On Sat, 2024-07-27 at 17:38 -0700, Andrew Pinski via Gcc-help wrote:
> 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.
revb.2w and revb.d are available since day one. I've no idea why they
weren't used. (Also strangely there's no revb.w, so on the [still
insubstantial] 32-bit LoongArch we would have to use revb.2h + rotri.w.
for bswap32, but it may change before a real 32-bit CPU is launched.)
I'm preparing a patch.
--
Xi Ruoyao <xry111@xxxxxxxxxxx>
School of Aerospace Science and Technology, Xidian University
