Re: [PATCH] Optimize manual byte swap implementations v3

[Robert Dewar <dewar at adacore dot com>]

Mark Mitchell wrote:
> Richard Sandiford wrote:
>
> > Sorry to fan the flames, but... you seem to be suggesting that the
> > optimisation shouldn't be on for either -O2 _or_ -O3.  I thought
> > we were trying to move away from having -f optimisations that
> > aren't enabled at any optimisation level?
>
> That may be the opinion of some.
>
> However, I think it's inevitable that we'll have optimizations that are
> useful to some people in some circumstances, but that aren't
> sufficiently beneficial to be enabled by default at any level.
>
> Some things that are somewhere between code-generation options and
> optimizations are certainly in that category; for example, something
> like the proposed -fno-implicit-fp.  But, even pure optimizations might
> well depend on the programmer making a promise about the code that goes
> beyond what ISO C guarantees.  

I would say that such a transformation is not an optimization, unless
the source contains some indication that it follows this guarantee,
and if possible the compiler checks that this is the case. For
example in Ada, we can have optimizations that depend on the
presence of a specific pragma Restriction, or other specialized
pragma.