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

[athena at fftw dot org]

------- Additional Comments From athena at fftw dot org  2005-02-16 20:44 -------
> Unfortunately, I doubt whether it'll be possible to siumultaneously address
> this performance regression without reintroducing the 3.x issue mentioned in
> the original "PS".  I doubt on many platforms a two multiply-adds are much
> faster than a single floating point multiplication whose result is shared by
> two additions.  Though again it might be possible to do something at the RTL
> level, especially if duplicating the multiplication is a win with -Os.

PowerPC is indeed a platform where an addition costs the same as a
multiplication and the same as a fused multiply-add.  The ia64 FPU does FMA's
only; you code A*B as A*B+(-0), and A+B as A*1+B.  (On a related
matter, altivec has FMA but not multiplication, and the same trick
applies.)

Bottom line: gcc should make an effort to respect FMAs, at least when
they appear explicitly in the source code.

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