asm constraints

[Chris Johns <cjohns@plessey.com.au>]

Hi

I know little about using constraints with asm statements and so would
like some clarification. With egcs-1.1b I now get bad code generated.
This did not happen with egcs-1.0.2. The code is C++. 

I use the following macro to access an 8bit device :

#define WRITE_REGISTER_8(addr, data) \
  asm volatile (" move.b  %0,%1@ " : : "g" (data), "a" (addr))

The target is a cpu32 (68360) and the compiler is egcs-1.1b.

The bad code is :

tsi_cmds.s: Assembler messages:
tsi_cmds.s:1117: Error: operands mismatch -- statement `move.b
%a6,%a1@' ignored

Can the 'g' constraint be used like this or is this some sort problem
with the compiler which I should report to the egcs people ?

I know I can use 'd' instead of 'g', how-ever 'd' generates register
pressure when doing the following types of operations :

 WRITE_REGISTER_8(0xf000000, 1);
 WRITE_REGISTER_8(0xf000001, 2);
 WRITE_REGISTER_8(0xf000002, 3);

The code is slower due to the need to load a 'd' register. The
required pushing and poping from the stack on entry and exit to a
function to free the 'd' registers slows the code even more. The 'g'
constraint drops the overhead to about 2 instructions.

-- 
 Chris Johns  Networks, Plessey Asia Pacfic Pty. Ltd.
  mailto:ccj@acm.org    mailto:cjohns@plessey.com.au
________________________________________________
To get help for the crossgcc list, send mail to
crossgcc-request@cygnus.com with the text 'help'
(without the quotes) in the body of the message.