Re: [PATCH v2] Add pattern to convert vector shift + bitwise and + multiply to vector compare in some cases.

[Richard Biener <richard.guenther@gmail.com>]

On Wed, Nov 30, 2022 at 9:59 AM Manolis Tsamis <manolis.tsamis@vrull.eu> wrote:
>
> On Wed, Nov 30, 2022 at 9:44 AM Richard Biener
> <richard.guenther@gmail.com> wrote:
> >
> > On Tue, Nov 29, 2022 at 11:05 AM Manolis Tsamis <manolis.tsamis@vrull.eu> wrote:
> > >
> > > When using SWAR (SIMD in a register) techniques a comparison operation within
> > > such a register can be made by using a combination of shifts, bitwise and and
> > > multiplication. If code using this scheme is vectorized then there is potential
> > > to replace all these operations with a single vector comparison, by reinterpreting
> > > the vector types to match the width of the SWAR register.
> > >
> > > For example, for the test function packed_cmp_16_32, the original generated code is:
> > >
> > >         ldr     q0, [x0]
> > >         add     w1, w1, 1
> > >         ushr    v0.4s, v0.4s, 15
> > >         and     v0.16b, v0.16b, v2.16b
> > >         shl     v1.4s, v0.4s, 16
> > >         sub     v0.4s, v1.4s, v0.4s
> > >         str     q0, [x0], 16
> > >         cmp     w2, w1
> > >         bhi     .L20
> > >
> > > with this pattern the above can be optimized to:
> > >
> > >         ldr     q0, [x0]
> > >         add     w1, w1, 1
> > >         cmlt    v0.8h, v0.8h, #0
> > >         str     q0, [x0], 16
> > >         cmp     w2, w1
> > >         bhi     .L20
> > >
> > > The effect is similar for x86-64.
> > >
> > > Signed-off-by: Manolis Tsamis <manolis.tsamis@vrull.eu>
> > >
> > > gcc/ChangeLog:
> > >
> > >         * match.pd: Simplify vector shift + bit_and + multiply in some cases.
> > >
> > > gcc/testsuite/ChangeLog:
> > >
> > >         * gcc.target/aarch64/swar_to_vec_cmp.c: New test.
> > >
> > > ---
> > >
> > > Changes in v2:
> > >         - Changed pattern to use vec_cond_expr.
> > >         - Changed pattern to work with VLA vector.
> > >         - Added more checks and comments.
> > >
> > >  gcc/match.pd                                  | 60 ++++++++++++++++
> > >  .../gcc.target/aarch64/swar_to_vec_cmp.c      | 72 +++++++++++++++++++
> > >  2 files changed, 132 insertions(+)
> > >  create mode 100644 gcc/testsuite/gcc.target/aarch64/swar_to_vec_cmp.c
> > >
> > > diff --git a/gcc/match.pd b/gcc/match.pd
> > > index 67a0a682f31..05e7fc79ba8 100644
> > > --- a/gcc/match.pd
> > > +++ b/gcc/match.pd
> > > @@ -301,6 +301,66 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
> > >      (view_convert (bit_and:itype (view_convert @0)
> > >                                  (ne @1 { build_zero_cst (type); })))))))
> > >
> > > +/* In SWAR (SIMD in a register) code a signed comparison of packed data can
> > > +   be constructed with a particular combination of shift, bitwise and,
> > > +   and multiplication by constants.  If that code is vectorized we can
> > > +   convert this pattern into a more efficient vector comparison.  */
> > > +(simplify
> > > + (mult (bit_and (rshift @0 uniform_integer_cst_p@1)
> > > +           uniform_integer_cst_p@2)
> > > +    uniform_integer_cst_p@3)
> >
> > Please use VECTOR_CST in the match instead of uniform_integer_cst_p
> > and instead ...
> >
>
> Will do.
>
> > > + (with {
> > > +   tree rshift_cst = uniform_integer_cst_p (@1);
> > > +   tree bit_and_cst = uniform_integer_cst_p (@2);
> > > +   tree mult_cst = uniform_integer_cst_p (@3);
> > > +  }
> > > +  /* Make sure we're working with vectors and uniform vector constants.  */
> > > +  (if (VECTOR_TYPE_P (type)
> >
> > ... test for non-NULL *_cst here where you can use uniform_vector_p instead
> > of uniform_integer_cst_p.  You can elide the VECTOR_TYPE_P check then
> > and instead do INTEGRAL_TYPE_P (TREE_TYPE (type)).
> >
>
> Will do.
>
> > > +       && tree_fits_uhwi_p (rshift_cst)
> > > +       && tree_fits_uhwi_p (mult_cst)
> > > +       && tree_fits_uhwi_p (bit_and_cst))
> > > +   /* Compute what constants would be needed for this to represent a packed
> > > +      comparison based on the shift amount denoted by RSHIFT_CST.  */
> > > +   (with {
