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Re: [PATCH] libio: Add small optimization on fmemopen

On 09/21/2016 08:57 PM, Paul Eggert wrote:
On 09/21/2016 11:47 AM, Florian Weimer wrote:
We don't know the nature of the GCC issue, so we cannot work around it
reliably.  The most likely explanation is that Address Sanitizer does
not account for a valid GCC optimization.

That's not the sense that I get from looking at Bug 66661. GCC is
assuming it can do an aligned word load of the trailing bytes of a
flexible array member in a properly-aligned structure.

But that assumption is completely correct! It's just that Address Sanitizer does not account for it (based on what I've seen so far).

Although the followups to Bug 66661 suggest that there may be
further problems with overaligned structures, the code here is not using
_Alignas so these further problems are not an issue here.

_Alignas is used only to demonstrate the issue more clearly.

glibc currently does not compile with Address Sanitizer anyway, so until that is fixed (or someone has a better explanation of the root cause of GCC PR66661), I would suggest to assume that this issue does not matter to glibc at all.


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