Re: [PATCH] gdb.dwarf2: Define and use gdb_target_symbol_prefix for symbol prefixes

[Pedro Alves <palves@redhat.com>]

On 11/05/2015 06:39 AM, Kevin Buettner wrote:
> On Fri, 30 Oct 2015 11:11:33 +0000
> Pedro Alves <palves@redhat.com> wrote:


> But for a .S file, we need to do something like this (as shown in
> gdb.arch/i386-float.S):
> 
> #define CONCAT1(a, b) CONCAT2(a, b)
> #define CONCAT2(a, b) a ## b
> 
> #ifdef SYMBOL_PREFIX
> # define SYMBOL(str)     CONCAT1(SYMBOL_PREFIX, str)
> #else
> # define SYMBOL(str)     str
> #endif
> 
> 	.text
> 	.globl SYMBOL(main)
> SYMBOL(main):
> 
> I don't understand the reason for both CONCAT1 and CONCAT2,

You need two levels in order to first expand SYMBOL_PREFIX, and
then concat the result.  Otherwise SYMBOL(main) expands to
literally "SYMBOL_PREFIXmain" (minus quotes).

> It seems to me that the way to fix this is to make
> gdb_target_symbol_prefix_flags return an unquoted prefix.  I.e.
> it should return:
> 
> additional_flags=-DSYMBOL_PREFIX=_
> 
> instead of:
> 
> additional_flags=-DSYMBOL_PREFIX="_"

Yup.

> 
> Then, within C files, SYMBOL is defined as follows:
> 
> #ifdef SYMBOL_PREFIX
> #define SYMBOL(str)     #SYMBOL_PREFIX #str
> #else
> #define SYMBOL(str)     #str
> #endif
> 
> Note that this is the same as before, except that I added a # to
> the front of SYMBOL_PREFIX.  It's possible that this won't work -

It wont.  # can only be used with a macro parameter.

> we might end up with "SYMBOL_PREFIX" instead of "_" for the prefix.
> If that's the case, then some layering is needed, perhaps something
> like this:
> 
> #define STRCAT1(a,b) #a #b
> #ifdef SYMBOL_PREFIX
> #define SYMBOL(str)     STRCAT1(SYMBOL_PREFIX,str)
> #else
> #define SYMBOL(str)     #str
> #endif
> 

That still needs double expansion.

> I'm willing to make these changes, but I want to first be sure that
> I'm not missing an easier fix.

IMO, it's clearer to separate the concerns -- concatenation
and final stringification.

Start with the usual concatenation:

#define CONCAT1(a, b) CONCAT2(a, b)
#define CONCAT2(a, b) a ## b

#ifdef SYMBOL_PREFIX
# define SYMBOL1(str)     CONCAT1(SYMBOL_PREFIX, str)
#else
# define SYMBOL1(str)     str
#endif


Then add a stringification layer:

#define STR1(s) #s
#define STR(s) STR1(s)

#define SYMBOL(str)     STR(SYMBOL1(str))


This way you have these expansions:

 SYMBOL1(main) -> _main
 SYMBOL(main) -> "_main"

Thanks,
Pedro Alves