Re: GNAT Optimization of Constant Expressions

[Florian Weimer <fw@deneb.enyo.de>]

* Randy Brukardt:

> As such, the compiler has to do a loop-hoisting optimization in order to
> pre-evaluate these function calls. I suspect that from your results, Gnat
> doesn't do such an optimization in this case. That doesn't surprise me, a
> loop hoist can make the program run slower if the loop is never executed.

Not really.  I suppose GCC special-cases the zero times case anyway
because it transforms the loop into a do-while-style loop, which is
easier to optimize.

It's more likely that the run-time library lacks a few Pure_Function
pragmas, so the compiler does not know that it's a constant
expression.  Ideally, GNAT would translate the function calls to GCC
built-in calls, so that they can be expanded at compile time.  This
optimization probably kicks in for C and Fortran, whose front ends are
more closely aligned with the rest of the compiler.