[cfe-dev] -fsanitize=undefined and alignment on x86_64

[Martin Martin]

Hi all,

sanitize=undefined is giving me an error message about unaligned access to
a size_t.  It's in a packed structure, and indeed is not 8 byte aligned
(although it is 4 byte aligned.)  This is in Linux on an x86_64
architecture.

My question is: why is this undefined behavior?  Can't modern Intel and AMD
processors accesses unaligned ints, although with a possible performance
penalty?  So this would be a performance problem, but not correctness?  Is
the problem that the compiler could use SSE or some other instructions that
require alignment and will seg fault or give wrong results with unaligned
access?

Best,
Martin
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