Re: [RFC PATCH v1 03/43] accel/tcg: Add gvec size changing operations

[Richard Henderson]

On 12/3/24 12:08, Anton Johansson wrote:
> On 22/11/24, Richard Henderson wrote:
> > On 11/20/24 19:49, Anton Johansson wrote:
> > > Adds new functions to the gvec API for truncating, sign- or zero
> > > extending vector elements.  Currently implemented as helper functions,
> > > these may be mapped onto host vector instructions in the future.
> > >
> > > For the time being, allows translation of more complicated vector
> > > instructions by helper-to-tcg.
> > >
> > > Signed-off-by: Anton Johansson <anjo@rev.ng>
> > > ---
> > >    accel/tcg/tcg-runtime-gvec.c     | 41 +++++++++++++++++
> > >    accel/tcg/tcg-runtime.h          | 22 +++++++++
> > >    include/tcg/tcg-op-gvec-common.h | 18 ++++++++
> > >    tcg/tcg-op-gvec.c                | 78 ++++++++++++++++++++++++++++++++
> > >    4 files changed, 159 insertions(+)
> > >
> > > diff --git a/accel/tcg/tcg-runtime-gvec.c b/accel/tcg/tcg-runtime-gvec.c
> > > index afca89baa1..685c991e6a 100644
> > > --- a/accel/tcg/tcg-runtime-gvec.c
> > > +++ b/accel/tcg/tcg-runtime-gvec.c
> > > @@ -1569,3 +1569,44 @@ void HELPER(gvec_bitsel)(void *d, void *a, void *b, void 
> > > *c, uint32_t desc)
> > >        }
> > >        clear_high(d, oprsz, desc);
> > >    }
> > > +
> > > +#define DO_SZ_OP1(NAME, DSTTY, SRCTY)                                      \
> > > +void HELPER(NAME)(void *d, void *a, uint32_t desc)                         \
> > > +{                                                                          \
> > > +    intptr_t oprsz = simd_oprsz(desc);                                     \
> > > +    intptr_t elsz = oprsz/sizeof(DSTTY);                                   \
> > > +    intptr_t i;                                                            \
> > > +                                                                           \
> > > +    for (i = 0; i < elsz; ++i) {                                           \
> > > +        SRCTY aa = *((SRCTY *) a + i);                                     \
> > > +        *((DSTTY *) d + i) = aa;                                           \
> > > +    }                                                                      \
> > > +    clear_high(d, oprsz, desc);                                            \
> >
> > This formulation is not valid.
> >
> > (1) Generic forms must *always* operate strictly on columns.  This
> > formulation is either expanding a narrow vector to a wider vector or
> > compressing a wider vector to a narrow vector.
> >
> > (2) This takes no care for byte ordering of the data between columns.  This
> > is where sticking strictly to columns helps, in that we can assume that data
> > is host-endian *within the column*, but we cannot assume anything about the
> > element indexing of ptr + i.
>
> Concerning (1) and (2), is this a limitation imposed on generic vector
> ops. to simplify mapping to host vector instructions where
> padding/alignment of elements might differ?  From my understanding, the
> helper above should be fine since we can assume contiguous elements?

This is a limitation imposed on generic vector ops, because different target/arch/ 
represent their vectors in different ways.

For instance, Arm and RISC-V chunk the vector in to host-endian uint64_t, with the chunks 
indexed little-endian.  But PPC puts the entire 128-bit vector in host-endian bit 
ordering, so the uint64_t chunks are host-endian.

On a big-endian host, ptr+1 may be addressing element i-1 or i-7 instead of i+1.

> I see, I don't think we can make this work for Hexagon vector ops., as
> an example consider V6_vadduwsat which performs an unsigned saturated
> add of 32-bit elements, currently we emit
>
>      void emit_V6_vadduwsat(intptr_t vec2, intptr_t vec7, intptr_t vec6) {
>          VectorMem mem = {0};
>          intptr_t vec5 = temp_new_gvec(&mem, 256);
>          tcg_gen_gvec_zext(MO_64, MO_32, vec5, vec7, 256, 128, 256);
>
>          intptr_t vec1 = temp_new_gvec(&mem, 256);
>          tcg_gen_gvec_zext(MO_64, MO_32, vec1, vec6, 256, 128, 256);
>
>          tcg_gen_gvec_add(MO_64, vec1, vec1, vec5, 256, 256);
>
>          intptr_t vec3 = temp_new_gvec(&mem, 256);
>          tcg_gen_gvec_dup_imm(MO_64, vec3, 256, 256, 4294967295ull);
>
>          tcg_gen_gvec_umin(MO_64, vec1, vec1, vec3, 256, 256);
>
>          tcg_gen_gvec_trunc(MO_32, MO_64, vec2, vec1, 128, 256, 128);
>      }
>
> so we really do rely on the size-changing property of zext here, the
> input vectors are 128-byte and we expand them to 256-byte.  We could
> expand vector operations within the instruction to the largest vector
> size, but would need to zext and trunc to destination and source
> registers anyway.
Yes, well, this is the output of llvm though, yes?

Did you forget to describe TCG's native saturating operations to the compiler? 
tcg_gen_gvec_usadd performs exactly this operation.

And if you'd like to improve llvm, usadd(a, b) equals umin(a, ~b) + b.
Fewer operations without having to change vector sizes.
Similarly for unsigned saturating subtract: ussub(a, b) equals umax(a, b) - b.


r~