llvm dev - Dec 2018 - __parityti2(), __paritydi2() and __paritysi2() vs. __builtin_parity

Hi @ll,

compiler-rt/lib/builtins/parityti2.c
compiler-rt/lib/builtins/paritydi2.c
compiler-rt/lib/builtins/paritysi2.c

implement the parity function as matroschka:

si_int
__paritysi2(si_int a)
{
    su_int x = (su_int)a;
    x ^= x >> 16;
    x ^= x >> 8;
    x ^= x >> 4;
    return (0x6996 >> (x & 0xF)) & 1; // see optimisation below!
}

si_int
__paritydi2(di_int a)
{
    dwords x;
    x.all = a;
    return __paritysi2(x.s.high ^ x.s.low);
}

si_int
__parityti2(ti_int a)
{
    twords x;
    x.all = a;
    return __paritydi2(x.s.high ^ x.s.low);
}

Questions:
~~~~~~~~~~

1. are these functions still needed, given that __builtin_parity is available?

2. will the optimiser "inline" the internal function calls (as part of
LTO)?

   If NOT, they should be inlined manually!

   JFTR: if the 3 functions are part of a single source or compilation unit,
         they are inlined by the compiler!

   Yes, parity is seldomly used, so this optimisation may not seem necessary.

si_int
__paritydi2(di_int a)
{
    su_int x = (su_int)a;
    x ^= (du_int)a >> 32;
    x ^= x >> 16;
    x ^= x >> 8;
    x ^= x >> 4;
    return (0x69966996 >> x) & 1;
}

si_int
__parityti2(ti_int a)
{
    du_int x = (du_int)a;
    x ^= (tu_int)a >> 64;
    x ^= x >> 32;
    x ^= x >> 16;
    x ^= x >> 8;
    x ^= x >> 4;
    return (0x69966996 >> x) & 1;
}

CAVEAT: the last right-shift MAY BE undefined behaviour, the optimisation
        shown here only works on CPUs which perform shifts modulo word-size!

stay tuned
Stefan Kanthak

I don't believe clang/llvm will ever emit a call to the parity library
routines today. But it might be needed so we can say that we match the
libgcc interface. gcc doesn't use it for x86 either as far as I know. And I
don't know if they have it in their x86 libgcc. But it might be easier for
compiler-rt to be a superset of libgcc rather than trying to track exactly
what they have on each target.

Not sure about the inlining question.

~Craig


On Tue, Dec 4, 2018 at 11:51 AM Stefan Kanthak via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> Hi @ll,
>
> compiler-rt/lib/builtins/parityti2.c
> compiler-rt/lib/builtins/paritydi2.c
> compiler-rt/lib/builtins/paritysi2.c
>
> implement the parity function as matroschka:
>
> si_int
> __paritysi2(si_int a)
> {
>     su_int x = (su_int)a;
>     x ^= x >> 16;
>     x ^= x >> 8;
>     x ^= x >> 4;
>     return (0x6996 >> (x & 0xF)) & 1; // see optimisation
below!
> }
>
> si_int
> __paritydi2(di_int a)
> {
>     dwords x;
>     x.all = a;
>     return __paritysi2(x.s.high ^ x.s.low);
> }
>
> si_int
> __parityti2(ti_int a)
> {
>     twords x;
>     x.all = a;
>     return __paritydi2(x.s.high ^ x.s.low);
> }
>
> Questions:
> ~~~~~~~~~~
>
> 1. are these functions still needed, given that __builtin_parity is
> available?
>
> 2. will the optimiser "inline" the internal function calls (as
part of
> LTO)?
>
>    If NOT, they should be inlined manually!
>
>    JFTR: if the 3 functions are part of a single source or compilation
> unit,
>          they are inlined by the compiler!
>
>    Yes, parity is seldomly used, so this optimisation may not seem
> necessary.
>
> si_int
> __paritydi2(di_int a)
> {
>     su_int x = (su_int)a;
>     x ^= (du_int)a >> 32;
>     x ^= x >> 16;
>     x ^= x >> 8;
>     x ^= x >> 4;
>     return (0x69966996 >> x) & 1;
> }
>
> si_int
> __parityti2(ti_int a)
> {
>     du_int x = (du_int)a;
>     x ^= (tu_int)a >> 64;
>     x ^= x >> 32;
>     x ^= x >> 16;
>     x ^= x >> 8;
>     x ^= x >> 4;
>     return (0x69966996 >> x) & 1;
> }
>
> CAVEAT: the last right-shift MAY BE undefined behaviour, the optimisation
>         shown here only works on CPUs which perform shifts modulo
> word-size!
>
> stay tuned
> Stefan Kanthak
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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