Re: [PATCH 11/12] tools: sync lib/find_bit implementation

Arnd Bergmann
2021-05-12T08:34:12Z

On Wed, May 12, 2021 at 10:16 AM Rasmus Villemoes
<linux@rasmusvillemoes.dk> wrote:
> 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@rasmusvillemoes.dk/
>
> 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.

Thanks for the detailed explanation. Checking the actual behavior of
a trivial example, I find that

int f(void)
{
const int i = 1;
return __builtin_choose_expr(__builtin_constant_p(i), 1, 2);
}

used to return '2' with gcc-7, which is what I remembered.
With gcc-8 and up as well as any version of clang, it returns '1' now:
https://godbolt.org/z/7eKjbMocb

I have also seen a couple of cases where __builtin_constant_p()
without a __builtin_choose_expr() ended up unexpectedly
returning true when gcc found a code path that it would be constant
(e.g. conditionally initializing a variable to one of two possible
ICEs), but then later turning that back into a non-constant
expression in a later optimization stage. There is probably also
a much more detailed explanation behind those.

Arnd