Problems with Template codegen

[Chris Nicholson-Sauls]

kris wrote:
> At this time, D templates are a bit finicky when it comes to argument 
> matching -- IFTI matching, to be explicit. For example:
> 
> # uint locate(T) (T[] source, T match, uint start=0);
> #
> # locate ("wumpus", 'p', 1);
> 
> will not compile because '1' is an integer literal, and does not match 
> the template parameter 'uint'.
> 
> To get around this, template writers will revert to tricks like this one:
> 
> # uint locate(T) (T[] source, T match, uint start=0);
> #
> # uint locate(T, U=uint) (T[] source, T match, U start=0)
> # {
> #     return locate!(T) (source, match, start);
> # }
> #
> # locate ("wumpus", 'p', 1);
> 
> 
> See what's going on? There's a stub added, which marshalls (for want of 
> a better term) the 'int' argument  across. This is done to (a) permit 
> use of both int & uint arguments and (b) to avoid template duplication 
> for both int and uint types.
> 
> So, that's a current shortcoming of IFTI -- but it's one that we hope 
> will be fixed. Moving onto the primary concern ...
> 
> As one would expect, the DM compiler generates code for the stub to 
> re-push the arguments and call the /real/ implementation. However, in 
> /all/ cases we really want to eliminate the stub since it is nothing but 
> a pass-through call; one generated to workaround what seems to be an 
> IFTI limitation.
> 
> Unfortunately, the DM tools will remove the stub /only/ when you use the 
> -inline switch. This is a very powerful switch, causing all kinds of 
> inlining to occur -- in some cases, things which many engineers would 
> balk at. Inline is often just too /big/ a switch to pull.
> 
> However, we'd still like darned stubs to be removed from the codegen. 
> How do we get that to happen?

One notion would be to use the (T, U=uint) notation for the 'real' template and leave the 
stub out, but it defeats the original intent somewhat and doesn't really *solve* the 
problem.  I'd say IFTI just needs to incorporate implicit casting rules such as int->uint 
and that would be a solution.

-- Chris Nicholson-Sauls