[PATCH] PR46598 confusion

[PATCH] PR46598 confusion

[Richard Guenther]

This clears up register pair constraint documentation by providing
an example.

Ok?

Thanks,
Richard.

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

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

Index: gcc/doc/md.texi
===================================================================
*** gcc/doc/md.texi	(revision 167076)
--- gcc/doc/md.texi	(working copy)
*************** The @code{di} register.
*** 2102,2108 ****
  
  @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).
  
  @item f
  Any 80387 floating-point (stack) register.
--- 2102,2130 ----
  
  @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).  For example
! for 32bit code 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 where you have to use the following variant:
! 
! @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.

Re: [PATCH] PR46598 confusion

[Jan Hubicka]

> --- 2102,2130 ----
>   
>   @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).  For example

I would probably just remove 
", as a pair (for instructions that
> ! return half the result in one and half in the other)"

and add something like "This class is used for instructions that return double
word result in ax:dx pair. Single word values will be allocated either in ax or
dx." To further prevent confusion that "A" imply register pair instead of class
containing ax and bx.

The change is OK with me and I believe it belongs to x86 maintainership.

thanks,
Honza

Re: [PATCH] PR46598 confusion

[Richard Guenther]

On Tue, 23 Nov 2010, Jan Hubicka wrote:

> > --- 2102,2130 ----
> >   
> >   @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).  For example
> 
> I would probably just remove 
> ", as a pair (for instructions that
> > ! return half the result in one and half in the other)"
> 
> and add something like "This class is used for instructions that return double
> word result in ax:dx pair. Single word values will be allocated either in ax or
> dx." To further prevent confusion that "A" imply register pair instead of class
> containing ax and bx.
> 
> The change is OK with me and I believe it belongs to x86 maintainership.

I have committed the following.

Thanks,
Richard.

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

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

Index: gcc/doc/md.texi
===================================================================
*** gcc/doc/md.texi	(revision 167079)
--- gcc/doc/md.texi	(working copy)
*************** The @code{si} register.
*** 2101,2108 ****
  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).
  
  @item f
  Any 80387 floating-point (stack) register.
--- 2101,2132 ----
  The @code{di} register.
  
  @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
! 
  
  @item f
  Any 80387 floating-point (stack) register.