x86_64 ABI question

John Fine johnsfine@verizon.net
Thu Sep 11 14:52:00 GMT 2008

Ian Lance Taylor wrote:
> John Fine <johnsfine@verizon.net> writes:
>> When an unsigned int is passed or returned in a 64-bit register, does
>> either the C or C++ ABI specify that the top half of the register
>> contain zero?
> There are threads on the subject at
> http://groups.google.com/group/ia32-abi/browse_thread/thread/f47e0106b21d9269
> http://gcc.gnu.org/ml/gcc/2008-01/msg00052.html
> Short answer: the ABI does not specify, and gcc's behaviour has
> recently changed.
Thankyou, but I don't believe those threads address the same question.

IIUC, those discuss extending a value smaller than int in a register 
whose size is the same as int, in an architecture that isn't x86_64.  
Also that is a situation where the performance optimal design choice for 
the ABI would be not extending.

I'm asking about values whose size is the same as int in registers that 
are bigger in an architecture where the performance optimal design 
choice would be extending.

I'm not saying I assume such choices have been made with enough 
attention to architecture specific considerations that the difference in 
the performance factor alone would make me disregard those threads.  I'm 
saying (and correct me if you are seeing something I'm missing) that 
those threads are very specific to are architecture and case that are 
quite different from the one I asked about.

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