linux-kernel - Re: [PATCH v2] statx: optimize copy of struct statx to userspace

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Message-ID: <dbbe9bca-15c5-5d3f-92f0-80cc69ec7768@redhat.com>
Date:   Mon, 13 Mar 2017 11:27:32 +0100
From:   Florian Weimer <fweimer@...hat.com>
To:     Andreas Dilger <adilger@...ger.ca>,
        Eric Biggers <ebiggers3@...il.com>
Cc:     Al Viro <viro@...IV.linux.org.uk>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        David Howells <dhowells@...hat.com>,
        linux-kernel@...r.kernel.org, Eric Biggers <ebiggers@...gle.com>
Subject: Re: [PATCH v2] statx: optimize copy of struct statx to userspace

On 03/13/2017 05:34 AM, Andreas Dilger wrote:
> Not that it is a huge deal either way, but I'd think it is harder for the
> compiler to optimize across a function call boundary like memset() vs. a
> struct initialization in the same function where it can see that all but
> a few of the fields are being overwritten immediately before they are used.

GCC treats memset as a function call only if options such as 
-ffreestanding or -fno-builtin are enabled, or if memset is redefined in 
a header file.  Does the kernel do this?

> I don't think the designated initializer is any less clear to the reader
> that the struct is zeroed out compared to using memset().  Possibly the
> best compromise is to use a designated initializer that specifies all of
> the known fields, and leaves it to the compiler to initialize unset fields
> or padding.

GCC will not always initialize padding if you specify a designated 
initializer because padding values are unspeficied and their value does 
not matter from a language point of view.

Thanks,
Florian