Re: testing that a Gimple call argument is a string...

[Basile Starynkevitch <basile at starynkevitch dot net>]

On Fri, Oct 19, 2012 at 10:26:58AM +0200, Richard Biener wrote:
> On Thu, Oct 18, 2012 at 10:33 PM, Basile Starynkevitch
> <basile@starynkevitch.net> wrote:
> > Hello
> >
> > I'm coding in MELT the ex06/ of https://github.com/bstarynk/melt-examples/
> > which should typecheck calls to variadic functions json_pack & json_unpack
> > from http://www.digip.org/jansson (a JSON library in C).
> >
> > I'm working on a MELT pass on Gimple/SSA after phiopt (maybe that place is wrong?)
> >
> > I don't understand well how to check that a given Gimple argument is a string.
> > I was thinking of using useless_type_conversion_p or types_compatible_p on the TREE_TYPE of some POINTER_TYPE with char_type_node, but it seems to not work as I would expect
> >
> > How would you (in C++ or C for a 4.6) code such a test (that argument 2 of some GIMPLE_CALL is a string, ie. a char* in C parlance)?
> 
> You would look at the function signature of the function called, not at its
> argument.  The argument can be a void * and still be a "string" (well, a pointer
> to a string).  GIMPLE doesn't care about the actual type pointed to.

GCC is using the syntactic type of the argument for printf. The following code
  void f (char *s)
  { printf ("%s\n", (void *) s); }
gives when compiled with gcc -Wall
  f.c:4:1: warning: format â%sâ expects argument of type âchar *â, but argument 2 has type âvoid *â [-Wformat]
and we are in the case where the argument is a string casted to (void*); I don't want to give more clever warnings.

So I don't follow you. I want to typecheck (at least in the common simple cases) the calls to that variadic function.
I don't care that much about its signature.

To be concrete, <jansson.h> declares 
   json_t *json_pack(const char *fmt, ...);

and the documentation http://www.digip.org/jansson/doc/2.4/apiref.html#building-values
suggests that
   /* Create the JSON integer 42 */
   json_pack("i", 42);
   /* Create the JSON array ["foo", "bar", true] */
   json_pack("[ssb]", "foo", "bar", 1);

I just want to extend GCC (thru a MELT extension, but that could be a GCC plugin in C++) to detect 
incorrect calls like

   json_pack("i", /*that should be an int:*/ 3.14);
or
   json_pack("[ss]", "foo", /* that should be a string*/ 3 
             /*and the arity is wrong an argument is missing*/);

I believe I should use 
   extern bool useless_type_conversion_p (tree, tree);
from gcc/gimple.h (it is funny that function is in gimple.h, I would expected it to be in tree.h)
with one of the argument being char_type_node

But I might have trouble in using it.

Thanks for reading. Regards.
-- 
Basile STARYNKEVITCH         http://starynkevitch.net/Basile/
email: basile<at>starynkevitch<dot>net mobile: +33 6 8501 2359
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