Re: pragma GCC optimize prevents inlining

[Segher Boessenkool <segher@kernel.crashing.org>]

On Sat, Jan 06, 2024 at 06:02:45PM +0100, David Brown wrote:
> On 05/01/2024 19:19, Segher Boessenkool wrote:
> >That's not the point.  A program can be perfectly fine, with bounded
> >errors and all, and then -ffast-math will typically completely destroy
> >all that, and replace all arithmetic by the equivalent of a dice roll.
> 
> The only difference between IEEE calculations and -ffast-math 
> calculations is that with IEEE, the ordering and rounding is controlled 
> and consistent.

No, that is not the only difference.

'-ffast-math'
     Sets the options '-fno-math-errno', '-funsafe-math-optimizations',
     '-ffinite-math-only', '-fno-rounding-math', '-fno-signaling-nans',
     '-fcx-limited-range' and '-fexcess-precision=fast'.

Many of those do much more than what you say, can result in the compiler
generating completely different code.

> For any given /single/ arithmetic operation that is 
> performed, each can have the same amount of rounding error or error due 
> to the limited length of the mantissa.  Agreed?

I don't understand what you mean to say even.

> >>The rounding errors in -ffast-math will be very similar to those in IEEE
> >>mode, for normal numbers.
> >
> >No, not at all.  Look at what -fassociative-math does, for example.
> >This can **and does** cause the loss of **all** bits of precision in
> >certain programs.  This is not theoretical.  This is real.
> 
> 	a = 1e120;
> 	b = 2;
> 
> 	x = (a + b) - a;
> 
> IEEE rules will give "x" equal to 1e120 - mathematically /completely/ 
> wrong.  -ffast-math will give "x" equal to 2, which is mathematically 
> precisely correct.

The IEEE result is 0.  Which is the **exactly correct** result.  This is
a computer program, not some formulas that you can manipulate at will.

> >The -ffast-math flag can only reasonably be used with programs that did
> >not want any specific results anyway.  It would be even faster (and just
> >as correct!) to always return 0.
> 
> That is simply wrong.

It is an exaggeration for dramatic effect, but it is fundamentally
correct.


Segher