[Bug tree-optimization/112296] __builtin_constant_p doesn't propagate through member functions

["rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112296

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |documentation
                 CC|                            |jsm28 at gcc dot gnu.org,
                   |                            |rth at gcc dot gnu.org

--- Comment #5 from Richard Biener <rguenth at gcc dot gnu.org> ---
+FAIL: gcc.c-torture/execute/bcp-1.c   -O1  (test for excess errors)
+UNRESOLVED: gcc.c-torture/execute/bcp-1.c   -O1  compilation failed to produce 
executable

that's because

bcp-1.c:(.text+0x6c): undefined reference to `func'^M
collect2: error: ld returned 1 exit status^M

the testcase can be reduced to

int func(void);
int bad7(void) { return __builtin_constant_p(func()); } 

and we want the side-effect to disappear!?  There's more fun in this testcase,
like verifying that

int bad1(void) { return __builtin_constant_p(global++); }

returns 0 (but not verifying whether 'global' is altered or not).

I don't see it documented that __builtin_constant_p discards side-effects on
operands - that would in fact make it behave like 'constexpr'?!

commit 54303b5cb3076f7bae9102c08b59d57b1a9f5104
Author: Richard Henderson <rth@cygnus.com>
Date:   Tue Jul 7 06:41:17 1998 -0700

    * gcc.c-torture/execute/bcp-1.c: New test.

So this also becomes a documentation thing and I doubt we can start to
retain side-effects we've thrown away for decades now.  But then
relying on _folding_ to implement what becomes a language extension this
way is definitely error-prone and fragile.

I think we should also definitely diagnose this somehow (which would also
mean moving discarding of side-effects to the frontend(s)).

Joseph, Richard, do you have anything to add or remember discussions about
this semantic detail of __builtin_constant_p?