Proper use of x86/x86_64 CPUID instruction with extended assembler

Jeffrey Walton
Wed Aug 19 09:56:00 GMT 2015

>>>> (I also looked at
>>>>, which is a
>>>> GCC patch for cpuid.h. But its not clear to me if the above is correct
>>>> because the operands are 32-bit in size. Naively, if I use "a" and "b"
>>>> with a 32-bit operand, then I would expect code for EAX and EBX; and
>>>> not RAX and RBX).
>>> It is correct.
>> Thanks Avi. Regarding the GCC patch... if the output register and the
>> ABI tells GCC that EBX needs to be preserved, then why does the patch
>> take special steps to preserve it?
> Well I just tried it, and gcc is indeed not clever enough to do this on its
> own.  So it is needed.

And one more related question... The Intel folks state to minimize the
use of the the XCHG instruction because it has lock overhead
( Should the GCC
patch example be using a simple push/pop instead of an XCHG?

Thanks again.

More information about the Gcc-help mailing list