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From: Jonathan Wakely <jwakely.gcc@gmail.com>
Date: Thu, 28 Oct 2021 10:55:11 +0100
Message-ID: <CAH6eHdS-wQsKUC+fZ7Mh4tNzySWR3uqgKWKBGQaOmxU6HDhk-A@mail.gmail.com>
Subject: Re: Generated assembly code for returning std::optional<uint32_t>
To: =?UTF-8?B?4piCSm9zaCBDaGlhICjorJ3ku7vkuK0p?= <joshchia@gmail.com>
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On Thu, 28 Oct 2021 at 08:11, =E2=98=82Josh Chia (=E8=AC=9D=E4=BB=BB=E4=B8=
=AD) via Gcc-help
<gcc-help@gcc.gnu.org> wrote:
>
> Under -O3 -std=3Dc++17, the assembly code generated by gcc 11.2 for some
> std::optional C++ code seems less optimal than I expect; it is writing th=
e
> content of optional<uint64_t> return values to both the stack and to the
> RAX register. What am I missing? I was under the impression that return
> values are put in RAX if they can fit. Why does gcc have to write the
> return value to both the stack and RAX instead of just one of them? Can t=
he
> writing to the stack be avoided?

Is this https://gcc.gnu.org/PR101326 ?