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digitalmars.D.learn - Thread local variables in betterC

reply Blatnik <blatblatnik gmail.com> writes:
Do thread local variables work in -betterC? Or maybe it's better 
to ask are they _supposed_ to work in -betterC?

Here's a simple test program:

```D
// test.d
auto a;

extern(C) void main() {
   printf("before... ");
   a = 42;
   printf("after");
}

import core.stdc.stdio;
```

And here is the output of compiling and running:

```
$ dmd -betterC test.d; ./test.exe
before... after

$ dmd -betterC -m64 test.d; ./test.exe
before... [program hangs for 3 seconds, and then exits without 
printing anything else]
```

So when I compile a 64-bit version, the program just hangs when 
writing to the thread local variable. But it does work when 
compiled for 32-bits. So what is the intended outcome here?
May 09 2021
next sibling parent Blatnik <blatblatnik gmail.com> writes:
On Sunday, 9 May 2021 at 18:41:33 UTC, Blatnik wrote:

 Here's a simple test program:


Whoops, `auto a` should be `int a`. Habits :P
May 09 2021
prev sibling parent reply rikki cattermole <rikki cattermole.co.nz> writes:
On 10/05/2021 6:41 AM, Blatnik wrote:

 Do thread local variables work in -betterC? Or maybe it's better to ask 
 are they _supposed_ to work in -betterC?


No.

It is tied to druntime. I.e. on Linux it calls __tls_get_addr to get the 
address.
May 09 2021
parent reply Blatnik <blatblatnik gmail.com> writes:
On Sunday, 9 May 2021 at 18:46:18 UTC, rikki cattermole wrote:

 No.

 It is tied to druntime. I.e. on Linux it calls __tls_get_addr 
 to get the address.


Oh. I wish I got a warning about that from the compiler then >.>

Also, why did it work for 32-bits then?
May 09 2021
parent reply =?UTF-8?Q?Ali_=c3=87ehreli?= <acehreli yahoo.com> writes:
On 5/9/21 11:49 AM, Blatnik wrote:


 Also, why did it work for 32-bits then?


This is a long shot, but are you on OSX? I think TLS is handled 
specially for 32-bit OSX and that may somehow work in this case. (?)

Ali
May 09 2021
parent reply Blatnik <blatblatnik gmail.com> writes:
On Sunday, 9 May 2021 at 20:56:01 UTC, Ali Çehreli wrote:

 This is a long shot, but are you on OSX? I think TLS is handled 
 specially for 32-bit OSX and that may somehow work in this 
 case. (?)


Nope, I'm on (64bit) windows.

Another really weird thing is that if I compile this code in 
VisualD (with the exact same flags, compiler and everything) it 
runs just fine either as 64-bit or 32-bit..

This really doesn't make any sense at all.

Is there any way at all I can initialize these variables myself 
without linking with druntime?
May 09 2021
parent reply Imperatorn <johan_forsberg_86 hotmail.com> writes:
On Sunday, 9 May 2021 at 21:42:54 UTC, Blatnik wrote:

 On Sunday, 9 May 2021 at 20:56:01 UTC, Ali Çehreli wrote:

 This is a long shot, but are you on OSX? I think TLS is 
 handled specially for 32-bit OSX and that may somehow work in 
 this case. (?)


 Nope, I'm on (64bit) windows.

 Another really weird thing is that if I compile this code in 
 VisualD (with the exact same flags, compiler and everything) it 
 runs just fine either as 64-bit or 32-bit..

 This really doesn't make any sense at all.

 Is there any way at all I can initialize these variables myself 
 without linking with druntime?


Check the assembly
May 09 2021
parent Blatnik <blatblatnik gmail.com> writes:
On Sunday, 9 May 2021 at 22:55:41 UTC, Imperatorn wrote:

 Check the assembly


I already did, here's the "wrong" version when I only run `$ dmd 
-betterC -m64 test.d`:

```
Dump of file test.obj

main:
   push        rbp
   mov         rbp,rsp
   mov         rax,qword ptr gs:[58h]

   mov         rcx,qword ptr [rax]
   mov         edx,offset _D4test1ai
   mov         dword ptr [rcx+rdx],42

   xor         eax,eax
   pop         rbp
   ret
```

And here it is when I run `$ dmd -betterC -m64 -c test.d`:

```
Dump of file test.obj

main:
   push        rbp
   mov         rbp,rsp
   push        rbx
   mov         eax,dword ptr [_tls_index] <--------
   mov         rcx,qword ptr gs:[58h]

   mov         rdx,qword ptr [rcx+rax*8]
   mov         ebx,offset _D4test1ai
   mov         dword ptr [rdx+rbx],42

   xor         eax,eax
   pop         rbx
   rbp
   ret
```

So this just seems like some sort of compiler bug. It doesn't 
emit the TLS index retrieval unless you compile with `-c` and 
link separately.

Also, I've looked into this now, and thread locals _should_ work 
in -betterC as far as I can tell. There's no mention of them not 
working on https://dlang.org/spec/betterc.html, and if they 
really didn't work it would have been mentioned there in bold all 
caps. It would be pretty important to mention that `int a = 0;` 
doesn't work correctly.
May 09 2021