Bug 731228

Description of problem:

When building a program with only 1 byte of thread local variables, the variable appears to not be initialised.

Version-Release number of selected component (if applicable):

[root@dxi0-02 ~]# gcc --version
gcc (GCC) 4.6.0 20110603 (Red Hat 4.6.0-10)
Copyright (C) 2011 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

[root@dxi0-02 ~]# /lib/libc.so.6 
GNU C Library stable release version 2.14, by Roland McGrath et al.
Copyright (C) 2011 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.
There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A
PARTICULAR PURPOSE.
Compiled by GNU CC version 4.6.0 20110530 (Red Hat 4.6.0-9).
Compiled on a Linux 2.6.38 system on 2011-06-28.
Available extensions:
	Support for some architectures added on, not maintained in glibc core.
	The C stubs add-on version 2.1.2.
	crypt add-on version 2.1 by Michael Glad and others
	GNU Libidn by Simon Josefsson
	Native POSIX Threads Library by Ulrich Drepper et al
	BIND-8.2.3-T5B
	RT using linux kernel aio
libc ABIs: UNIQUE IFUNC
For bug reporting instructions, please see:
<http://www.gnu.org/software/libc/bugs.html>.


How reproducible:

Every time.


Steps to Reproduce:
1. Write this simple example to tls-byte.c:

[root@dxi0-02 ~]# cat > tls-byte.c
#include <stdio.h>
#include <stdint.h>

__thread uint8_t foo = 1;

int main (void)
{
 printf("foo is %hhu.\n", foo);
 return 0;
}

2. Compile it to produce tls-byte.

[root@dxi0-02 ~]# gcc -lpthread tls-byte.c -o tls-byte

3. Run it.

[root@dxi0-02 ~]# ./tls-byte 
  
Actual results:
The program prints "foo is 0."

Expected results:
The program should print "foo is 1."

Interestingly, if foo is changed to a uint16_t, or a second '__thread uint8_t bar' variable is declared (even if it is not initialised), I get the expected result.

I have filed this bug on glibc rather than gcc because the TLS header appears to show 1 byte of data, which seems to me to be set correctly:

[root@dxi0-02 ~]# readelf -l ./tls-byte | grep TLS -A1
  TLS            0x0000000000000714 0x0000000000600714 0x0000000000600714
                 0x0000000000000001 0x0000000000000001  R      1

[root@dxi0-02 ~]# od --address-radix=x --format=x1 --skip-bytes=0x714 --read-bytes=1 ./tls-byte 
000714 01
000715

Apologies if I have misinterpreted this readelf output.