md.texi (386 constraints): Clarify A constraint documentation.

2010-11-23  Richard Guenther  <rguenther@suse.de>

	* doc/md.texi (386 constraints): Clarify A constraint documentation.

From-SVN: r167081

2 files changed, 30 insertions, 2 deletions

diff --git a/gcc/ChangeLog b/gcc/ChangeLog
index 09053c3..9fcd937 100644
--- a/gcc/ChangeLog
+++ b/gcc/ChangeLog
@@ -1,3 +1,7 @@
+2010-11-23  Richard Guenther  <rguenther@suse.de>
+
+	* doc/md.texi (386 constraints): Clarify A constraint documentation.
+
 2010-11-23  Basile Starynkevitch  <basile@starynkevitch.net>
 	    Jeremie Salvucci  <jeremie.salvucci@free.fr>
 
diff --git a/gcc/doc/md.texi b/gcc/doc/md.texi
index bdf42f1..677ea02 100644
--- a/gcc/doc/md.texi
+++ b/gcc/doc/md.texi
@@ -2101,8 +2101,32 @@ The @code{si} register.
 The @code{di} register.
 
 @item A
-The @code{a} and @code{d} registers, as a pair (for instructions that
-return half the result in one and half in the other).
+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
+
 
 @item f
 Any 80387 floating-point (stack) register.