>>>> (I also looked at >>>> https://gcc.gnu.org/ml/gcc-patches/2007-09/msg00324.html, 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 (https://software.intel.com/en-us/forums/topic/393587). Should the GCC patch example be using a simple push/pop instead of an XCHG? Thanks again.
