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From: "steffen.hirschmann at ipvs dot uni-stuttgart.de"
 <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/95405] New: Unnecessary stores with std::optional
Date: Fri, 29 May 2020 08:38:55 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D95405

            Bug ID: 95405
           Summary: Unnecessary stores with std::optional
           Product: gcc
           Version: 10.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: steffen.hirschmann at ipvs dot uni-stuttgart.de
  Target Milestone: ---

I posted this to the gcc-help mailing list a few days ago
(https://gcc.gnu.org/pipermail/gcc-help/2020-May/138978.html).

GCC produces stores that don't seem to be required for std::optional.

Code:
--------
#include <optional>
std::optional<long> foo();
long bar()
{
    auto r =3D foo();
    if (r)
        return *r;
    else
        return 0L;
}
--------

What gcc 10.1 with -std=3Dc++17 -O3 produces is:
bar():
        sub     rsp, 24
        call    foo()
        mov     QWORD PTR [rsp+8], rdx
        cmp     BYTE PTR [rsp+8], 0
        mov     QWORD PTR [rsp], rax
        mov     rax, QWORD PTR [rsp]
        jne     .L1
        xor     eax, eax
.L1:
        add     rsp, 24
        ret

(see: https://godbolt.org/z/uHE6QB)

I don't understand the stores (and loads) after the call to foo. They
don't seem necessary to me.


Marc Glisse pointed out
(https://gcc.gnu.org/pipermail/gcc-help/2020-May/138982.html) that the first
pair of store/load seems to be a tuning choice and can be removed with the
correct tuning flags.


What I expected is:
        mov     QWORD PTR [rsp+8], rdx
        cmp     BYTE PTR [rsp+8], 0
should be a compare/test directly of dl.

And:
        mov     QWORD PTR [rsp], rax
        mov     rax, QWORD PTR [rsp]
is not present at all.

Can someone explain this behavior? Shouldn't the optimizer produce what I
pointed out?=