Reason for the “ impossible constraint in ‘asm” error in below code

Question

Could anybody help me in identifying reason for the below compiler error.

" ..\\TestRT\\TDP\\PPCGNU\\lib\\custom.h:428: error: impossible constraint in 'asm' "

Code:

Thanks in advance!

Answer 1

I think documentation answers this question. You probably have x86 code that you are building for x86-64. A is valid for x86, but not for x86-64. GCC documentation explains how to replace A by a for x86-64:

"This is not correct on x86-64 as it would allocate tick in either ax or dx. You have to use the following variant instead:"

unsigned long long rdtsc (void)
{
  unsigned int tickl, tickh;
  __asm__ __volatile__("rdtsc":"=a"(tickl),"=d"(tickh));
  return ((unsigned long long)tickh << 32)|tickl;
}