Re: [RFC] Modeling the behavior of function calls

[Joe Buck <Joe dot Buck at synopsys dot COM>]

On Mon, Apr 28, 2008 at 03:04:56PM -0400, Diego Novillo wrote:
> [ Apologies if this comes out twice.  I posted this message last week,
>    but I think it was rejected because of a .pdf attachment. ]
> 
> We have been bouncing ideas for a new mechanism to describe the behavior
> of function calls so that optimizers can be more aggressive at call
> sites.  Currently, GCC supports the notion of pure/impure,
> const/non-const, but that is not enough for various cases.
> 
> The main application for this would be stable library code like libc,
> that the compiler generally doesn't get to process.
...
> The main idea is to add a variety of attributes to describe contracts
> for function calls.  When the optimizers read in the function
> declaration, they can take advantage of the attributes and adjust the
> clobbering effects of call sites.

Such a facility can have other uses, particularly for static analysis,
by allowing simple preconditions and postconditions to be specified.
For example:

* a returned pointer is guaranteed to be non-null.
* a supplied pointer is always dereferenced.
* a supplied pointer must be dereferenceable on input, and that pointer
  is no longer dereferenceable after return, e.g. free().

Of course, there's a tradeoff between implementation complexity and
features, as always.  While these facilities might help the optimizer,
the compiler could also issue warnings if it detects that a precondition
must be violated (and this can also be used to check the correctness
of any user-supplied annotations).