Mason <mpeg.blue@xxxxxxx> writes:
> I've been looking for the GCC built-in to swap two octets
> within a 16-bit word. I looked at the list of GCC built-ins,
> and found bswap32 and bswap64, but no bswap16.
I don't think anybody has implemented bswap16 as a general builtin
function (it does exist for PPC).
> On x86, GCC is smart enough to optimize the operation to
> a single instruction.
>
> $ cat swap16.c
> #include <stdint.h>
> uint16_t swap16(uint16_t n) { return (n >> 8) | (n << 8); }
> $ gcc -O3 -fomit-frame-pointer -S swap16.c
> _swap16:
> movzwl 4(%esp), %eax
> rolw $8, %ax
> ret
>
> Is my swap16 function recognized as a bswap16 operation
> internally, therefore hand-written assembly code is provided?
That is one way to view what is happening. I would describe this as a
target specific compiler optimization.
> On my platform (SH-4) the operation is not optimized:
>
> $ sh-superh-elf-gcc -O3 -S swap16.c
> _swap16:
> extu.w r4,r4
> mov r4,r0
> shlr8 r4
> shll8 r0
> or r4,r0
> rts
> extu.w r0,r0
>
> Even though the operation could be done in a single swap.b
> instruction (and possibly an extu.w for ABI compliance)
>
> So, if I understand the situation correctly, the
> bswap16 built-in may not be needed because GCC can
> recognize the C pattern for bswap16, but no optimized
> assembly code has been provided for my platform.
That sounds about right, although personally I would say that gcc ought
to provide __builtin_bswap16 just for consistency.
Ian
