Re: Unnecessary stores with std::optional?

Marc Glisse <marc.glisse@xxxxxxxx> · Thu, 28 May 2020 17:00:18 +0200 (CEST)

On Tue, 26 May 2020, Steffen Hirschmann wrote:

Dear GCC community,

I was testing std::optional when I noticed that gcc produces stores that
don't seem to be required.

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

What gcc 10.1 with -std=c++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. Can anyone explain them to me?

One of them seems to be a tuning choice (it disappears with some 
-march/-mtune flags), but not the other. Could you file an issue in gcc's 
bugzilla please?

--
Marc Glisse