Re: PR111754

[Prathamesh Kulkarni <prathamesh.kulkarni@linaro.org>]

On Wed, 8 Nov 2023 at 21:57, Prathamesh Kulkarni
<prathamesh.kulkarni@linaro.org> wrote:
>
> On Thu, 26 Oct 2023 at 09:43, Prathamesh Kulkarni
> <prathamesh.kulkarni@linaro.org> wrote:
> >
> > On Thu, 26 Oct 2023 at 04:09, Richard Sandiford
> > <richard.sandiford@arm.com> wrote:
> > >
> > > Prathamesh Kulkarni <prathamesh.kulkarni@linaro.org> writes:
> > > > On Wed, 25 Oct 2023 at 02:58, Richard Sandiford
> > > > <richard.sandiford@arm.com> wrote:
> > > >>
> > > >> Hi,
> > > >>
> > > >> Sorry the slow review.  I clearly didn't think this through properly
> > > >> when doing the review of the original patch, so I wanted to spend
> > > >> some time working on the code to get a better understanding of
> > > >> the problem.
> > > >>
> > > >> Prathamesh Kulkarni <prathamesh.kulkarni@linaro.org> writes:
> > > >> > Hi,
> > > >> > For the following test-case:
> > > >> >
> > > >> > typedef float __attribute__((__vector_size__ (16))) F;
> > > >> > F foo (F a, F b)
> > > >> > {
> > > >> >   F v = (F) { 9 };
> > > >> >   return __builtin_shufflevector (v, v, 1, 0, 1, 2);
> > > >> > }
> > > >> >
> > > >> > Compiling with -O2 results in following ICE:
> > > >> > foo.c: In function ‘foo’:
> > > >> > foo.c:6:10: internal compiler error: in decompose, at rtl.h:2314
> > > >> >     6 |   return __builtin_shufflevector (v, v, 1, 0, 1, 2);
> > > >> >       |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > > >> > 0x7f3185 wi::int_traits<std::pair<rtx_def*, machine_mode>
> > > >> >>::decompose(long*, unsigned int, std::pair<rtx_def*, machine_mode>
> > > >> > const&)
> > > >> >         ../../gcc/gcc/rtl.h:2314
> > > >> > 0x7f3185 wide_int_ref_storage<false,
> > > >> > false>::wide_int_ref_storage<std::pair<rtx_def*, machine_mode>
> > > >> >>(std::pair<rtx_def*, machine_mode> const&)
> > > >> >         ../../gcc/gcc/wide-int.h:1089
> > > >> > 0x7f3185 generic_wide_int<wide_int_ref_storage<false, false>
> > > >> >>::generic_wide_int<std::pair<rtx_def*, machine_mode>
> > > >> >>(std::pair<rtx_def*, machine_mode> const&)
> > > >> >         ../../gcc/gcc/wide-int.h:847
> > > >> > 0x7f3185 poly_int<1u, generic_wide_int<wide_int_ref_storage<false,
> > > >> > false> > >::poly_int<std::pair<rtx_def*, machine_mode>
> > > >> >>(poly_int_full, std::pair<rtx_def*, machine_mode> const&)
> > > >> >         ../../gcc/gcc/poly-int.h:467
> > > >> > 0x7f3185 poly_int<1u, generic_wide_int<wide_int_ref_storage<false,
> > > >> > false> > >::poly_int<std::pair<rtx_def*, machine_mode>
> > > >> >>(std::pair<rtx_def*, machine_mode> const&)
> > > >> >         ../../gcc/gcc/poly-int.h:453
> > > >> > 0x7f3185 wi::to_poly_wide(rtx_def const*, machine_mode)
> > > >> >         ../../gcc/gcc/rtl.h:2383
> > > >> > 0x7f3185 rtx_vector_builder::step(rtx_def*, rtx_def*) const
> > > >> >         ../../gcc/gcc/rtx-vector-builder.h:122
> > > >> > 0xfd4e1b vector_builder<rtx_def*, machine_mode,
> > > >> > rtx_vector_builder>::elt(unsigned int) const
> > > >> >         ../../gcc/gcc/vector-builder.h:253
> > > >> > 0xfd4d11 rtx_vector_builder::build()
> > > >> >         ../../gcc/gcc/rtx-vector-builder.cc:73
> > > >> > 0xc21d9c const_vector_from_tree
> > > >> >         ../../gcc/gcc/expr.cc:13487
> > > >> > 0xc21d9c expand_expr_real_1(tree_node*, rtx_def*, machine_mode,
> > > >> > expand_modifier, rtx_def**, bool)
> > > >> >         ../../gcc/gcc/expr.cc:11059
> > > >> > 0xaee682 expand_expr(tree_node*, rtx_def*, machine_mode, expand_modifier)
> > > >> >         ../../gcc/gcc/expr.h:310
> > > >> > 0xaee682 expand_return
> > > >> >         ../../gcc/gcc/cfgexpand.cc:3809
> > > >> > 0xaee682 expand_gimple_stmt_1
> > > >> >         ../../gcc/gcc/cfgexpand.cc:3918
> > > >> > 0xaee682 expand_gimple_stmt
> > > >> >         ../../gcc/gcc/cfgexpand.cc:4044
> > > >> > 0xaf28f0 expand_gimple_basic_block
> > > >> >         ../../gcc/gcc/cfgexpand.cc:6100
> > > >> > 0xaf4996 execute
> > > >> >         ../../gcc/gcc/cfgexpand.cc:6835
> > > >> >
> > > >> > IIUC, the issue is that fold_vec_perm returns a vector having float element
> > > >> > type with res_nelts_per_pattern == 3, and later ICE's when it tries
> > > >> > to derive element v[3], not present in the encoding, while trying to
> > > >> > build rtx vector
> > > >> > in rtx_vector_builder::build():
> > > >> >  for (unsigned int i = 0; i < nelts; ++i)
> > > >> >     RTVEC_ELT (v, i) = elt (i);
> > > >> >
> > > >> > The attached patch tries to fix this by returning false from
> > > >> > valid_mask_for_fold_vec_perm_cst if sel has a stepped sequence and
> > > >> > input vector has non-integral element type, so for VLA vectors, it
> > > >> > will only build result with dup sequence (nelts_per_pattern < 3) for
> > > >> > non-integral element type.
