[fpc-pascal] Reversing bit-order of byte

[Thomas Schatzl]

Hi,

On Fri, 2012-05-04 at 14:14 +0200, Koenraad Lelong wrote:
> On 03-05-12 13:27, Thomas Schatzl wrote:
> ...
> > function reverse(b : byte) : byte; assembler; nostackframe;
> > asm
> >    .long 0xe6ff0f30 // rbit r0, r0
> >    lsr r0, r0, #23
> > end;
> >
> 
> Hi,
> 
> I've been looking a bit further at this.
> I disassembled the code and this is the result :
> 
>   8000150:       e6ff0f30        rbit    r0, r0
>   8000154:       ea4f 50d0       mov.w   r0, r0, lsr #23

The constant is wrong here, it should be #24, not #23 imo... typo.

>   8000158:       46f7            mov     pc, lr
> 

It might be good to compile with -Cparmv6 (or -Cparmv7, do not know what
type the processor you use is), this generates a "better" return
instruction ("bx lr") - and for the rbit instruction you need armv6
already.

> Wouldn't it be possible to do
> 	rbit	r0, r0, lsr #23
> When reading the ARM programming manual I believe it should.
> 

No, rbit does not allow the "flexible operand" encoding.

> But do I gain something ?
> 

If it were possible, at most a few cpu cycles.

Compared to the other variants (e.g. with the multiplies, except for the
table lookup) you already save a lot of cycles - although I guess they
would be sufficiently fast anyway, considering the typical uart speeds.

Thomas