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

[Alan Stern]

On Sun, Dec 06, 2020 at 06:24:05PM +0700, Bui Quang Minh wrote:
> On Sat, Dec 05, 2020 at 10:15:11AM -0500, Alan Stern wrote:
> > 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@xxxxxxxxxxxxxxxxxxx> 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
> > 
> 
> Thank you for your clarification, I didn't think about it before.
> 
> As I check when compiling the code, with MAX_NUM_UDC=32 the initialization
> becomes
> 
>         xor    eax,eax
>         mov    ecx,0x40
>         rep stos DWORD PTR es:[rdi],eax
> 
> With MAX_NUM_UDC=2, the initialization becomes
> 
>         mov    QWORD PTR [rbp-0x30],0x0
>         mov    QWORD PTR [rbp-0x28],0x0
> 
> As I see, initialization does not require additional static data space.
> Am I right?

Ah, okay, good.  I didn't realize the compiler was smart enough to do 
this.  Thanks for checking.

Alan Stern