Re: [Minios-devel] [UNIKRAFT PATCH v2 2/2] lib/uknetdev: Fix compliation warning about signed/unsigned comparision

["Justin He (Arm Technology China)" <Justin.He@xxxxxxx>]

Hi Julien

> -----Original Message-----
> From: Minios-devel <minios-devel-bounces@xxxxxxxxxxxxxxxxxxxx> On Behalf
> Of Julien Grall
> Sent: Monday, October 14, 2019 5:01 PM
> To: Justin He (Arm Technology China) <Justin.He@xxxxxxx>; minios-
> devel@xxxxxxxxxxxxxxxxxxxx; Simon Kuenzer <simon.kuenzer@xxxxxxxxx>
> Cc: Felipe Huici <felipe.huici@xxxxxxxxx>; Kaly Xin (Arm Technology China)
> <Kaly.Xin@xxxxxxx>; Sharan.Santhanam@xxxxxxxxx;
> Santiago.Pagani@xxxxxxxxx; nd <nd@xxxxxxx>; Costin Lupu
> <costin.lupu@xxxxxxxxx>
> Subject: Re: [Minios-devel] [UNIKRAFT PATCH v2 2/2] lib/uknetdev: Fix
> compliation warning about signed/unsigned comparision
> 
> Hi,
> 
> Title: s/compliation/compilation/
> 

Okay
> On 12/10/2019 07:39, Jia He wrote:
> > This fixes the compilation warning when compiling libuknetdev
> >
> > /root/hj/UK/unikraft/lib/uknetdev/netbuf.c: In function
> > 'uk_netbuf_alloc_buf':
> > /root/hj/UK/unikraft/lib/uknetdev/netbuf.c:120:35: warning:
> comparison
> > between signed and unsigned integer expressions [-Wsign-compare]
> >    if (likely(UINT16_MAX - headroom > NETBUF_ADDR_ALIGNMENT)) {
> >                                     ^
> > /root/hj/UK/unikraft/include/uk/arch/lcpu.h:48:43: note: in definition
> > of macro 'likely'
> >   #define likely(x)   (__builtin_expect((!!(x)), 1))
> >                                             ^
> >
> > The UINT16_MAX is 0xffff (signed int) on my X86 host (gcc version 7.4.0).
> 
> I am a bit confused, is this cross-compilation or native compilation? I guess
> the later as you mention x86 host. So I guess you are referring to
> /usr/include/stdint.h, right?
It is a native compilation. This warnings occurred when I reviewed Felipe's WAMR
lib patches.  As for your latter question, unikraft won't include any glibc 
head files
(-nostdinc option)
> 
> > Integer types smaller than int are promoted when an operation is
> performed
> > on them. Thus, the left side of equation is "int", and the right side is
> > size_t.
> 
> I am not sure to understand this. Above you say UINT16_MAX is signed int
> (aka
> int), so why do you speak about promotion to int here?

The original code line:
(UINT16_MAX - headroom) > NETBUF_ADDR_ALIGNMENT
IIUC,  UINT16_MAX  is int, headroom  is unsigned short (u16), and 
NETBUF_ADDR_ALIGNMENT
is unsigned int (u32).
So headroom  is promoted to int and the left of the equation is int, the right 
is unsigned int.
 
--
Cheers,
Justin (Jia He)


> 
> Cheers,
> 
> --
> Julien Grall
> 
> 
> 
> 
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