TypeFunction example creatiing a conversion matrix

[Stefan Koch]

On Thursday, 1 October 2020 at 21:34:54 UTC, Walter Bright wrote:
> On 10/1/2020 1:57 AM, Stefan Koch wrote:
>> However there is a difference in file size.
>> 
>> Type function version:
>> 
>> stat  makeConvMatrix.o
>>    File: 'makeConvMatrix.o'
>>    Size: 2876
>> 
>> VS template version
>> 
>> stat  makeConvMatrix_tmpl.o
>>    File: 'makeConvMatrix_tmpl.o'
>>    Size: 11760
>
> Indeed there is, as the compiler generates code for the 
> function and writes the code to the object file. The code it 
> generates is placed into a COMDAT section which, since it is 
> unreferenced, is supposed to be elided by the linker.

Linkers are not under our control.
The compiler is.

> However, many linkers don't seem to do that, and besides, the 
> compiler is wasting effort generating unused code. This is a 
> more general inefficiency in the design of the compiler 
> internals, and is not necessarily a language design defect.

So you are saying having a mechanism to replace a macro/template 
with a regular function
does not address a language issue?

> Internally, there is a function called `needsCodegen()` which 
> decides if a template instantiation needs code generated for it 
> (duh!). It clearly errs on the side of being a bit too 
> conservative.

I am intimately familiar with needsCodegen() (- and I still don't 
fully understand it)
It leads to a stack overflow within the compiler when compiling 
one our apps with -allinst.
Atila tries to fix it, and it has taken him a couple of weeks 
already.
Right now the likely solution is to make it much more 
conservative, because that's the only thing we can think of to 
avoid linker errors.

> I suggest an improvement to it where it will return false if 
> the only use of a template instantiation is in CTFE. If that is 
> unimplementable (the separate compilation model may make it 
> impractical), a fallback position is to have a pragma or some 
> other mechanism that says "don't generate code for this 
> function".

Been there. Done that.

> I suspect such a compiler improvement would produce 
> considerable benefit to existing template-heavy code.

It requires the programmer to decide if a function needs to be 
elided which in a template, he may not be able to tell.
Because it may depend on which instance is produced.