Richard Sandiford <richard.sandif...@arm.com> writes:
> Prathamesh Kulkarni <prathamesh.kulka...@linaro.org> writes:
>> On Tue, 25 Jul 2023 at 18:25, Richard Sandiford
>> <richard.sandif...@arm.com> wrote:
>>>
>>> Hi,
>>>
>>> Thanks for the rework and sorry for the slow review.
>> Hi Richard,
>> Thanks for the suggestions! Please find my responses inline below.
>>>
>>> Prathamesh Kulkarni <prathamesh.kulka...@linaro.org> writes:
>>> > Hi Richard,
>>> > This is reworking of patch to extend fold_vec_perm to handle VLA vectors.
>>> > The attached patch unifies handling of VLS and VLA vector_csts, while
>>> > using fallback code
>>> > for ctors.
>>> >
>>> > For VLS vector, the patch ignores underlying encoding, and
>>> > uses npatterns = nelts, and nelts_per_pattern = 1.
>>> >
>>> > For VLA patterns, if sel has a stepped sequence, then it
>>> > only chooses elements from a particular pattern of a particular
>>> > input vector.
>>> >
>>> > To make things simpler, the patch imposes following constraints:
>>> > (a) op0_npatterns, op1_npatterns and sel_npatterns are powers of 2.
>>> > (b) The step size for a stepped sequence is a power of 2, and
>>> > multiple of npatterns of chosen input vector.
>>> > (c) Runtime vector length of sel is a multiple of sel_npatterns.
>>> > So, we don't handle sel.length = 2 + 2x and npatterns = 4.
>>> >
>>> > Eg:
>>> > op0, op1: npatterns = 2, nelts_per_pattern = 3
>>> > op0_len = op1_len = 16 + 16x.
>>> > sel = { 0, 0, 2, 0, 4, 0, ... }
>>> > npatterns = 2, nelts_per_pattern = 3.
>>> >
>>> > For pattern {0, 2, 4, ...}
>>> > Let,
>>> > a1 = 2
>>> > S = step size = 2
>>> >
>>> > Let Esel denote number of elements per pattern in sel at runtime.
>>> > Esel = (16 + 16x) / npatterns_sel
>>> > = (16 + 16x) / 2
>>> > = (8 + 8x)
>>> >
>>> > So, last element of pattern:
>>> > ae = a1 + (Esel - 2) * S
>>> > = 2 + (8 + 8x - 2) * 2
>>> > = 14 + 16x
>>> >
>>> > a1 /trunc arg0_len = 2 / (16 + 16x) = 0
>>> > ae /trunc arg0_len = (14 + 16x) / (16 + 16x) = 0
>>> > Since both are equal with quotient = 0, we select elements from op0.
>>> >
>>> > Since step size (S) is a multiple of npatterns(op0), we select
>>> > all elements from same pattern of op0.
>>> >
>>> > res_npatterns = max (op0_npatterns, max (op1_npatterns, sel_npatterns))
>>> > = max (2, max (2, 2)
>>> > = 2
>>> >
>>> > res_nelts_per_pattern = max (op0_nelts_per_pattern,
>>> > max
>>> > (op1_nelts_per_pattern,
>>> >
>>> > sel_nelts_per_pattern))
>>> > = max (3, max (3, 3))
>>> > = 3
>>> >
>>> > So res has encoding with npatterns = 2, nelts_per_pattern = 3.
>>> > res: { op0[0], op0[0], op0[2], op0[0], op0[4], op0[0], ... }
>>> >
>>> > Unfortunately, this results in an issue for poly_int_cst index:
>>> > For example,
>>> > op0, op1: npatterns = 1, nelts_per_pattern = 3
>>> > op0_len = op1_len = 4 + 4x
>>> >
>>> > sel: { 4 + 4x, 5 + 4x, 6 + 4x, ... } // should choose op1
>>> >
>>> > In this case,
>>> > a1 = 5 + 4x
>>> > S = (6 + 4x) - (5 + 4x) = 1
>>> > Esel = 4 + 4x
>>> >
>>> > ae = a1 + (esel - 2) * S
>>> > = (5 + 4x) + (4 + 4x - 2) * 1
>>> > = 7 + 8x
>>> >
>>> > IIUC, 7 + 8x will always be index for last element of op1 ?
