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I am running Python 2.7.1 under Valgrind 3.6.1 on a x86_64 GNU/Linux box. Valgrind is flagging errors that are not covered by the standard suppressions file (Misc/valgrind-python.supp; after uncommenting the part I'm supposed to), e.g.:
==5804== Use of uninitialised value of size 8
==5804== at 0x4EBA956: PyObject_Free (obmalloc.c:958)
==5804== by 0x4E85D65: code_dealloc (codeobject.c:317)
==5804== by 0x4ECE23C: tupledealloc (tupleobject.c:220)
==5804== by 0x4E85E42: code_dealloc (codeobject.c:308)
==5804== by 0x4F3315D: load_source_module (import.c:1022)
==5804== by 0x4F33F7E: import_submodule (import.c:2596)
==5804== by 0x4F3420E: load_next (import.c:2416)
==5804== by 0x4F3484F: import_module_level (import.c:2137)
==5804== by 0x4F34DA3: PyImport_ImportModuleLevel (import.c:2189)
==5804== by 0x4F16ABE: builtin___import__ (bltinmodule.c:49)
==5804== by 0x4E6DA42: PyObject_Call (abstract.c:2529)
==5804== by 0x4F16F82: PyEval_CallObjectWithKeywords (ceval.c:3881)
==5804== Use of uninitialised value of size 8
==5804== at 0x4EBA956: PyObject_Free (obmalloc.c:958)
==5804== by 0x4E85D65: code_dealloc (codeobject.c:317)
==5804== by 0x4F3315D: load_source_module (import.c:1022)
==5804== by 0x4F33F7E: import_submodule (import.c:2596)
==5804== by 0x4F3420E: load_next (import.c:2416)
==5804== by 0x4F3484F: import_module_level (import.c:2137)
==5804== by 0x4F34DA3: PyImport_ImportModuleLevel (import.c:2189)
==5804== by 0x4F16ABE: builtin___import__ (bltinmodule.c:49)
==5804== by 0x4E6DA42: PyObject_Call (abstract.c:2529)
==5804== by 0x4F16F82: PyEval_CallObjectWithKeywords (ceval.c:3881)
==5804== by 0x4F1ACD3: PyEval_EvalFrameEx (ceval.c:2332)
==5804== by 0x4F1F347: PyEval_EvalCodeEx (ceval.c:3252)
etc.
Perhaps the suppressions file covers only 32-bit machines, because if I add:
{
 ADDRESS_IN_RANGE/Use of uninitialised value of size 8
 Memcheck:Addr8
 fun:PyObject_Free
}
{
 ADDRESS_IN_RANGE/Use of uninitialised value of size 8
 Memcheck:Value8
 fun:PyObject_Free
}
{
 ADDRESS_IN_RANGE/Use of uninitialised value of size 8
 Memcheck:Addr8
 fun:PyObject_Realloc
}
{
 ADDRESS_IN_RANGE/Use of uninitialised value of size 8
 Memcheck:Value8
 fun:PyObject_Realloc
}
(i.e., "Value8" instead of "Value4") then all is clear.

It works for me!

Could you please provide a diff ?
Also, they should probably be commented by default (as other obmalloc-related calls).

Here's the diff with the added sections commented out.

New changeset 3eb73f45a614 by Charles-François Natali in branch 'default':
Issue #12612: Add some Valgrind suppressions for 64-bit machines. Patch by Paul
http://hg.python.org/cpython/rev/3eb73f45a614 

Committed, thanks for the patch!

Welcome. Thank you!

Could this fix be backported to the 2.7 branch as well?

New changeset 92dda5f00b0f by doko in branch '2.7':
Issue #12612: Add some Valgrind suppressions for 64-bit machines.
https://hg.python.org/cpython/rev/92dda5f00b0f