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From: "eric-bugs at omnifarious dot org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/113527] New: Missed optimization [[assume]] attribute
Date: Sun, 21 Jan 2024 02:55:47 +0000
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: c++
X-Bugzilla-Version: 14.0
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D113527

            Bug ID: 113527
           Summary: Missed optimization [[assume]] attribute
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: eric-bugs at omnifarious dot org
  Target Milestone: ---

This code https://godbolt.org/z/jeYGcr5sv still generates all the exception
handling code. It treats the assume attribute more like 'likely' than 'assu=
me'.
It's supposed to be undefined behavior for the assumption to be false.

Here is the code replicated here:

#include <variant>

struct Potato
{
    explicit Potato(int);
    Potato(const Potato & other) noexcept;
};

using V =3D std::variant<Potato, int, double>;

template <class... Ts_>
struct overloaded : Ts_... { using Ts_::operator()...; };

auto f(const V & v) -> int
{
    [[assume(! v.valueless_by_exception())]];
    return visit(overloaded{
        [](int) { return 144; },
        [](double) { return 27; },
        [](const Potato &) { return 50; }
    }, v);
}

You can make the exception handling code go away by simply using "if
(v.valueless_by_exception())" and returning some random value in the false
case. But, if the spirit of the keyword is to be observed, you should be fi=
ne
just doing a straight up table lookup and using the result without even
checking the type tag at all.=