From: Steffen Hirschmann <steffen.hirschmann@ipvs.uni-stuttgart.de>
To: gcc-help@gcc.gnu.org
Subject: Unnecessary stores with std::optional?
Date: Tue, 26 May 2020 15:07:10 +0200 [thread overview]
Message-ID: <20200526130710.GB362962@lapsgs17> (raw)
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?
Greetings,
Steffen
next reply other threads:[~2020-05-26 13:07 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-05-26 13:07 Steffen Hirschmann [this message]
2020-05-28 15:00 ` Marc Glisse
2020-05-29 8:45 ` Steffen Hirschmann
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