Re: -ffast-math and floating point reordering

[Dale Johannesen <dalej@apple.com>]

On Mar 26, 2004, at 4:41 PM, Robert Dewar wrote:
> Dale Johannesen wrote:
>> One point I haven't seen yet:
>> A couple of strict-constructionists have asserted that people
>> who want reassociation done in a nonstandard way can easily rewrite 
>> their
>> code to achieve this.  This is not really true, because such 
>> expressions
>> often arise as a result of other optimizations, in a way that isn't 
>> obvious
>> from the source.  Offhand, inlining, unrolling, and constant/copy 
>> propagation
>> can all do it, and there are probably more.
>
> Well in the absence of the (very annoying) extra precision phenomenon,
> none of these optimizations should have observable effects.

Untrue.  If two of a+b+c are constant, for example, you want to add the
constants at compile time (assuming your requirements for accuracy
are satisfied).  This is exactly the sort of thing that's exposed by 
other
optimiziations.