Re: Compile time executed functions

[Robert A Duff <bobduff@world.std.com>]

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Mats Karlssohn <mats@mida.se> writes:

> Robert A Duff wrote:
> > 
> > Mats Karlssohn <mats@mida.se> writes:
> > 
> > > Maybe you could see my proposed pragma as a representation pragma
> > > for code then ?
> > 
> > OK, but only as a hint.  Rep pragmas or clauses for data can be tested,
> > because you can Unchecked_Convert the data, and see if it's represented
> > the way you asked.  But how do you test a rep pragma for code?  For
> > example, how can you write an Ada program that behaves differently
> > depending on whether pragma Inline was obeyed?  No fair looking at the
> > generated code.
> 
> How to check it... there should't be more than initialize data left
> at the place the function was called and since the function is not
> (at least in my case) called from anywhee else the body shoule _not_
> be included in the output. When writing the previous sentence I
> really felt thah my english isn't what it needs to be to discuss
> really complicated matters, so maybe I didn't understand your point ? 

You seem to be saying "look at the generated code" to see if the
compiler obeyed your pragma.  But that's not meaningful in a formal
language-definition sense.

> As for the test for inlined code... hmmm, I guess it'd be kind of
> hard.

Right, and the same is true for any other pragma that (pretends to)
require something about the generated code.

> > > This leads me to another thing that I've sometimes have found annoy�ng:
> > > Why isn't it required by the language that there should be a n option
> > > to force the compiler to tell you wich pragma;s it cant obey ?
> > > I just hate auditing machine code just to check that the compiler
> > > hasn't generated silly code. Although I must admit that this stuff has
> > > only occured a few times during the past 7 years.
> > 
> > But how would you know if *that* option were obeyed?  Suppose the
> > compiler says "yes, I inlined your function", how do you know if it's
> > lying?  You still have to inspect the generated code.
> 
> Well, until proven otherwise I trust the compiler vendor (a trust
> that have proven false quite often), so if such an option was
> availible I'd trust it (on my own projects, clients may have other
> ideas) until I suspect that it lies, then som investigation is
> needed.

It would be useful to have a compiler option that warns you whenever you
asked for inlining (or other code-generation-specific pragmas) and it
didn't obey.  I'm just saying it's not really appropriate as a
requirement in the RM, because as a requirement, it's meaningless -- it
can't be formally tested.  So ask your compiler vendor for such a
feature.

> > In practise, though, compilers do give this kind of information.
> > Eg, the documentation will normally tell you "we obey pragma Inline in
> > the following circumstances...".
> 
> Yes, aside from it beeing inconvenient to have to look for such
> things in the documentation, I have found that the documentation
> is often buried in some archive somwhere or just missing :(

:-(

- Bob