builtins: Store unspecified value to *exp for inf/nan [PR114877]

The fold_builtin_frexp folding for NaN/Inf just returned the first argument
with evaluating second arguments side-effects, rather than storing something
to what the second argument points to.

The PR argues that the C standard requires the function to store something
there but what exactly is stored is unspecified, so not storing there
anything can result in UB if the value isn't initialized and is read later.

glibc and newlib store there 0, musl apparently doesn't store anything.

The following patch stores there zero (or would you prefer storing there
some other value, 42, INT_MAX, INT_MIN, etc.?; zero is cheapest to form
in assembly though) and adjusts the test so that it
doesn't rely on not storing there anything but instead checks for
-Wmaybe-uninitialized warning to find out that something has been stored
there.
Unfortunately I had to disable the NaN tests for -O0, while we can fold
__builtin_isnan (__builtin_nan ("")) at compile time, we can't fold
__builtin_isnan ((i = 0, __builtin_nan (""))) at compile time.
fold_builtin_classify uses just tree_expr_nan_p and if that isn't true
(because expr is a COMPOUND_EXPR with tree_expr_nan_p on the second arg),
it does
      arg = builtin_save_expr (arg);
      return fold_build2_loc (loc, UNORDERED_EXPR, type, arg, arg);
and that isn't folded at -O0 further, as we wrap it into SAVE_EXPR and
nothing propagates the NAN to the comparison.
I think perhaps tree_expr_nan_p etc. could have case COMPOUND_EXPR:
added and recurse on the second argument, but that feels like stage1
material to me if we want to do that at all.

2025-01-23  Jakub Jelinek  <jakub@redhat.com>

	PR middle-end/114877
	* builtins.cc (fold_builtin_frexp): Handle rvc_nan and rvc_inf cases
	like rvc_zero, return passed in arg and set *exp = 0.

	* gcc.dg/torture/builtin-frexp-1.c: Add -Wmaybe-uninitialized as
	dg-additional-options.
	(bar): New function.
	(TESTIT_FREXP2): Rework the macro so that it doesn't test whether
	nothing has been stored to what the second argument points to, but
	instead that something has been stored there, whatever it is.
	(main): Temporarily don't enable the nan tests for -O0.