[Valgrind-developers] clang callstacks

[Paul F. <pj...@wa...>]

Hi
 
I've been looking at some differences that I get when building with clang.
 
One kind of difference that I see is that Helgrind displays one less element in callstacks. For instance with a GCC build I might get
 
==34086== ---Thread-Announcement------------------------------------------
==34086==
==34086== Thread #2 was created
==34086== at 0x4D144BA: thr_new (in /lib/libc.so.7)
==34086== by 0x4C6639C: pthread_create (in /lib/libthr.so.3)
==34086== by 0x4A5098A: pthread_create_WRK (hg_intercepts.c:433)
==34086== by 0x4A5199C: pthread_create (hg_intercepts.c:472)
==34086== by 0x400935: main (tc01_simple_race.c:22)
 
but the same with a clang build gives
 
==37539== ---Thread-Announcement------------------------------------------
==37539==
==37539== Thread #3 was created
==37539== at 0x491E4BA: thr_new (in /lib/libc.so.7)
==37539== by 0x487039C: pthread_create (in /lib/libthr.so.3)
==37539== by 0x4855B44: pthread_create_WRK (hg_intercepts.c:434)
==37539== by 0x400B7A: main (tc21_pthonce.c:87)
 
(note there is no pthread_create/hg_intercepts.c line).
 
I think that the cause of this is the clang codegen in the helgrind preload lib.
 
Here is the GCC codegen, a classic function call


 000000000000999f <_vgw00000ZZ_libthrZdsoZa_pthreadZujoin>:
999f: 55 push %rbp
99a0: 48 89 e5 mov %rsp,%rbp
99a3: e8 a5 c1 ff ff callq 5b4d 

99a8: 5d pop %rbp
99a9: c3 retq 

Clang optimizes the call and uses a jump

000000000000a9f0 <_vgw00000ZZ_libthrZdsoZa_pthreadZucreate>:
a9f0: 55 push %rbp
a9f1: 48 89 e5 mov %rsp,%rbp
a9f4: 5d pop %rbp
a9f5: eb 09 jmp aa00 

a9f7: 66 0f 1f 84 00 00 00 nopw 0x0(%rax,%rax,1)
a9fe: 00 00 

Am I right in assuming it is this 'jmp' rather than 'callq / retq' that is causing "VG_(get_StackTrace_wrk)" to see one less element in the callstack?

The difference goes away if I force 'pthread_create' to be not optimized.

#define PTH_FUNC(ret_ty, f, args...) \
ret_ty I_WRAP_SONAME_FNNAME_ZZ(VG_Z_LIBPTHREAD_SONAME,f)(args); \
__attribute__((optnone)) \
ret_ty I_WRAP_SONAME_FNNAME_ZZ(VG_Z_LIBPTHREAD_SONAME,f)(args)

(Not that I'm seriously suggesting that).

A+
Paul