[csmith-dev] Alignment of struct members cause target dependent checksum

[Kees Bakker]

On 05-06-13 19:16, John Regehr wrote:
>> Incidentally, John, I was also surprised that Csmith would generate
>> code that writes to a union member and then reads back from a
>> different (and not even equivalent) member. On reflection I agree this
>> might be useful, but shouldn't there at least be a command-line switch
>> to prevent Csmith from generating undefined behavior? (If Kees
>> described the problem correctly, the test program was definitely UB.)
>
> This is one of those things that is technically undefined but common compilers (at least GCC and Clang, probably more) provide well-defined behavior that we wanted to test.  In other words, the GCC 
> developers guarantee that type punning via union works "as expected."  In fact, there are optimization passes that propagate values through these sorts of type puns (I'm actually working on a blog 
> post about this).
>
> There are a few random union-related command line options, I'm not sure if the one you want is there, but if it's not I would imagine that we could add it.  We spent quite a bit of time on unions!
>
> John

Can you perhaps explain what this option does?

   --union-read-type-sensitive | --no-union-read-type-sensitive: allow | disallow reading an union field when there is no risk of reading padding bits (enabled by default).