Re: Destructors not called for c++11 thread_local objects

[Larry Hall (Cygwin)]

On 08/01/2014 01:32 PM, Ryan Johnson wrote:

Hi all,

(Please CC me in replies, I'm not subscribed to the list)

Question: is this a Cygwin bug, gcc bug, or somebody else's bug entirely?

The following STC shows the problem:

$ cat bug.cpp
#include <cstdio>
static thread_local struct X {
     int x;
     X() { puts("hi"); }
     ~X() { puts("bye!"); }
} x;
int main() { x.x = 0; }

$ g++ -std=gnu++11 -Wall -g bug.cpp && ./a
hi

A quick inspection of the assembly code shows no call to __cxa_thread_atexit
in __tls_init, where the same code compiled on linux, with the same version
of gcc, has it right. This is odd, because the function does seem to be
available in cygwin's libstdc++:

$ nm /usr/lib/gcc/x86_64-pc-cygwin/4.8.3/libstdc++.a | grep thread_atexit
0000000000000000 d .data$_ZZ19__cxa_thread_atexitE4once
0000000000000000 p .pdata$__cxa_thread_atexit
0000000000000000 t .text$__cxa_thread_atexit
0000000000000000 r .xdata$__cxa_thread_atexit
0000000000000000 T __cxa_thread_atexit
0000000000000000 d _ZZ19__cxa_thread_atexitE4once

Here's the linux version of __tls_init:

__tls_init:
.LFB19:
        cmpb    $0, %fs:__tls_guard@tpoff
        je      .L7
        rep; ret
.L7:
        subq    $8, %rsp
        movl    $.LC1, %edi
        movb    $1, %fs:__tls_guard@tpoff
        call    puts
        movq    %fs:0, %rsi
        movl    $__dso_handle, %edx
        movl    $_ZN1XD1Ev, %edi
        addq    $8, %rsp
        addq    $_ZL1x@tpoff, %rsi
        jmp     __cxa_thread_atexit

vs. the same for cygwin:

__tls_init:
        subq    $40, %rsp
        leaq    __emutls_v.__tls_guard(%rip), %rcx
        call    __emutls_get_address
        cmpb    $0, (%rax)
        je      .L27
        addq    $40, %rsp
        ret
.L27:
        leaq    .LC0(%rip), %rcx
        movb    $1, (%rax)
        addq    $40, %rsp
        jmp     puts

Relevant packages installed:
binutils                              2.24.51-4
cygwin                                1.7.30-1
gcc-core                              4.8.3-2
gcc-g++                               4.8.3-2
libstdc++6                            4.8.3-2

(I realize I'm a version behind on cygwin1.dll, but I don't think that's the
problem here)

Thoughts?

Well it's not a general gcc problem because the same code on Linux with
version 4.8.3 works for me.

--
Larry

_____________________________________________________________________

A: Yes.
> Q: Are you sure?
>> A: Because it reverses the logical flow of conversation.
>>> Q: Why is top posting annoying in email?

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