> > > +     HOST_WIDE_INT vec_elem_bits = vector_element_bits (type);
> > > +     poly_int64 vec_nelts = TYPE_VECTOR_SUBPARTS (type);
> > > +     poly_int64 vec_bits = vec_elem_bits * vec_nelts;
> > > +
> > > +     unsigned HOST_WIDE_INT cmp_bits_i, bit_and_i, mult_i;
> > > +     unsigned HOST_WIDE_INT target_mult_i, target_bit_and_i;
> > > +     cmp_bits_i = tree_to_uhwi (rshift_cst) + 1;
> > > +     target_mult_i = (HOST_WIDE_INT_1U << cmp_bits_i) - 1;
> > > +
> > > +     mult_i = tree_to_uhwi (mult_cst);
> > > +     bit_and_i = tree_to_uhwi (bit_and_cst);
> > > +     target_bit_and_i = 0;
> > > +
> > > +     /* The bit pattern in BIT_AND_I should be a mask for the least
> > > +        significant bit of each packed element that is CMP_BITS wide.  */
> > > +     for (unsigned i = 0; i < vec_elem_bits / cmp_bits_i; i++)
> > > +       target_bit_and_i = (target_bit_and_i << cmp_bits_i) | 1U;
> > > +    }
> > > +    (if ((exact_log2 (cmp_bits_i)) >= 0
> > > +        && cmp_bits_i < HOST_BITS_PER_WIDE_INT
> > > +        && multiple_p (vec_bits, cmp_bits_i)
> > > +        && vec_elem_bits <= HOST_BITS_PER_WIDE_INT
> > > +        && target_mult_i == mult_i
> > > +        && target_bit_and_i == bit_and_i)
> > > +     /* Compute the vector shape for the comparison and check if the target is
> > > +       able to expand the comparison with that type.  */
> > > +     (with {
> > > +       /* We're doing a signed comparison.  */
> > > +       tree cmp_type = build_nonstandard_integer_type (cmp_bits_i, 0);
> > > +       poly_int64 vector_type_nelts = exact_div (vec_bits, cmp_bits_i);
> > > +       tree vector_cmp_type = build_vector_type (cmp_type, vector_type_nelts);
> > > +       tree zeros = build_zero_cst (vector_cmp_type);
> > > +       tree ones = build_all_ones_cst (vector_cmp_type);
> > > +      }
> > > +      (if (expand_vec_cmp_expr_p (vector_cmp_type, vector_cmp_type, LT_EXPR))
> > > +       (view_convert:type (vec_cond (lt (view_convert:vector_cmp_type @0)
> > > +                                    { zeros; })
> > > +                          { ones; } { zeros; })))))))))
> >
> > You are testing whether we can expand the comparison but not whether we can
> > expand the vector condition here?  The set of combinations supported are
> > tricky, I think your test is conservatively OK but it might fail to match AVX512
> > and will fail targets with masks (SVE, GCN?).  I think adding
> >
> >   || expand_vec_cond_expr_p (vector_cmp_type, vector_cmp_type, LT_EXPR)
> >
> > would fix that (but there will be extra cost in producing the final
> > result vector
> > from the comparison mask).
> >
>
> I didn't understand that these needed separate checks, thanks for
> pointing it out.
> Will do.
>
> > I do wonder if, as this is targeted at vectorization, this shouldn't
> > be a vectorizer pattern instead of a post-processing transform?  That
>
> My initial implementation for this was actually targeting the vectorization
> patterns but it didn't fit with it very well. One issue that was
> raised is that these
> patterns need the internal vec calls and adding one internal function for this
> optimization would be hardly justifiable and possibly ugly. The way I see it
> this is a peephole optimization and not something to make a pattern for.
> After that match.pd was looking like a better place to achieve the optimization
> in a clean way and not pollute the vectorizer.
> My understanding of the vectorization machinery is limited so please correct
> me if I'm wrong.
>
> > way we would
> > get the costing in the vectorizer correct and not rely on integer
> > vector shift or
> > multiplication and also get the cost of producing the result vector correct?  I
> > can't fully decipher the trick though, but assume that vector_cmp_type
> > always has
> > smaller elements than 'type'.
> >
>
> That is correct, all this applies when vector_cmp_type has smaller elements
> than type. Otherwise if vector_cmp_type == type there is no SWAR and GCC
> optimizes it nicely already). But I haven't added any special case for that as
> the pattern is general and works nicely in both cases.

