[Bug c++/122779] New: "[[assume(x)]];" vs "if (!x) __builtin_unreachable();"

[terra at gnome dot org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=122779

            Bug ID: 122779
           Summary: "[[assume(x)]];" vs "if (!x) __builtin_unreachable();"
           Product: gcc
           Version: 15.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: terra at gnome dot org
  Target Milestone: ---

Here are two ways of telling the compiler that it may assume the truth of some
statement:

    [[assume(x)]];
    if (!x) __builtin_unreachable();

I naively thought they would act identically, provided the evaluation of x
doesn't change anything. They do not.

Consider:

https://godbolt.org/z/Gh18e4bGc
and
https://godbolt.org/z/5o5Ys6Eff

(sources to be attached.)

Gcc is able to make great use of

  if (!(res.size() + s.size() <= res.capacity()))
    __builtin_unreachable();

but no use of

  [[assume(res.size() + s.size() <= res.capacity())]];

(This is spun off from bug 122727.)