[LLVMbugs] [Bug 13720] New: Clang allows global initializers to take the address of __thread (TLS) variables

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=13720

             Bug #: 13720
           Summary: Clang allows global initializers to take the address
                    of __thread (TLS) variables
           Product: clang
           Version: trunk
          Platform: PC
        OS/Version: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: Frontend
        AssignedTo: unassignedclangbugs at nondot.org
        ReportedBy: mseaborn at chromium.org
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified


Clang accepts the following initializers for p1 and p2, but it should reject
them:

__thread int x;
/* gcc rejects these with "error: initializer element is not constant": */
__thread int *p1 = &x;
int *p2 = &x;

int main() {
  printf("&x = %p\n", &x);
  printf("p1 = %p\n", p1);
  printf("p2 = %p\n", p2);
  return 0;
}

The program prints the following, which suggests that p1 and p2 end up pointing
to the TLS template for x rather than the thread-local instantiation of x:

&x = 0x7f89083716f8
p1 = 0x600e18
p2 = 0x600e18

Arguably LLVM should not allow the code that Clang generates here, which is:

@x = thread_local global i32 0
@p2 = global i32* @x
...

LLVM treats @x as a constant expression, but the address of @x is not a
constant because it varies between threads.

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