>>> > if x = 0, len = 4, 7 + 8x = 7
>>> > if x = 1, len = 8, 7 + 8x = 15, etc.
>>> > So the stepped sequence will always choose elements
>>> > from op1 regardless of vector length for above case ?
>>> >
>>> > However,
>>> > ae /trunc op0_len
>>> > = (7 + 8x) / (4 + 4x)
>>> > which is not defined because 7/4 != 8/4
>>> > and we return NULL_TREE, but I suppose the expected result would be:
>>> > res: { op1[0], op1[1], op1[2], ... } ?
>>> >
>>> > The patch passes bootstrap+test on aarch64-linux-gnu with and without sve,
>>> > and on x86_64-unknown-linux-gnu.
>>> > I would be grateful for suggestions on how to proceed.
>>> >
>>> > Thanks,
>>> > Prathamesh
>>> >
>>> > diff --git a/gcc/fold-const.cc b/gcc/fold-const.cc
>>> > index a02ede79fed..8028b3e8e9a 100644
>>> > --- a/gcc/fold-const.cc
>>> > +++ b/gcc/fold-const.cc
>>> > @@ -85,6 +85,10 @@ along with GCC; see the file COPYING3. If not see
>>> > #include "vec-perm-indices.h"
>>> > #include "asan.h"
>>> > #include "gimple-range.h"
>>> > +#include <algorithm>
>>> > +#include "tree-pretty-print.h"
>>> > +#include "gimple-pretty-print.h"
>>> > +#include "print-tree.h"
>>> >
>>> > /* Nonzero if we are folding constants inside an initializer or a C++
>>> > manifestly-constant-evaluated context; zero otherwise.
>>> > @@ -10493,15 +10497,9 @@ fold_mult_zconjz (location_t loc, tree type,
>>> > tree expr)
>>> > static bool
>>> > vec_cst_ctor_to_array (tree arg, unsigned int nelts, tree *elts)
>>> > {
>>> > - unsigned HOST_WIDE_INT i, nunits;
>>> > + unsigned HOST_WIDE_INT i;
>>> >
>>> > - if (TREE_CODE (arg) == VECTOR_CST
>>> > - && VECTOR_CST_NELTS (arg).is_constant (&nunits))
>>> > - {
>>> > - for (i = 0; i < nunits; ++i)
>>> > - elts[i] = VECTOR_CST_ELT (arg, i);
>>> > - }
>>> > - else if (TREE_CODE (arg) == CONSTRUCTOR)
>>> > + if (TREE_CODE (arg) == CONSTRUCTOR)
>>> > {
>>> > constructor_elt *elt;
>>> >
>>> > @@ -10519,6 +10517,230 @@ vec_cst_ctor_to_array (tree arg, unsigned int
>>> > nelts, tree *elts)
>>> > return true;
>>> > }
>>> >
>>> > +/* Return a vector with (NPATTERNS, NELTS_PER_PATTERN) encoding. */
>>> > +
>>> > +static tree
>>> > +vector_cst_reshape (tree vec, unsigned npatterns, unsigned
>>> > nelts_per_pattern)
>>> > +{
>>> > + gcc_assert (pow2p_hwi (npatterns));
>>> > +
>>> > + if (VECTOR_CST_NPATTERNS (vec) == npatterns
>>> > + && VECTOR_CST_NELTS_PER_PATTERN (vec) == nelts_per_pattern)
>>> > + return vec;
>>> > +
>>> > + tree v = make_vector (exact_log2 (npatterns), nelts_per_pattern);
>>> > + TREE_TYPE (v) = TREE_TYPE (vec);
>>> > +
>>> > + unsigned nelts = npatterns * nelts_per_pattern;
>>> > + for (unsigned i = 0; i < nelts; i++)
>>> > + VECTOR_CST_ENCODED_ELT(v, i) = vector_cst_elt (vec, i);
>>> > + return v;
>>> > +}
>>> > +
>>> > +/* Helper routine for fold_vec_perm_vla to check if ARG is a suitable
