On 6 October 2012 01:33, Sascha Hauer <s.hauer@xxxxxxxxxxxxxx> wrote:
> On Fri, Oct 05, 2012 at 09:55:04PM +0200, Robert Jarzmik wrote:
>> Sascha Hauer <s.hauer@xxxxxxxxxxxxxx> writes:
>>
>> > To check if two regions overlap
>> >
>> > Signed-off-by: Sascha Hauer <s.hauer@xxxxxxxxxxxxxx>
>> > ---
>> > include/common.h | 13 +++++++++++++
>> > 1 file changed, 13 insertions(+)
>> >
>> > diff --git a/include/common.h b/include/common.h
>> > index c1f44b4..e30774a 100644
>> > --- a/include/common.h
>> > +++ b/include/common.h
>> > @@ -256,4 +256,17 @@ static inline void barebox_banner(void) {}
>> > (__x < 0) ? -__x : __x; \
>> > })
>> >
>> > +/*
>> > + * Check if two regions overlap. returns true if they do, false otherwise
>> > + */
>> > +static inline bool region_overlap(unsigned long starta, unsigned long lena,
>> > + unsigned long startb, unsigned long lenb)
>> > +{
>> > + if (starta + lena <= startb)
>> > + return 0;
>> > + if (startb + lenb <= starta)
>> > + return 0;
>> > + return 1;
>> > +}
>> > +
>> > #endif /* __COMMON_H_ */
>>
>> Or if you look for perfomance (I presume not in barebox) :
>> static inline bool region_overlap(unsigned long starta, unsigned long lena,
>> unsigned long startb, unsigned long lenb)
>> {
>> return starta <= startb + lenb && starta + lena >= startb;
>> }
>>
>> It's a bit more obfuscated, but performance wise no branch prediction :)
>
> You made me curious. I tried to compile both and here is the result on
> ARM (I swapped the arguments left and right of the &&):
>
> 00025000 <_region_overlap>:
> 25000: e0811000 add r1, r1, r0
> 25004: e1510002 cmp r1, r2
> 25008: 9a000004 bls 25020 <_region_overlap+0x20>
> 2500c: e0832002 add r2, r3, r2
> 25010: e1520000 cmp r2, r0
> 25014: 93a00000 movls r0, #0
> 25018: 83a00001 movhi r0, #1
> 2501c: e12fff1e bx lr
> 25020: e3a00000 mov r0, #0
> 25024: e12fff1e bx lr
>
> 00025000 <__region_overlap>:
> 25000: e0811000 add r1, r1, r0
> 25004: e1510002 cmp r1, r2
> 25008: 3a000004 bcc 25020 <__region_overlap+0x20>
> 2500c: e0832002 add r2, r3, r2
> 25010: e1500002 cmp r0, r2
> 25014: 83a00000 movhi r0, #0
> 25018: 93a00001 movls r0, #1
> 2501c: e12fff1e bx lr
> 25020: e3a00000 mov r0, #0
> 25024: e12fff1e bx lr
>
> Maybe gcc isn't so clever on other architectures, I don't know ;)
You made me curious too.
I compiled this piece of code for MIPS:
--- code ---
#include <stdbool.h>
bool _region_overlap(unsigned long starta, unsigned long lena,
unsigned long startb, unsigned long lenb)
{
if (starta + lena <= startb)
return 0;
if (startb + lenb <= starta)
return 0;
return 1;
}
bool __region_overlap(unsigned long starta, unsigned long lena,
unsigned long startb, unsigned long lenb)
{
return starta <= startb + lenb && starta + lena >= startb;
}
--- /code ---
I used gcc 4.6.2 with the '-O2' option.
Here is the result:
00000000 <_region_overlap>:
0: 00a42821 addu a1,a1,a0
4: 00c5282b sltu a1,a2,a1
8: 10a00003 beqz a1,18 <_region_overlap+0x18>
c: 00e63021 addu a2,a3,a2
10: 03e00008 jr ra
14: 0086102b sltu v0,a0,a2
18: 03e00008 jr ra
1c: 00001021 move v0,zero
00000020 <__region_overlap>:
20: 00e63821 addu a3,a3,a2
24: 00e4382b sltu a3,a3,a0
28: 14e00004 bnez a3,3c <__region_overlap+0x1c>
2c: 00a42021 addu a0,a1,a0
30: 0086302b sltu a2,a0,a2
34: 03e00008 jr ra
38: 38c20001 xori v0,a2,0x1
3c: 03e00008 jr ra
40: 00001021 move v0,zero
You can see that the shorten obfuscated function (__region_overlap)
has ONE MORE processor instruction!
--
Best regards,
Antony Pavlov
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