> On Jun 12, 2015, at 12:51 AM, Ulrich Weigand <uweig...@de.ibm.com> wrote:
>
> Hello,
>
> since the new type-based pool allocator was merged, the SPU toolchain
> automated build / regression test has been failing due to crashes of
> the compiled GCC due to random memory corruption.
>
> Debugging this, it seems this is caused by GCC being miscompiled by
> the host compiler. As one example, var-tracking.c contains this
> code in vt_initialize:
>
> empty_shared_hash = new shared_hash_def;
> empty_shared_hash->refcount = 1;
>
> Via operator new, these bits of alloc-pool.h are inlined:
>
> template <typename T>
> inline T *
> pool_allocator<T>::allocate ()
> {
> [...]
> /* Pull the first free element from the free list, and return it. */
> header = m_returned_free_list;
> VALGRIND_DISCARD (VALGRIND_MAKE_MEM_DEFINED (header, sizeof (*header)));
> m_returned_free_list = header->next;
> m_elts_free--;
> [...]
> /* Call default constructor. */
> return (T *)(header);
> }
>
> Note that the read from "header->next" accesses the same memory location
> as the write to "empty_shared_hash->refcount" above.
>
> Type-based aliasing now assumes that those accesses cannot alias, since
> they use incompatible types. The scheduler then moves the write to
> before the read, which causes m_returned_free_list to be set to 1,
> causing a crash later on.
>
> Now -- I'm only seeing this effect on my SPU toolchain regression test
> machine, which uses a RHEL 5 distribution with its default GCC 4.1.2
> system compiler. (Using -fno-strict-aliasing fixes the symptom.)
>
> Maybe this old compiler simply has a bug in type-based aliasing. But
> on the other hand, I'm not completely sure whether this *is* a bug --
> is the code above supposed to be valid C++, or does it in fact violate
> the aliasing rules after all?
This is just a bug in the older compiler. There was a change to fix in
placement new operator. I can't find the reference right now but this is the
same issue as that.
Thanks,
Andrew
>
> Bye,
> Ulrich
>
> --
> Dr. Ulrich Weigand
> GNU/Linux compilers and toolchain
> ulrich.weig...@de.ibm.com
>