> > > >> >
> > > >> > For VLS vectors, this will still work for stepped sequence since it
> > > >> > will then use the "VLS exception" in fold_vec_perm_cst, and set:
> > > >> > res_npattern = res_nelts and
> > > >> > res_nelts_per_pattern = 1
> > > >> >
> > > >> > and fold the above case to:
> > > >> > F foo (F a, F b)
> > > >> > {
> > > >> >   <bb 2> [local count: 1073741824]:
> > > >> >   return { 0.0, 9.0e+0, 0.0, 0.0 };
> > > >> > }
> > > >> >
> > > >> > But I am not sure if this is entirely correct, since:
> > > >> > tree res = out_elts.build ();
> > > >> > will canonicalize the encoding and may result in a stepped sequence
> > > >> > (vector_builder::finalize() may reduce npatterns at the cost of increasing
> > > >> > nelts_per_pattern)  ?
> > > >> >
> > > >> > PS: This issue is now latent after PR111648 fix, since
> > > >> > valid_mask_for_fold_vec_perm_cst with  sel = {1, 0, 1, ...} returns
> > > >> > false because the corresponding pattern in arg0 is not a natural
> > > >> > stepped sequence, and folds correctly using VLS exception. However, I
> > > >> > guess the underlying issue of dealing with non-integral element types
> > > >> > in fold_vec_perm_cst still remains ?
> > > >> >
> > > >> > The patch passes bootstrap+test with and without SVE on aarch64-linux-gnu,
> > > >> > and on x86_64-linux-gnu.
> > > >>
> > > >> I think the problem is instead in the way that we're calculating
> > > >> res_npatterns and res_nelts_per_pattern.
> > > >>
> > > >> If the selector is a duplication of { a1, ..., an }, then the
> > > >> result will be a duplication of n elements, regardless of the shape
> > > >> of the other arguments.
> > > >>
> > > >> Similarly, if the selector is { a1, ...., an } followed by a
> > > >> duplication of { b1, ..., bn }, the result be n elements followed
> > > >> by a duplication of n elements, regardless of the shape of the other
> > > >> arguments.
> > > >>
> > > >> So for these two cases, res_npatterns and res_nelts_per_pattern
> > > >> can come directly from the selector's encoding.
> > > >>
> > > >> If:
> > > >>
> > > >> (1) the selector is an n-pattern stepped sequence
> > > >> (2) the stepped part of each pattern selects from the same input pattern
> > > >> (3) the stepped part of each pattern does not select the first element
> > > >>     of the input pattern, or the full input pattern is stepped
> > > >>     (your previous patch)
> > > >>
> > > >> then the result is stepped only if one of the inputs is stepped.
> > > >> This is because, if an input pattern has 1 or 2 elements, (3) means
> > > >> that each element of the stepped sequence will select the same value,
> > > >> as if the selector step had been 0.
> > > > Hi Richard,
> > > > Thanks for the suggestions! I agree when selector is dup of {a1, ... an, ...} or
> > > > base elements followed up dup {a1, .. an, b1, ... bn, ...} in that
> > > > case we can set
> > > > res_nelts_per_pattern from selector's encoding. However even if we provide
> > > > more nelts_per_pattern that necessary, I guess vector_builder::finalize() will
> > > > canonicalize it to the correct encoding for result ?
> > >
> > > Right.  The elements before finalize is called have to be correct,
> > > but they don't need to be canonical or minimal.
> > >
> > > After the change, we'll build no more elements than we did before.
> > >
> > > >> So I think the PR could be solved by something like the attached.
