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From: "khim at google dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/114305] New: GCC doesn't use [[gnu::pure]] attribute on the
 inline function
Date: Mon, 11 Mar 2024 11:43:36 +0000
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: c++
X-Bugzilla-Version: 14.0
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X-Bugzilla-Severity: normal
X-Bugzilla-Who: khim at google dot com
X-Bugzilla-Status: UNCONFIRMED
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X-Bugzilla-Priority: P3
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D114305

            Bug ID: 114305
           Summary: GCC doesn't use [[gnu::pure]] attribute on the inline
                    function
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: khim at google dot com
  Target Milestone: ---

The following program is an illustration (https://godbolt.org/z/PYW34ccvP):

#include <cstddef>

GNU_PURE1 extern size_t foo(size_t);

template <size_t size>
GNU_PURE2 inline size_t test() {
    return foo(size);
}

int test_func(int x, int y) {
    return test<sizeof x>() + test<sizeof y>();
}

If GNU_PURE1 is [[gnu::pure]] then both gcc and clang optimize away second
call, if GNU_PURE2 is [[gnu::pure]] then only clang does that.

That means that it's impossible to add [[gnu::pure]] optimization on top of
function that's pure according to the docs but is not declared as such in t=
he
header file.=