On Friday, 28 January 2022 at 18:02:27 UTC, Iain Buclaw wrote:
For example, druntime depends on this behaviour.
Template:
https://github.com/dlang/druntime/blob/a0ad8c42c15942faeeafb016e81a360113ae1b6b/src/rt/config.d#L46-L58
Ouch. From where I stand, this looks like some really ugly hack
abusing both the template keyword and mangle pragma. Presumably
intended to implement this part of the spec:
https://dlang.org/library/rt/config.html
Moreover, these are even global variables rather than functions.
Wouldn't it make more sense to use a special "weak" attribute for
this particular use case? I see that there was a related
discussion here:
https://forum.dlang.org/post/rgmp5d$198g$1...@digitalmars.com
Regular symbol:
https://github.com/dlang/druntime/blob/a17bb23b418405e1ce8e4a317651039758013f39/test/config/src/test19433.d#L1
If we can rely on instantiated symbols to not violate ODR, then
you would be able to put symbols in the .link-once section.
However all duplicates must also be in the .link-once section,
else you'll get duplicate definition errors.
Duplicate definition errors are surely better than something
fishy silently happening under the hood. They can be solved
when/if we encounter them. That said, I can confirm that GDC 10
indeed fails with `multiple definition of 'rt_cmdline_enabled'`
linker error when trying to compile:
```D
extern(C) __gshared bool rt_cmdline_enabled = false;
void main() { }
```
But can't GDC just use something like this in `rt/config.d` to
solve the problem?
```D
version(GNU) {
import gcc.attribute;
pragma(mangle, "rt_envvars_enabled") @attribute("weak")
__gshared bool rt_envvars_enabled_ = false;
pragma(mangle, "rt_cmdline_enabled") @attribute("weak")
__gshared bool rt_cmdline_enabled_ = true;
pragma(mangle, "rt_options") @attribute("weak") __gshared
string[] rt_options_ = [];
bool rt_envvars_enabled()() { return rt_envvars_enabled_; }
bool rt_cmdline_enabled()() { return rt_cmdline_enabled_; }
bool rt_options()() { return rt_options_; }
} else {
// put each variable in its own COMDAT by making them
template instances
template rt_envvars_enabled()
{
pragma(mangle, "rt_envvars_enabled") __gshared bool
rt_envvars_enabled = false;
}
template rt_cmdline_enabled()
{
pragma(mangle, "rt_cmdline_enabled") __gshared bool
rt_cmdline_enabled = true;
}
template rt_options()
{
pragma(mangle, "rt_options") __gshared string[]
rt_options = [];
}
}
```
