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SubjectRe: [PATCH net-next 01/14] etherdevice: Add eth_<foo>_addr CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS code
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On Tue, 2015-03-03 at 05:19 -0800, Eric Dumazet wrote:
> On Mon, 2015-03-02 at 19:54 -0800, Joe Perches wrote:
> > Make the memset possibly more efficient with the appropriate
> > CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS #ifdef
[]
> > diff --git a/include/linux/etherdevice.h b/include/linux/etherdevice.h
[]
> > @@ -192,7 +192,12 @@ static inline void eth_random_addr(u8 *addr)
> > */
> > static inline void eth_broadcast_addr(u8 *addr)
> > {
> > +#if defined(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS)
> > + *(u32 *)addr = 0xffffffff;
> > + *(u16 *)(addr + 4) = 0xffff;
> > +#else
> > memset(addr, 0xff, ETH_ALEN);
> > +#endif
> > }
> >
>
> I am pretty sure compiler already does an inline, and might chose to
> emit equivalent instructions for :
> *(u16 *) addr = 0xffff;
> *(u32 *)(addr + 2) = 0xffffffff;
>
> if (addr % 4) is known to be 2

(cross-compiled on x86)

At least for arm gcc 4.6.3, it emits different code
for net/l2tp/l2tp_eth.o

old:

- 1cc0: e3a010ff mov r1, #255 ; 0xff
- 1cc4: e3a02006 mov r2, #6
- 1cc8: e3c33001 bic r3, r3, #1
- 1ccc: e3833002 orr r3, r3, #2
- 1cd0: e5c53000 strb r3, [r5]
- 1cd4: ebfffffe bl 0 <memset>
- 1cc8: e3c33001 bic r3, r3, #1

new:

+ 1cc0: e3a02f8a mov r2, #552 ; 0x228
+ 1cc4: e3c33001 bic r3, r3, #1
+ 1cc8: e3833002 orr r3, r3, #2
+ 1ccc: e5c53000 strb r3, [r5]
+ 1cd0: e3e03000 mvn r3, #0
+ 1cd4: e5843224 str r3, [r4, #548] ; 0x224
+ 1cd8: e18430b2 strh r3, [r4, r2]




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