From: Marc Glisse <marc.glisse@inria.fr>
To: Steffen Hirschmann <steffen.hirschmann@ipvs.uni-stuttgart.de>
Cc: gcc-help@gcc.gnu.org
Subject: Re: Unnecessary stores with std::optional?
Date: Thu, 28 May 2020 17:00:18 +0200 (CEST) [thread overview]
Message-ID: <alpine.DEB.2.22.394.2005281658550.6869@stedding.saclay.inria.fr> (raw)
In-Reply-To: <20200526130710.GB362962@lapsgs17>
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
next prev parent reply other threads:[~2020-05-28 15:00 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-05-26 13:07 Steffen Hirschmann
2020-05-28 15:00 ` Marc Glisse [this message]
2020-05-29 8:45 ` Steffen Hirschmann
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