TLS/GC issues on macOS

[David Nadlinger]

On 3 Jan 2019, at 12:58, Sönke Ludwig via digitalmars-d-ldc wrote:
> There appears to be a GC visibility issue of TLS sections on macOS 
> when multiple threads are involved. I added a test case to what looks 
> like the same issue: https://github.com/ldc-developers/ldc/issues/2187

Try this patch:

```
diff --git a/src/rt/sections_elf_shared.d b/src/rt/sections_elf_shared.d
index a7b3336a..bc84a116 100644
--- a/src/rt/sections_elf_shared.d
+++ b/src/rt/sections_elf_shared.d
@@ -296,6 +296,20 @@ else
          {
              _tlsRanges = cast(Array!(void[])*)calloc(1, 
Array!(void[]).sizeof);
              _tlsRanges || assert(0, "Could not allocate TLS range 
storage");
+
+            version (Shared) {} else
+            {
+                version (Linux)
+                {
+                    // Nothing to do; glibc allocates the TLS area for 
additional
+                    // threads at the beginning of the stack space, so 
they will already
+                    // be scanned.
+                }
+                else version (OSX)
+                {
+                    
_tlsRanges.insertBack(getTLSRange(&dummyTlsSymbol));
+                }
+            }
          }
          return _tlsRanges;
      }
```

Still trying to figure out how much of that code can be stripped away 
for non-Shared builds (it's a mess, as DMD partially supports shared 
libraries on Linux with a static runtime), or how on earth nobody has 
found this before.

Best,
David