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From: "gcc.gnu at vvalter dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug ipa/106816] New: noreturn/pure attributes are not set correctly
 on multiversioned functions
Date: Fri, 02 Sep 2022 15:34:43 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D106816

            Bug ID: 106816
           Summary: noreturn/pure attributes are not set correctly on
                    multiversioned functions
           Product: gcc
           Version: 12.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: ipa
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gcc.gnu at vvalter dot com
                CC: marxin at gcc dot gnu.org
  Target Milestone: ---

During the investigation of PR106627, Richard noted (
https://gcc.gnu.org/pipermail/gcc-patches/2022-September/600777.html ) that
attributes like noreturn and pure on multiversioned functions are lost when=
 the
function declaration is replaced by the dispatcher declaration in the same =
way
that the TREE_NOTHROW attribute got lost, which was fixed in PR106627.

Example:

__attribute__((noreturn,target("default")))
void f() {
    for (;;) {}
}

__attribute__((noreturn,target("sse4.2,bmi")))
void f() {
    for (;;) {}
}

int main()
{
    f();
    return 1;
}


Gcc should create no code after the call to f(), but the assembly output wi=
th
-O3 looks like the following:

        call    _Z5_Z1fvv@PLT
        movl    $1, %eax
        addq    $8, %rsp
        .cfi_def_cfa_offset 8
        ret

For a non-multiversioned function, no assembly instructions are generated a=
fter
the call instruction. Similar problems happen if the function is marked pur=
e.
I reproduced this on 11.2.0 and 12.2, but this most likely affects all gcc
versions.=