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From: Jonathan Wakely <jwakely.gcc@gmail.com>
Date: Tue, 24 Jan 2023 14:26:24 +0000
Message-ID: <CAH6eHdSVcFW_Xvhpiy9vOe1POyipTmq5Ca3bpb3aZrPrADVbvg@mail.gmail.com>
Subject: Re: Avoiding stack buffer clear being optimised out
To: Jonny Grant <jg@jguk.org>
Cc: gcc-help <gcc-help@gcc.gnu.org>
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On Tue, 24 Jan 2023 at 13:52, Jonny Grant <jg@jguk.org> wrote:
>
> Probably other functions be "compiled out" like memset() is, would memcpy() be compiled out?

The compiler can do anything which does not change the observable side
effects of a valid program.

https://en.cppreference.com/w/cpp/language/as_if

So yes, the compiler can omit a memcpy that doesn't do anything. Or
any other function call that doesn't do anything.