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From: "s_gccbugzilla at nedprod dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/95001] New: std::terminate() and abort() do not have
 __builtin_unreachable() semantics
Date: Fri, 08 May 2020 09:07:14 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D95001

            Bug ID: 95001
           Summary: std::terminate() and abort() do not have
                    __builtin_unreachable() semantics
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: s_gccbugzilla at nedprod dot com
  Target Milestone: ---

Consider the codegen from https://godbolt.org/z/xhmBrL:

```
#include <cstddef>
#include <cstdint>
#include <cstdlib>
#include <exception>

void sum(uint32_t *__restrict a, const uint32_t *__restrict b, const uint32=
_t
*__restrict c, size_t count)
{
    auto invoke_terminate =3D []{
#ifdef USE_UNREACHABLE
        __builtin_unreachable();
#else
        std::terminate();
#endif
    };
    if((((uintptr_t)a) & 15)!=3D0) invoke_terminate();
    if((((uintptr_t)b) & 15)!=3D0) invoke_terminate();
    if((((uintptr_t)c) & 15)!=3D0) invoke_terminate();
    if((count & 15) !=3D 0) invoke_terminate();
    while(count !=3D 0)
    {
        *a++ =3D *b++ + *c++;
        count--;
    }
}
```

It would seem that functions marked with both [[noreturn]] and noexcept do =
not
have the same improvements on codegen as __builtin_unreachable() has. This =
is
despite that [[noreturn]] functions returning is explicitly required to be =
UB
in the standard, and if they are noexcept then they cannot throw an excepti=
on
either.

Can noexcept functions marked [[noreturn]] please gain the same effects on
codegen as __builtin_unreachable() has please?=