diff options
Diffstat (limited to 'src/fma.c')
| -rw-r--r-- | src/fma.c | 8 |
1 files changed, 5 insertions, 3 deletions
@@ -234,7 +234,7 @@ mpfr_fma (mpfr_ptr s, mpfr_srcptr x, mpfr_srcptr y, mpfr_srcptr z, /* Let's eliminate the obvious case where x*y and z have the same sign. No possible cancellation -> real overflow. Also, we know that |z| < 2^emax. If E(x) + E(y) >= emax+3, - then |x*y| >= 2^(emax+1), and |x*y + z| >= 2^emax. This case + then |x*y| >= 2^(emax+1), and |x*y + z| > 2^emax. This case is also an overflow. */ if (MPFR_SIGN (u) == MPFR_SIGN (z) || e >= __gmpfr_emax + 3) { @@ -253,7 +253,8 @@ mpfr_fma (mpfr_ptr s, mpfr_srcptr x, mpfr_srcptr y, mpfr_srcptr z, inexact = mpfr_mul (u, u, y, MPFR_RNDN); MPFR_ASSERTN (inexact == 0); - /* Now, we need to add z/4... But it may underflow! */ + /* Now, we need to add z/4, which is in fact a subtraction since + x*y and z have different signs. But it may underflow! */ { mpfr_t zo4; mpfr_srcptr zz; @@ -262,7 +263,8 @@ mpfr_fma (mpfr_ptr s, mpfr_srcptr x, mpfr_srcptr y, mpfr_srcptr z, if (MPFR_GET_EXP (u) > MPFR_GET_EXP (z) && MPFR_GET_EXP (u) - MPFR_GET_EXP (z) > MPFR_PREC (u)) { - /* |z| < ulp(u)/2, therefore one can use z instead of z/4. */ + /* |z| < ulp(u)/2, therefore one can use z instead of z/4, + except maybe when u is a power of 2! */ zz = z; } else |