Hi Jakub,
On Mon, Mar 03, 2014 at 10:08:53AM +0100, Jakub Jelinek wrote:
> On Mon, Mar 03, 2014 at 05:04:45PM +0800, lin zuojian wrote:
> > > No. As I wrote earlier, the alternative is to use unaligned stores for
> > > ARM,
> > > I've asked Lin to benchmark that compared to his patch, but haven't seen
> > > that done yet.
>
> > I have not benchmark yet.But according to what I hear from an ARM Engineer
> > in Huawei,
> > unaligned accessing usually slow.And not recommand to use too much.
>
> It is expected it will not be as fast as aligned store, the question is if
> an unaligned 32-bit store is faster than 4 8-bit stores, and/or if the cost of
> the unaligned stores is bad enough (note, usually it is just a few stores in
> the prologue and epilogue) to offset for the penalties introduced by
> realigning the stack (typically one extra register that has to be live, plus
> the cost of the realignment itself).
Will I run some microbenchmarks?Like
char * a = new char[100];
a++;
int * b = reinterpret_cast<int*>(a);
b[0] = xxx;
b[4] = xxx;
b[1] = xxx;
b[2] = xxx;
b[3] = xxx;
...
I don't know if this is accurate.
>
> Jakub
