[PATCH] __builtin_constant_p inside an initializer

[Mark Shinwell]

Consider the following code:

int main (int argc, char *argv[])
{
   static int a[] = { __builtin_constant_p (argc) ? 1 : 0 };
   return a[0];
}

With 4.x series compilers this compiles successfully at -O0 but not
at -O2, for example, failing with an error of the form

foo.c: In function 'main':
foo.c:3: error: initializer element is not constant
foo.c:3: error: (near initialization for 'a[0]')

The underlying reason for this is that the folding function
fold_builtin_constant_p defers the decision to evaluate the
builtin (to a constant zero or one expression) until optimization.
A special case catches this at -O0 and changes that decision to
"evaluate this to a zero".  However at other optimization levels, the
__builtin_constant_p expression is not known to be a constant early
enough for the above error to be suppressed.

To fix this, the attached patch notes when folding inside an initializer,
and makes fold_builtin_constant_p give a definite answer rather than
deferring until a later stage.  This does not fix all of the situations
where similar problems could manifest themselves, of course: use of
__builtin_constant_p inside an array size bound would be one such.

OK for mainline?

Mark

--

gcc/ChangeLog:

2006-06-01  Mark Shinwell  <shinwell@codesourcery.com>

	* tree.h: Declare folding_initializer.
	* builtins.c (fold_builtin_constant_p): Give definite answer
	if folding inside an initializer.
	* fold-const.c: Define folding_initializer.
	(START_FOLD_INIT): Save and then set folding_initializer.
	(END_FOLD_INIT): Restore folding_initializer.

gcc/testsuite/ChangeLog:

2006-06-01  Mark Shinwell  <shinwell@codesourcery.com>

	* gcc.c-torture/compile/builtin_constant_p.c: New test.
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