So a vectorizer pattern would be to replace the multiplication with
(with smaller type T and original type U)

  (T)@0 < 0 ? (U)-1 :(U)0

?  That is, this should also work on the scalar side?  (if it doesn't then
it indeed gets more tricky)

> The trick is that in scalar code where simd-within-a-register
> techniques are used
> a signed comparison can be created by 1) right shifting the sign bits to the lsb
> 2) masking off everything else and 3) multiply by an appropriate
> 0xffff... constant
> to create a mask. GCC computes this as 4 instructions since the
> multiply is usually
> replaced by shift and subtract. But if this sequence gets vectorized
> then all this
> sequence can be replaced by a single vector compare-less, since the
> original code
> was emulating what a vector comapre-less does anyway.
>
> Thanks!
> Manolis
>
> > Thanks,
> > Richard.
> >
> > > +
> > >  (for cmp (gt ge lt le)
> > >       outp (convert convert negate negate)
> > >       outn (negate negate convert convert)
> > > diff --git a/gcc/testsuite/gcc.target/aarch64/swar_to_vec_cmp.c b/gcc/testsuite/gcc.target/aarch64/swar_to_vec_cmp.c
> > > new file mode 100644
> > > index 00000000000..26f9ad9ef28
> > > --- /dev/null
> > > +++ b/gcc/testsuite/gcc.target/aarch64/swar_to_vec_cmp.c
> > > @@ -0,0 +1,72 @@
> > > +/* { dg-do compile } */
> > > +/* { dg-options "-O2 -ftree-vectorize" } */
> > > +
> > > +typedef unsigned char uint8_t;
> > > +typedef unsigned short uint16_t;
> > > +typedef unsigned int uint32_t;
> > > +
> > > +/* 8-bit SWAR tests.  */
> > > +
> > > +static uint8_t packed_cmp_8_8(uint8_t a)
> > > +{
> > > +  return ((a >> 7) & 0x1U) * 0xffU;
> > > +}
> > > +
> > > +/* 16-bit SWAR tests.  */
> > > +
> > > +static uint16_t packed_cmp_8_16(uint16_t a)
> > > +{
> > > +  return ((a >> 7) & 0x101U) * 0xffU;
> > > +}
> > > +
> > > +static uint16_t packed_cmp_16_16(uint16_t a)
> > > +{
> > > +  return ((a >> 15) & 0x1U) * 0xffffU;
> > > +}
> > > +
> > > +/* 32-bit SWAR tests.  */
> > > +
> > > +static uint32_t packed_cmp_8_32(uint32_t a)
> > > +{
> > > +  return ((a >> 7) & 0x1010101U) * 0xffU;
> > > +}
> > > +
> > > +static uint32_t packed_cmp_16_32(uint32_t a)
> > > +{
> > > +  return ((a >> 15) & 0x10001U) * 0xffffU;
> > > +}
> > > +
> > > +static uint32_t packed_cmp_32_32(uint32_t a)
> > > +{
> > > +  return ((a >> 31) & 0x1U) * 0xffffffffU;
> > > +}
> > > +
> > > +/* Driver function to test the vectorized code generated for the different
> > > +   packed_cmp variants.  */
> > > +
> > > +#define VECTORIZED_PACKED_CMP(T, FUNC)         \
> > > +  void vectorized_cmp_##FUNC(T* a, int n)      \
> > > +  {                                            \
> > > +    n = (n / 32) * 32;                         \
> > > +    for(int i = 0; i < n; i += 4)              \
> > > +    {                                          \
> > > +      a[i + 0] = FUNC(a[i + 0]);               \
> > > +      a[i + 1] = FUNC(a[i + 1]);               \
> > > +      a[i + 2] = FUNC(a[i + 2]);               \
> > > +      a[i + 3] = FUNC(a[i + 3]);               \
> > > +    }                                          \
> > > +  }
> > > +
> > > +VECTORIZED_PACKED_CMP(uint8_t, packed_cmp_8_8);
> > > +
> > > +VECTORIZED_PACKED_CMP(uint16_t, packed_cmp_8_16);
> > > +VECTORIZED_PACKED_CMP(uint16_t, packed_cmp_16_16);
> > > +
> > > +VECTORIZED_PACKED_CMP(uint32_t, packed_cmp_8_32);
> > > +VECTORIZED_PACKED_CMP(uint32_t, packed_cmp_16_32);
> > > +VECTORIZED_PACKED_CMP(uint32_t, packed_cmp_32_32);
> > > +
> > > +/* { dg-final { scan-assembler {\tcmlt\t} } } */
> > > +/* { dg-final { scan-assembler-not {\tushr\t} } } */
> > > +/* { dg-final { scan-assembler-not {\tshl\t} } } */
> > > +/* { dg-final { scan-assembler-not {\tmul\t} } } */
> > > --
> > > 2.34.1
> > >