math: add fma TODO comments about the underflow issue

[musl] / src / math / fmal.c

diff --git a/src/math/fmal.c b/src/math/fmal.c
index 3944c29..87e30fc 100644 (file)
--- a/src/math/fmal.c
+++ b/src/math/fmal.c
@@ -115,7 +115,7 @@ static inline long double add_and_denormalize(long double a, long double b, int
                if (bits_lost != 1 ^ (int)(u.bits.manl & 1))
                        sum.hi = nextafterl(sum.hi, INFINITY * sum.lo);
        }
-       return (ldexp(sum.hi, scale));
+       return scalbnl(sum.hi, scale);
 }
 
 /*
@@ -162,6 +162,7 @@ static inline struct dd dd_mul(long double a, long double b)
  */
 long double fmal(long double x, long double y, long double z)
 {
+       #pragma STDC FENV_ACCESS ON
        long double xs, ys, zs, adj;
        struct dd xy, r;
        int oround;
@@ -173,14 +174,14 @@ long double fmal(long double x, long double y, long double z)
         * return values here are crucial in handling special cases involving
         * infinities, NaNs, overflows, and signed zeroes correctly.
         */
-       if (x == 0.0 || y == 0.0)
-               return (x * y + z);
-       if (z == 0.0)
-               return (x * y);
        if (!isfinite(x) || !isfinite(y))
                return (x * y + z);
        if (!isfinite(z))
                return (z);
+       if (x == 0.0 || y == 0.0)
+               return (x * y + z);
+       if (z == 0.0)
+               return (x * y);
 
        xs = frexpl(x, &ex);
        ys = frexpl(y, &ey);
@@ -228,7 +229,7 @@ long double fmal(long double x, long double y, long double z)
                }
        }
        if (spread <= LDBL_MANT_DIG * 2)
-               zs = ldexpl(zs, -spread);
+               zs = scalbnl(zs, -spread);
        else
                zs = copysignl(LDBL_MIN, zs);
 
@@ -254,23 +255,25 @@ long double fmal(long double x, long double y, long double z)
                 */
                fesetround(oround);
                volatile long double vzs = zs; /* XXX gcc CSE bug workaround */
-               return (xy.hi + vzs + ldexpl(xy.lo, spread));
+               return xy.hi + vzs + scalbnl(xy.lo, spread);
        }
 
        if (oround != FE_TONEAREST) {
                /*
                 * There is no need to worry about double rounding in directed
                 * rounding modes.
+                * TODO: underflow is not raised correctly, example in downward rounding:
+                * fmal(0x1.0000000001p-16000L, 0x1.0000000001p-400L, -0x1p-16440L)
                 */
                fesetround(oround);
                adj = r.lo + xy.lo;
-               return (ldexpl(r.hi + adj, spread));
+               return scalbnl(r.hi + adj, spread);
        }
 
        adj = add_adjusted(r.lo, xy.lo);
        if (spread + ilogbl(r.hi) > -16383)
-               return (ldexpl(r.hi + adj, spread));
+               return scalbnl(r.hi + adj, spread);
        else
-               return (add_and_denormalize(r.hi, adj, spread));
+               return add_and_denormalize(r.hi, adj, spread);
 }
 #endif