Re: Floating poing 'non-bug'? Or is this a bug?

[Jim Wilson <wilson at specifixinc dot com>]

Yuval Kfir wrote:
> The short program below gives '5*1.2=5' when compiled
> without -DDOUBLE_INTER, but '5*1.2=6' with the double-precision intermediate
> variable.

This is yet another instance of the "excess precision" problem.  This is 
partly a problem with the x86 FPU design which provides only 
double-extended precision operations. And this is partly a problem with 
the design of the x86 gcc backend, which lies, and claims that we have 
float and double precision operations.

You should get the same result in both cases, but you get a different 
result for one because an intermediate value is written to the stack 
(fstpl) which rounds it to a double, and then loaded back (fldl).  In 
the other case, the intermediate value is not written to the stack, and 
remains a double-extended value with excess precision.

This will probably only be fixed when the entire world switches to AMD 
AMD64 and/or Intel EM64T machines, and stops using x87 FP register-stack 
instructions.
--
Jim Wilson, GNU Tools Support, http://www.SpecifixInc.com