Re: macro expansion and operator &&

[Alan Modra <amodra at gmail dot com>]

On Sat, Aug 14, 2010 at 02:24:35PM +0300, Alex Druinsky wrote:
> Hi,
> 
> I am writing code in GCC's inline assembly, and I stumbled upon what
> looks like a bug in GAS. It appears that when GAS expands macros, it
> will sometimes replace expressions with operator && by expressions
> with operator &, changing the original meaning of the code.
> 
> I made a tiny program to illustrate the problem. To reproduce this,
> run the attached program and verify that the return code is 10. The
> return code should actually be 0.
> 
> Is this indeed a bug?

Yes, I think it is, but I'm not at all comfortable making changes to
macro.c.  There are a lot or things happening in this file that are
not documented in either the code or the gas info, so it's hard to
know what should be done about bugs like this one.  To get some idea,
it helps to go back to gas-2.3 and look at the gasp code and info.  If
you do that, you discover that a single quote could be used to
separate a gasp preprocessor variable from following text.  So the
calls to sub_actual that pass kind='\'' are arranging to have
get_apost_token remove a single quote from the input stream.  The
sub_actual call causing a problem is the one that passes kind='&'.
This seems to allow macros like the following:

  .macro x arg
  .long &arg&,&arg,0
  .endm

ie. deliniating macro parameter substitutions by an ampersand prefix,
or both an ampersand prefix and suffix like masm.  We don't document
this feature as far as I can see.

Ian, Nick, does this look reasonable?

	* macro.c (sub_actual): Add back ampersand suffix when no
	substitution.

Index: gas/macro.c
===================================================================
RCS file: /cvs/src/src/gas/macro.c,v
retrieving revision 1.53
diff -u -p -r1.53 macro.c
--- gas/macro.c	23 Jul 2010 06:44:25 -0000	1.53
+++ gas/macro.c	16 Aug 2010 04:53:52 -0000
@@ -748,6 +748,8 @@ sub_actual (int start, sb *in, sb *t, st
       /* Doing this permits people to use & in macro bodies.  */
       sb_add_char (out, '&');
       sb_add_sb (out, t);
+      if (src != start && in->ptr[src - 1] == '&')
+	sb_add_char (out, '&');
     }
   else if (copyifnotthere)
     {

-- 
Alan Modra
Australia Development Lab, IBM