Re: [PATCH] Initialize the entire obstack struct [BZ #17919]

[Siddhesh Poyarekar <siddhesh at redhat dot com>]

[Also looping in bug-gnulib]

On Tue, Feb 03, 2015 at 11:30:17AM -0500, Carlos O'Donell wrote:
> Is this a valgrind bug, false positive in valgrind, or bug in glibc?
> 
> It clearly says we have a move that depends on an uninitizlied value,
> so something read the value and tried to do something with it.

It is a combination of multiple things actually.  The uninitialized
value here is the padding in the structure and we have fixed such
warnings in the past; see the nscd stats warning[1] for example.

This warning doesn't trigger on x86 though due to the code that gcc
generates for it - it explicitly ANDs the bit field in the struct
before testing it.  On s390, the optimizer does away with the AND
operation and tests the entire thing for some reason - I haven't
figured out why but the instruction combiner RTL pass does away with
the AND operation.

This is also why I started out filing a gcc bug, but then changed my
mind and fixed it in glibc instead, since I presumed that the compiler
can assume that the padding is also appropriately initialized.  But
then, I am not completely sure if the compiler is allowed to make that
assumption.

Siddhesh

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