[Bug middle-end/19988] [4.0 Regression] pessimizes fp multiply-add/subtract combo

["roger at eyesopen dot com" <gcc-bugzilla@gcc.gnu.org>]

------- Additional Comments From roger at eyesopen dot com  2005-02-19 05:41 -------
Re: comment #5
For floating point expressions, -(A+B) is only transformed into (-A)-B or
(-B)-A when the user explicitly specifies -ffast-math, i.e. only when
flag_unsafe_math_optimizations is true.

Re: comment #6
Interesting.  Although on a handful of rs6000 cores (mpccore, 601 and 603),
a fused-multiply-add is more expensive that an addition, its always a win
to perform two fma's rather than a mult and two adds.  It might be possible
(with some work) to teach combine to un-CSE the following:

double x;
double y;

void foo(double p, double q, double r, double s)
{
  double t = p * q;
  x = t + r;
  y = t + s;
}


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19988