PATCH - [tree-ssa] regrouping of expression tree for single multiply add.

[Bradley Lucier]

In

http://gcc.gnu.org/ml/gcc/2004-03/msg01426.html

Ian suggests

> My vote, at least right now, would be to add -fassociative-math to
> mean that gcc is permitted to reassociate math expressions at will,
> regardless of the presence of parentheses.  And -ffast-math should
> imply -fassociative-math.  That is, with -ffast-math, (a+b)+c may be
> evaluated as a+(b+c).

I would like to second that suggestion, not because -fassociative-math 
is a particularly bad option for many algorithms, but that it's a 
particularly *benign* option for many programs.  For example, the usual 
error analysis for numerical linear algebra programs does not depend on 
the order that the additions are performed in; and Colin Percival's 
beautiful analysis of the FFT using floating point arithmetic is not 
invalidated by re-association (while it depends critically on the 
accuracy of the sin/cos computations used).

So, now, I'd like to be able to use this flag in places where I cannot 
otherwise use -funsafe-math-optimizations.  And in the future, will 
there not be pragmas that one can use to designate that certain blocks 
of code will be compiled with certain floating-point properties?  If 
so, this is a great property to put around a block of code where 
re-association doesn't matter.

Brad