[Issue 23806] [REG 2.099.0] Link error with -betterC

March 24, 2023
Posted by RazvanN
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RazvanN
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https://issues.dlang.org/show_bug.cgi?id=23806

RazvanN <razvan.nitu1305@gmail.com> changed:

           What    |Removed                     |Added
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                 CC|                            |razvan.nitu1305@gmail.com

--- Comment #2 from RazvanN <razvan.nitu1305@gmail.com> ---
(In reply to SHOO from comment #0)
> After the dmd-2.099.0, the following code causes a link error when built with the -betterC compile option:
> ------------------
> struct Foo
> {
> 	~this(){}
> }
> struct Bar
> {
> 	this(ref Bar){}
> }
> struct Data
> {
> 	Foo[2] _foo;
> 	Bar    _bar;
> }
> extern (C) void main()
> {
> 	Data pool;
> }
> ------------------
> 
> (dmd-2.099.0)dev@cc4fa74d49d1:~/work$ dmd -betterC main.d
> main.o:main.d:function
> _D4core8internal5array12construction__T12_d_arrayctorHTANgS4main3FooTNgQnZQBj
> FNeNkMQBcMQBgPaZ27enforceRawArraysConformableMFNaNbNiNexAaxmxAvxQdZv: error:
> undefined reference to
> '_D4core8internal4util5array31enforceRawArraysConformableNogcFNbNfxAaxmxAvxQd
> xbZv'
> collect2: error: ld returned 1 exit status
> Error: linker exited with status 1
> 
> This error does not occur before dmd-2.098.1 and in ldc-1.32.0

Unfortunately, this is an annoying issue. Here's what happens:

The copy constructor of Bar triggers the generation of a copy constructor in Data with the signature `this(ref inout Data src) inout`. The body of the copy constructor is:

this._foo = src._foo;
this._bar = src._bar;

The first statement is lowered to _d_arrayctor(this._foo, src._foo) which calls the druntime template, which in turn calls some druntime internal functions (not templates). This is the fundamental issue, the hook should only call other templates otherwise it is not usable in betterC.

However, what happens next is that the second statement is lowered to a copy constructor call: _bar.__ctor(src._bar). Note that src is `inout` whereas the parameter of Bar's copy constructor is mutable. This issues an error, but since we're in a generated method, it gets gagged and the generated copy constructor is disabled. However, the body of _d_arrayctor has already been attached to AST and therefore it will get codegened (even if it's not used). That will cause the linker error.

I think the only way to fix this is to somehow mark the template instances that are instantiated from a disabled context so that they are not codegened. Although it's not really obvious how to do that.

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