On Sat, Jul 27, 2024 at 3:37 PM pifminns deettnta via Gcc
<gcc@xxxxxxxxxxx> wrote:
>
> using uint_least64_t = __UINT_LEAST64_TYPE__;
>
> uint_least64_t testbswap(uint_least64_t a) noexcept
> {
> return __builtin_bswap64(a);
> }
>
> clang:
> https://godbolt.org/z/z8GTsazf4
>
>
> _Z9testbswapm:
> revb.d $a0, $a0
> ret
>
>
> GCC:
>
> https://godbolt.org/z/PabfxP9ve
>
> _Z9testbswapm:
> revb.4h $r4,$r4
> revh.d $r4,$r4
> jr $r1
>
> It should just use revb.d for bswap, not separate them into two.
The code generation is not wrong, just not as good.
GCC swaps bytes inside 4 half words and then swaps the half words.
I looked into the history of GCC's code generation here and noticed it
comes from the original port when committed upstream. even the bswap32
is also done using 2 instructions. Now I am suspecting is the original
Loongson ISA didn't have revb.d/revb.2w when the GCC port was done and
it was added afterwards and GCC port was never updated to use the new
instructions.
