Re: Efficiency of code generated by Ada compilers

[Robert A Duff <bobduff@shell01.TheWorld.com>]

Elias Salom�o Helou Neto <eshneto@gmail.com> writes:

>> Anyway, someone prematurely implied that I may be doing premature
>> optimization. The fact is that I do know, from profiling, how
>> important is _not_ to range check in my specific application, so I
>> will try to give you a "bottom line" of what have been said that
>> really matters to me.
>>
>> 1) You can, in more than one way, tell the compiler to suppress most
>> (any?) checks, but people do not advise to do so. Even if I say that I
>> do need that :(

Even if it's true that it is "important _not_ to range check",
that does not necessarily mean that you need to suppress checks.
As Dmitry pointed out, many checks will be removed by the
compiler even if you don't suppress them -- it will remove the
ones it can prove will not fail.  The only way to determine
whether that's good enough is to measure it, and/or look
at the machine code.

As for the advice you mention, if it said "Never suppress checks",
it's bad advice.

> ask for the suppression, but it does NOT mandate the compiler to
> actually
> skip such checks. ...

Of course!  That's true of all higher-level languages, including C.
Higher-level language never formally define what machine code gets
generated, nor how efficient it has to be -- there's no way to do so.
A C compiler is allowed by the C standard to do array bounds checking,
and I assure you that if such a C compiler exists, the checks are
FAR more expensive than for Ada.

The Ada RM has some informal rules, called "Implementation Advice"
and "Implementation Requirements", and there's one in there
somewhere saying compilers really ought to skip the checks
when it makes sense.

Don't worry, Ada compiler writers do not deliberately try
to harm their customers -- of course they actually skip the
checks when it makes sense.  Note that it doesn't always make
sense, since some checks are done for free by the hardware.

- Bob