linux-kernel - Re: [PATCH] USB: dummy-hcd: Fix uninitialized array use in init()

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Message-ID: <20201205151511.GA1179536@rowland.harvard.edu>
Date:   Sat, 5 Dec 2020 10:15:11 -0500
From:   Alan Stern <stern@...land.harvard.edu>
To:     Minh Bùi Quang <minhquangbui99@...il.com>
Cc:     Felipe Balbi <balbi@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Corentin Labbe <clabbe@...libre.com>,
        Jules Irenge <jbi.octave@...il.com>,
        Andrey Konovalov <andreyknvl@...il.com>,
        linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] USB: dummy-hcd: Fix uninitialized array use in init()

On Sat, Dec 05, 2020 at 07:47:01PM +0700, Minh Bùi Quang wrote:
> Vào Th 6, 4 thg 12, 2020 vào lúc 23:12 Alan Stern
> <stern@...land.harvard.edu> đã viết:
> > Does this initialization end up using less memory than an explicit
> > memset() call?
> 
> You mean speed?

No, I mean memory space.

A memset call requires a certain amount of instruction space (to push 
the arguments and make the call) but no static data space.  
Initialization requires some instruction space (to copy the data) and 
static data space as well (to hold the data that is to be copied).

Alan Stern

> In my opinion, there is no difference in speed between 2 ways.
> When I compile this array initialization using gcc 5.4.0,  this
> initialization becomes
> mov instructions when MAX_NUM_UDC=2 and becomes rep stos when
> MAX_NUM_UDC=32. I think it makes no difference when comparing with memset()
> 
> Thanks,
> Quang Minh