Re: [PATCH] PR libstdc++/13045 / C++ demangler, floating values and function pointer type return type postfix fix.

[Ian Lance Taylor <ian at wasabisystems dot com>]

Gabriel Dos Reis <gdr@integrable-solutions.net> writes:

> Ian Lance Taylor <ian@wasabisystems.com> writes:
> 
> | Or, if you are suggesting that the mangling ABI be changed to use C99
> | hexadecimal floating point constants, then that sounds like a good
> | idea to me, certainly better than the current situation. 
> 
> That is essentially what I'm suggesting, but wanted to know its
> drawbacks compared to current choice.

Drawbacks:

1) Obviously, it is an ABI change, which means that code compiled with
   gcc 3.3.1 and 3.3.2 may not link with code compiled with gcc 3.4
   and higher (versions of gcc before 3.3 are broken anyhow; support
   for gcc 3.3 itself is available with -fabi-version=1).

2) The same floating point number has multiple representations as a
   C99 hex floating point constant.  The ABI would have to be written
   to constrain this in some way, such as requiring that the exponent
   be a multiple of 4 and the significand have no more than three
   trailing zero bits (i.e., no trailing hex zero digit).  It should
   be possible to guarantee interoperability given a specific floating
   point format.  (Note that it would still not be possible to
   guarantee the same name given different floating point formats, but
   it's not clear why anybody would care, since the functions would
   most likely not be call compatible anyhow.)

3) Given such a constraint, it would be slightly more difficult to
   implement this specification in the compiler than it is to
   implement the current specification.

Advantages:

1) It would be possible to write a target-independent demangler.

2) ???

Do the advantages outweigh the drawbacks?  Actually, I think they do.
But it's not like there are a lot of advantages.

Ian