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

["athena at fftw dot org" <gcc-bugzilla at gcc dot gnu dot org>]

------- Additional Comments From athena at fftw dot org  2005-02-16 20:37 -------
(In reply to comment #3)
> Ok, this is the patch which caused this code generation regression:
> 2004-02-07  Roger Sayle  <roger@eyesopen.com>
> 
>         * fold-const.c (negate_expr_p, negate_expr): Optimize -(A+B) into
>         either (-A)-B or (-B)-A, if A or B is easily negated respectively.
>         (fold) <MINUS_EXPR>: Optimize (A*C) - (B*C) -> (A-B)*C for both
>         integer types and floating point with unsafe_math_optimizations.
>         Add similar optimization for (A*C1) - (A*C2) -> A*(C1-C2).
>         Optimize A - B as A + (-B), if B is easily negated.

Note that the straightforward transformation -(A+B) => (-A)-B is illegal
according to the ieee-754 standard.  For A=-0 and B=+0, -(A+B) = -(+0) = -0,
whereas (-A)-B = +0 - +0 = +0 + -0 = +0.

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