>>> > + operand for VLA vec_perm folding. If arg is VLS, then set
>>> > + NPATTERNS = nelts and NELTS_PER_PATTERN = 1. */
>>> > +
>>> > +static tree
>>> > +valid_operand_for_fold_vec_perm_cst_p (tree arg)
>>> > +{
>>> > + if (TREE_CODE (arg) != VECTOR_CST)
>>> > + return NULL_TREE;
>>> > +
>>> > + unsigned HOST_WIDE_INT nelts;
>>> > + unsigned npatterns, nelts_per_pattern;
>>> > + if (TYPE_VECTOR_SUBPARTS (TREE_TYPE (arg)).is_constant (&nelts))
>>> > + {
>>> > + npatterns = nelts;
>>> > + nelts_per_pattern = 1;
>>> > + }
>>> > + else
>>> > + {
>>> > + npatterns = VECTOR_CST_NPATTERNS (arg);
>>> > + nelts_per_pattern = VECTOR_CST_NELTS_PER_PATTERN (arg);
>>> > + }
>>> > +
>>> > + if (!pow2p_hwi (npatterns))
>>> > + return NULL_TREE;
>>> > +
>>> > + return vector_cst_reshape (arg, npatterns, nelts_per_pattern);
>>> > +}
>>>
>>> I don't think we should reshape the vectors for VLS, since it would
>>> create more nodes for GC to clean up later. Also, the "compact" encoding
>>> is canonical even for VLS, so the reshaping would effectively create
>>> noncanonical constants (even if only temporarily).
>> Removed in the attached patch.
>>>
>>> Instead, I think we should change the later:
>>>
>>> > + if (!valid_mask_for_fold_vec_perm_cst_p (arg0, arg1, sel,
>>> > sel_npatterns,
>>> > + sel_nelts_per_pattern, reason,
>>> > verbose))
>>> > + return NULL_TREE;
>>>
>>> so that it comes after the computation of res_npatterns and
>>> res_nelts_per_pattern. Then, if valid_mask_for_fold_vec_perm_cst_p
>>> returns false, and if the result type has a constant number of elements,
>>> we should:
>>>
>>> * set res_npatterns to that number of elements
>>> * set res_nelts_per_pattern to 1
>>> * continue instead of returning null
>> Assuming we don't enforce only VLA or only VLS for input vectors and sel,
>> won't that be still an issue if res (and sel) is VLS, and input
>> vectors are VLA ?
>> For eg:
>> arg0, arg1 are type VNx4SI with npatterns = 2, nelts_per_pattern = 3, step =
>> 2
>> sel is V4SI constant with encoding { 0, 2, 4, ... }
>> and res_type is V4SI.
>> In this case, when it comes to index 4, the vector selection becomes
>> ambiguous,
>> since it can be arg1 for len = 4 + 4x, and arg0 for lengths > 4 + 4x ?
>
> Ah, right. So the condition is whether the result type and the data
> input type have a constant number of elements, rather than just the result.
Actually, I take that back. The reason:
>>> The loop that follows will then do the correct thing for each element.
is true is that:
+ if (!can_div_trunc_p (sel[i], len, &q, &r))
+ {
+ if (reason)
+ strcpy (reason, "cannot divide selector element by arg len");
+ return NULL_TREE;
+ }
will return false if q isn't computable at compile time (that is,
if we can't decide at compile time which input the element comes from).
So I think checking the result is enough.
Thanks,
Richard
