bugs with missing
underflow exceptions currently filed in Bugzilla.

Tested for x86_64, x86, mips64 and powerpc.

	[BZ #18825]
	* sysdeps/i386/fpu/i386-math-asm.h (FLT_NARROW_EVAL_UFLOW_NONNAN):
	New macro.
	(DBL_NARROW_EVAL_UFLOW_NONNAN): Likewise.
	(LDBL_CHECK_FORCE_UFLOW_NONNAN): Likewise.
	* sysdeps/i386/fpu/e_pow.S: Use DEFINE_DBL_MIN.
	(__ieee754_pow): Use DBL_NARROW_EVAL_UFLOW_NONNAN instead of
	DBL_NARROW_EVAL, reloading the PIC register as needed.
	* sysdeps/i386/fpu/e_powf.S: Use DEFINE_FLT_MIN.
	(__ieee754_powf): Use FLT_NARROW_EVAL_UFLOW_NONNAN instead of
	FLT_NARROW_EVAL.  Use separate return path for case when first
	argument is NaN.
	* sysdeps/i386/fpu/e_powl.S: Include <i386-math-asm.h>.  Use
	DEFINE_LDBL_MIN.
	(__ieee754_powl): Use LDBL_CHECK_FORCE_UFLOW_NONNAN, reloading the
	PIC register.
	* sysdeps/ieee754/dbl-64/e_pow.c (__ieee754_pow): Use
	math_check_force_underflow_nonneg.
	* sysdeps/ieee754/flt-32/e_powf.c (__ieee754_powf): Force
	underflow for subnormal result.
	* sysdeps/ieee754/ldbl-128/e_powl.c (__ieee754_powl): Likewise.
	* sysdeps/ieee754/ldbl-128ibm/e_powl.c (__ieee754_powl): Use
	math_check_force_underflow_nonneg.
	* sysdeps/x86/fpu/powl_helper.c (__powl_helper): Use
	math_check_force_underflow.
	* sysdeps/x86_64/fpu/x86_64-math-asm.h
	(LDBL_CHECK_FORCE_UFLOW_NONNAN): New macro.
	* sysdeps/x86_64/fpu/e_powl.S: Include <x86_64-math-asm.h>.  Use
	DEFINE_LDBL_MIN.
	(__ieee754_powl): Use LDBL_CHECK_FORCE_UFLOW_NONNAN.
	* math/auto-libm-test-in: Add more tests of pow.
	* math/auto-libm-test-out: Regenerated.

4 files changed, 14 insertions, 2 deletions

diff --git a/sysdeps/ieee754/dbl-64/e_pow.c b/sysdeps/ieee754/dbl-64/e_pow.c
index 3c027fe7f5..3b0bbe3103 100644
--- a/sysdeps/ieee754/dbl-64/e_pow.c
+++ b/sysdeps/ieee754/dbl-64/e_pow.c
@@ -119,6 +119,8 @@ __ieee754_pow (double x, double y)
 	retval = huge * huge;
       else if (retval == 0)
 	retval = tiny * tiny;
+      else
+	math_check_force_underflow_nonneg (retval);
       return retval;
     }
 
diff --git a/sysdeps/ieee754/flt-32/e_powf.c b/sysdeps/ieee754/flt-32/e_powf.c
index 8e8d918456..18042960c2 100644
--- a/sysdeps/ieee754/flt-32/e_powf.c
+++ b/sysdeps/ieee754/flt-32/e_powf.c
@@ -244,7 +244,12 @@ __ieee754_powf(float x, float y)
 	z  = one-(r-z);
 	GET_FLOAT_WORD(j,z);
 	j += (n<<23);
-	if((j>>23)<=0) z = __scalbnf(z,n);	/* subnormal output */
+	if((j>>23)<=0)	/* subnormal output */
+	  {
+	    z = __scalbnf (z, n);
+	    float force_underflow = z * z;
+	    math_force_eval (force_underflow);
+	  }
 	else SET_FLOAT_WORD(z,j);
 	return s*z;
 }
diff --git a/sysdeps/ieee754/ldbl-128/e_powl.c b/sysdeps/ieee754/ldbl-128/e_powl.c
index f531385232..7f3037fb51 100644
--- a/sysdeps/ieee754/ldbl-128/e_powl.c
+++ b/sysdeps/ieee754/ldbl-128/e_powl.c
@@ -435,7 +435,11 @@ __ieee754_powl (long double x, long double y)
   j = o.parts32.w0;
   j += (n << 16);
   if ((j >> 16) <= 0)
-    z = __scalbnl (z, n);	/* subnormal output */
+    {
+      z = __scalbnl (z, n);	/* subnormal output */
+      long double force_underflow = z * z;
+      math_force_eval (force_underflow);
+    }
   else
     {
       o.parts32.w0 = j;
diff --git a/sysdeps/ieee754/ldbl-128ibm/e_powl.c b/sysdeps/ieee754/ldbl-128ibm/e_powl.c
index c942f2f249..90340e890e 100644
--- a/sysdeps/ieee754/ldbl-128ibm/e_powl.c
+++ b/sysdeps/ieee754/ldbl-128ibm/e_powl.c
@@ -409,6 +409,7 @@ __ieee754_powl (long double x, long double y)
   r = (z * t1) / (t1 - two) - (w + z * w);
   z = one - (r - z);
   z = __scalbnl (z, n);
+  math_check_force_underflow_nonneg (z);
   return s * z;
 }
 strong_alias (__ieee754_powl, __powl_finite)