GDC dont optimize calls to template functions

[Iain Buclaw]

On Sunday, 5 October 2025 at 03:26:21 UTC, qxi wrote:
> as expected. Now changing 'pass' to template function
>
> ```d
> int pass()(int a) { return a; } //template function
>
> int add(int num) { return pass(num) + num;}
> ```
>
> Output:

-- snip --

> way worst output.

Tracked in https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102765

DMD emits templates as weak symbols, that it chooses to inline 
such functions violates ODR.

There was a proposal to fix druntime that would allow reversing 
the behaviour change: https://github.com/dlang/druntime/pull/3716.

As it stands, one potential way forward would be to restrict weak 
linkage to just extern(C) declarations in templates. All other 
symbols then get thrown into the linkonce section.

> Now we get expected output.
> But lack of inlining is not the only problem because
>
> ```d
> pragma(inline,false)
> int pass(int a) { return a; } //normal function (no inline)
>
> int add(int num) { return pass(num) + num;}
> ```
>
> Output:

--snip--

>
> which is still better output than templated one.

These two code blocks are not equivalent.  G++ and GDC produce 
the same code when the regular function is annotated correctly.

https://compiler-explorer.com/z/YqoG918GK