You could always equate the macro to some inline assembly like what is done for a number of avr-libc macros.
Eric Weddington > -----Original Message----- > From: > [EMAIL PROTECTED] > [mailto:[EMAIL PROTECTED] > org] On Behalf Of Shaun Jackman > Sent: Friday, November 17, 2006 4:31 PM > To: avr-gcc-list@nongnu.org; gcc@gcc.gnu.org > Subject: [avr-gcc-list] AVR byte swap optimization > > The following macro expands to some rather frightful code on the AVR: > > #define BSWAP_16(x) \ > ((((x) >> 8) & 0xff) | (((x) & 0xff) << 8)) > > uint16_t bswap_16(uint16_t x) > { > 0: 9c 01 movw r18, r24 > 2: 89 2f mov r24, r25 > 4: 99 27 eor r25, r25 > 6: 32 2f mov r19, r18 > 8: 22 27 eor r18, r18 > return BSWAP_16(x); > } > a: 82 2b or r24, r18 > c: 93 2b or r25, r19 > e: 08 95 ret > > Ideally, this macro would expand to three mov instructions and a ret. > Is there anything I can do to help GCC along here? I'm using GCC 4.1.0 > with -O2. > > I won't bother to show bswap_32 here, which produces a real disaster! > Think 47 instructions, for what should be 6. > > Cheers, > Shaun > > $ avr-gcc --version |head -1 > avr-gcc (GCC) 4.1.0 > > > _______________________________________________ > AVR-GCC-list mailing list > AVR-GCC-list@nongnu.org > http://lists.nongnu.org/mailman/listinfo/avr-gcc-list >
