On 01/17/2012 11:31 PM, Jonathan M Davis wrote:
On Tuesday, January 17, 2012 14:14:36 H. S. Teoh wrote:
On Tue, Jan 17, 2012 at 05:04:18PM -0500, Jonathan M Davis wrote:
[...]
Attribute inferrence is a big step forward in making as much as
possible @safe and pure, but there's still plenty to do there.
[...]
Funny you should mention that, I was just starting to wonder if I should
start littering my code with 'pure', and whether it's possible to make
the compiler infer it for me. From the little that I know, it seems that
in most cases 'pure' can be automatically inferred. The compiler already
distinguishes between weakly pure and strongly pure internally, so why
not take it all the way? Not sure how this will affect inter-module
analysis, though.
But since this is apparently not yet implemented, just what *is*
implemented currently when you specify 'pure'? Common subexpression
factorization? Hoisting? Not (yet) memoization, apparently.
_pure_ is implemented. It's @safe that isn't fully implemented. pure, @safe,
and nothrow are inferred for templated functions when they're instantiated so
that they can be pure (or @safe or nothrow) based on the code that's generated
rather than always forcing it to be one or the other, since that would be far
too restrictive. But that's completely unnecessary for normal functions. You
_do_ need to mark those pure, @safe, or nothrow yourself.
If attributes were inferred for normal functions, the compiler would always
have to have the full source of every function. And even then, it might be an
instance of the halting problem. Every function is and must be pure (or @safe
or nothrow) or not when it's declared, and that's part of its signature, so it
can be known even when the full source isn't. Inference works with templates
only because they're generating code, and the compiler needs their full source
anyway.
- Jonathan M Davis
I think he is interested in the state of implementation of specific
compiler _optimisations_ that make use of function purity in order to
prove their correctness. IIRC ldc has CSE for pure functions, but I
don't know exactly.
