math: fix pow(x,-1) to raise underflow properly

if FLT_EVAL_METHOD!=0 check if (double)(1/x) is subnormal and not a
power of 2 (if 1/x is power of 2 then either it is exact or the
long double to double rounding already raised inexact and underflow)

diff --git a/src/math/pow.c b/src/math/pow.c
index ac3abc0fdbb81b4d4d4f082ac62f6521a920e6cb..82f684bdb0e363c6846d0f7dde445cf4b5818618 100644 (file)
--- a/src/math/pow.c
+++ b/src/math/pow.c
@@ -146,8 +146,20 @@ double pow(double x, double y)
                        else if ((ix|lx) != 0)     /* (|x|<1)**+-inf = 0,inf if x!=0 */
                                return hy >= 0 ? 0.0 : -y;
                }
-               if (iy == 0x3ff00000)    /* y is +-1 */
-                       return hy >= 0 ? x : 1.0/x;
+               if (iy == 0x3ff00000) {    /* y is +-1 */
+                       if (hy >= 0)
+                               return x;
+                       y = 1/x;
+#if FLT_EVAL_METHOD!=0
+                       {
+                               union {double f; uint64_t i;} u = {y};
+                               uint64_t i = u.i & -1ULL/2;
+                               if (i>>52 == 0 && (i&(i-1)))
+                                       FORCE_EVAL((float)y);
+                       }
+#endif
+                       return y;
+               }
                if (hy == 0x40000000)    /* y is 2 */
                        return x*x;
                if (hy == 0x3fe00000) {  /* y is 0.5 */