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Re: AVR byte swap optimization
- From: "Shaun Jackman" <sjackman at gmail dot com>
- To: "Denis Vlasenko" <vda dot linux at googlemail dot com>
- Cc: gcc at gcc dot gnu dot org, avr-gcc-list at nongnu dot org
- Date: Mon, 18 Dec 2006 11:28:31 -0700
- Subject: Re: AVR byte swap optimization
- References: <7f45d9390611171530g4c12f68dua3ade87a8b4147b0@mail.gmail.com> <200611270219.56427.vda.linux@googlemail.com>
- Reply-to: "Shaun Jackman" <sjackman at gmail dot com>
On 11/26/06, Denis Vlasenko <vda.linux@googlemail.com> wrote:
On Saturday 18 November 2006 00:30, Shaun Jackman wrote:
> The following macro expands to some rather frightful code on the AVR:
>
> #define BSWAP_16(x) \
> ((((x) >> 8) & 0xff) | (((x) & 0xff) << 8))
Sometimes gcc is generating better code if you cast
values instead of masking. Try:
( (uint8_t)((x) >> 8) | ((uint8_t)(x)) << 8 )
Your suggestion seemed like a good one and gave me some hope.
Unfortunately, it produces identical results to my BSWAP_16 macro. I
also tried the following:
uint8_t a = x >> 8, b = x;
return b << 8 | a;
Different register allocations this time, but identical instructions.
Cheers,
Shaun