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Message-ID:	<53DBCF39.9050309@cs.utoronto.ca>
Date:	Fri, 01 Aug 2014 13:32:41 -0400
From:	Ryan Johnson <ryan DOT johnson AT cs DOT utoronto DOT ca>
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To:	"cygwin AT cygwin DOT com" <cygwin AT cygwin DOT com>
Subject:	Destructors not called for c++11 thread_local objects

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 AT tpoff
>         je      .L7
>         rep; ret
> .L7:
>         subq    $8, %rsp
>         movl    $.LC1, %edi
>         movb    $1, %fs:__tls_guard AT tpoff
>         call    puts
>         movq    %fs:0, %rsi
>         movl    $__dso_handle, %edx
>         movl    $_ZN1XD1Ev, %edi
>         addq    $8, %rsp
>         addq    $_ZL1x AT 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?
Ryan


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