On 12/05/2021 09.48, Arnd Bergmann wrote: > On Tue, May 11, 2021 at 10:39 PM Rikard Falkeborn > <rikard.falkeborn@xxxxxxxxx> wrote: >> On Tue, May 11, 2021 at 08:53:53PM +0900, Tetsuo Handa wrote: > >>> #define GENMASK_INPUT_CHECK(h, l) \ >>> (BUILD_BUG_ON_ZERO(__builtin_choose_expr( \ >>> __builtin_constant_p((l) > (h)), (l) > (h), 0))) >>> >>> __GENMASK() does not need "h" and "l" being a constant. >>> >>> Yes, small_const_nbits(size) in find_next_bit() can guarantee that "size" is a >>> constant and hence "h" argument in GENMASK_INPUT_CHECK() call is also a constant. >>> But nothing can guarantee that "offset" is a constant, and hence nothing can >>> guarantee that "l" argument in GENMASK_INPUT_CHECK() call is also a constant. >>> >>> Then, how can (l) > (h) in __builtin_constant_p((l) > (h)) be evaluated at build time >>> if either l or h (i.e. "offset" and "size - 1" in find_next_bit()) lacks a guarantee of >>> being a constant? >>> >> >> So the idea is that if (l > h) is constant, __builtin_constant_p should >> evaluate that, and if it is not it should use zero instead as input to >> __builtin_chose_expr(). This works with non-const inputs in many other >> places in the kernel, but apparently in this case with a certain >> compiler, it doesn't so I guess we need to work around it. > > I have a vague memory that __builtin_constant_p() inside of > __builtin_choose_expr() > always evaluates to false because of the order in which the compiler processes > those: If constant-folding only happens after __builtin_choose_expr(), then > __builtin_constant_p() has to be false. It's more complicated than that. __builtin_constant_p on something which is a bona-fide Integer Constant Expression (ICE) gets folded early to a 1. And then it turns out that such a __builtin_constant_p() that folds early to a 1 can be "stronger" than a literal 1, in the sense that when used as the controlling expression of a ?: with nonsense in the false branch, the former is OK but the latter fails: https://lore.kernel.org/lkml/c68a0f46-346c-70a0-a9b8-31747888f05f@xxxxxxxxxxxxxxxxxx/ Now what happens when the argument to __builtin_constant_p is not an ICE is a lot more complicated. The argument _may_ be so obviously non-constant that it can be folded early to a 0, hence still be suitable as first argument to __b_c_e. But it is also possible that the compiler leaves it unevaluated, in the "hope" that a later optimization stage could prove the argument constant. And that's the case where __b_c_e will then break, because that can't be left unevaluated for very long - the very _type_ of the result depends on which branch is chosen. tl;dr: there's no "order in which the compiler processes those", __b_c_p can get evaluated (folded) early, before __b_c_e inspects it, or be left for later stages. Rasmus
