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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug inline-asm/50772] Inline assembler "A" constrain works non-expectedly on 64-bits target
Date: Tue, 18 Oct 2011 09:28:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50772

Richard Guenther <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |INVALID

--- Comment #1 from Richard Guenther <rguenth at gcc dot gnu.org> 2011-10-18 09:27:34 UTC ---
Documentation says:

@item A
The @code{a} and @code{d} registers.  This class is used for instructions
that return double word results in the @code{ax:dx} register pair.  Single
word values will be allocated either in @code{ax} or @code{dx}.
For example on i386 the following implements @code{rdtsc}:

@smallexample
unsigned long long rdtsc (void)
@{
  unsigned long long tick;
  __asm__ __volatile__("rdtsc":"=A"(tick));
  return tick;
@}
@end smallexample

This is not correct on x86_64 as it would allocate tick in either @code{ax}
or @code{dx}.  You have to use the following variant instead:

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