When glibc is configured with --enable-kernel=2.6.38 on x86-64,
R15 may be uninitialized in x86_64/pthread_cond_timedwait.S
when called from nptl/tst-abstime.c. The backtrace is


Breakpoint 5, pthread_cond_timedwait@@GLIBC_2.3.2 ()
    at ../nptl/sysdeps/unix/sysv/linux/x86_64/pthread_cond_timedwait.S:50
50		pushq	%r12
(gdb) bt
#0  pthread_cond_timedwait@@GLIBC_2.3.2 ()
    at ../nptl/sysdeps/unix/sysv/linux/x86_64/pthread_cond_timedwait.S:50
#1  0x0040081d in main () at fixed.c:85
(gdb) 

and sequence is

(gdb) next
53		pushq	%r13
(gdb) 
56		pushq	%r14
(gdb) 
59		pushq	%r15
(gdb) 
[Thread 0xf7a42940 (LWP 2393) exited]
67		subq	$FRAME_SIZE, %rsp
(gdb) 
71		cmpq	$1000000000, 8(%rdx)
(gdb) 
72		movl	$EINVAL, %eax
(gdb) 
73		jae	48f
(gdb) 
93		CMP_LP	$-1, dep_mutex(%rdi)
(gdb) 
96		movq	%rdi, 8(%rsp)
(gdb) 
97		movq	%rsi, 16(%rsp)
(gdb) 
98		movq	%rdx, %r13
(gdb) 
100		je	22f
(gdb) 
101		mov	%RSI_LP, dep_mutex(%rdi)
(gdb) 
114		movl	$1, %esi
(gdb) 
115		xorl	%eax, %eax
(gdb) 
116		LOCK
(gdb) 
122		jnz	31f
(gdb) 
125	32:	movq	16(%rsp), %rdi
(gdb) 
126		xorl	%esi, %esi
(gdb) 
127		callq	__pthread_mutex_unlock_usercnt
(gdb) 
129		testl	%eax, %eax
(gdb) 
130		jne	46f
(gdb) 
132		movq	8(%rsp), %rdi
(gdb) 
133		incq	total_seq(%rdi)
(gdb) 
134		incl	cond_futex(%rdi)
(gdb) 
135		addl	$(1 << nwaiters_shift), cond_nwaiters(%rdi)
(gdb) 
138		movq	8(%rsp), %rdi
(gdb) 
139		movq	wakeup_seq(%rdi), %r9
(gdb) 
140		movl	broadcast_seq(%rdi), %edx
(gdb) 
141		movq	%r9, 24(%rsp)
(gdb) 
142		movl	%edx, 4(%rsp)
(gdb) 
144		cmpq	$0, (%r13)
(gdb) 
145		movq	$-ETIMEDOUT, %r14
(gdb) 
146		js	36f
(gdb) 
231	36:	movl	broadcast_seq(%rdi), %edx
(gdb) 
233		movq	woken_seq(%rdi), %rax
(gdb) 
235		movq	wakeup_seq(%rdi), %r9
(gdb) 
237		cmpl	4(%rsp), %edx
(gdb) 
238		jne	53f
(gdb) 
240		cmpq	24(%rsp), %r9
(gdb) 
241		jbe	45f
(gdb) 
246	45:	cmpq	$-ETIMEDOUT, %r14
(gdb) 
247		jne	38b
(gdb) 
249	99:	incq	wakeup_seq(%rdi)
(gdb) 
250		incl	cond_futex(%rdi)
(gdb) 
251		movl	$ETIMEDOUT, %r14d
(gdb) 
252		jmp	44f
(gdb) 
258	44:	incq	woken_seq(%rdi)
(gdb) 
260	54:	subl	$(1 << nwaiters_shift), cond_nwaiters(%rdi)
(gdb) 
263		cmpq	$0xffffffffffffffff, total_seq(%rdi)
(gdb) 
264		jne	55f
(gdb) 
286	55:	LOCK
(gdb) 
292		jne	40f
(gdb) 
296	41:	movq	16(%rsp), %rdi
(gdb) 

Breakpoint 6, pthread_cond_timedwait@@GLIBC_2.3.2 ()
    at ../nptl/sysdeps/unix/sysv/linux/x86_64/pthread_cond_timedwait.S:297
297		testl	%r15d, %r15d
(gdb)