> > > >> Do you agree?  If so, could you base the patch on this instead?
> > > >>
> > > >> Only tested against the self-tests.
> > > >>
> > > >> Thanks,
> > > >> Richard
> > > >>
> > > >> diff --git a/gcc/fold-const.cc b/gcc/fold-const.cc
> > > >> index 40767736389..00fce4945a7 100644
> > > >> --- a/gcc/fold-const.cc
> > > >> +++ b/gcc/fold-const.cc
> > > >> @@ -10743,27 +10743,37 @@ fold_vec_perm_cst (tree type, tree arg0, tree arg1, const vec_perm_indices &sel,
> > > >>    unsigned res_npatterns, res_nelts_per_pattern;
> > > >>    unsigned HOST_WIDE_INT res_nelts;
> > > >>
> > > >> -  /* (1) If SEL is a suitable mask as determined by
> > > >> -     valid_mask_for_fold_vec_perm_cst_p, then:
> > > >> -     res_npatterns = max of npatterns between ARG0, ARG1, and SEL
> > > >> -     res_nelts_per_pattern = max of nelts_per_pattern between
> > > >> -                            ARG0, ARG1 and SEL.
> > > >> -     (2) If SEL is not a suitable mask, and TYPE is VLS then:
> > > >> -     res_npatterns = nelts in result vector.
> > > >> -     res_nelts_per_pattern = 1.
> > > >> -     This exception is made so that VLS ARG0, ARG1 and SEL work as before.  */
> > > >> -  if (valid_mask_for_fold_vec_perm_cst_p (arg0, arg1, sel, reason))
> > > >> -    {
> > > >> -      res_npatterns
> > > >> -       = std::max (VECTOR_CST_NPATTERNS (arg0),
> > > >> -                   std::max (VECTOR_CST_NPATTERNS (arg1),
> > > >> -                             sel.encoding ().npatterns ()));
> > > >> +  /* First try to implement the fold in a VLA-friendly way.
> > > >> +
> > > >> +     (1) If the selector is simply a duplication of N elements, the
> > > >> +        result is likewise a duplication of N elements.
> > > >> +
> > > >> +     (2) If the selector is N elements followed by a duplication
> > > >> +        of N elements, the result is too.
> > > >>
> > > >> -      res_nelts_per_pattern
> > > >> -       = std::max (VECTOR_CST_NELTS_PER_PATTERN (arg0),
> > > >> -                   std::max (VECTOR_CST_NELTS_PER_PATTERN (arg1),
> > > >> -                             sel.encoding ().nelts_per_pattern ()));
> > > >> +     (3) If the selector is N elements followed by an interleaving
> > > >> +        of N linear series, the situation is more complex.
> > > >>
> > > >> +        valid_mask_for_fold_vec_perm_cst_p detects whether we
> > > >> +        can handle this case.  If we can, then each of the N linear
> > > >> +        series either (a) selects the same element each time or
> > > >> +        (b) selects a linear series from one of the input patterns.
> > > >> +
> > > >> +        If (b) holds for one of the linear series, the result
> > > >> +        will contain a linear series, and so the result will have
> > > >> +        the same shape as the selector.  If (a) holds for all of
> > > >> +        the lienar series, the result will be the same as (2) above.
> > > >> +
> > > >> +        (b) can only hold if one of the inputs pattern has a
> > > >> +        stepped encoding.  */
> > > >> +  if (valid_mask_for_fold_vec_perm_cst_p (arg0, arg1, sel, reason))
> > > >> +    {
> > > >> +      res_npatterns = sel.encoding ().npatterns ();
> > > >> +      res_nelts_per_pattern = sel.encoding ().nelts_per_pattern ();
> > > >> +      if (res_nelts_per_pattern == 3
> > > >> +         && VECTOR_CST_NELTS_PER_PATTERN (arg0) < 3
> > > >> +         && VECTOR_CST_NELTS_PER_PATTERN (arg1) < 3)
> > > >> +       res_nelts_per_pattern = 2;
> > > > Um, in this case, should we set:
> > > > res_nelts_per_pattern = max (nelts_per_pattern (arg0), nelts_per_pattern(arg1))
> > > > if both have nelts_per_pattern == 1 ?
> > >
> > > No, it still needs to be 2 even if arg0 and arg1 are duplicates.
> > > E.g. consider a selector that picks the first element of arg0
> > > followed by a duplicate of the first element of arg1.
> > >
> > > > Also I suppose this matters only for non-integral element type, since
> > > > for integral element type,
> > > > vector_cst_elt will return the correct value even if the element is
> > > > not explicitly encoded and input vector is dup ?
> > >
> > > Yeah, but it might help even for integers.  If we build fewer
> > > elements explicitly, and so read fewer implicitly-encoded inputs,
> > > there's less risk of running into:
> > >
> > >       if (!can_div_trunc_p (sel[i], len, &q, &r))
> > >         {
> > >           if (reason)
> > >             *reason = "cannot divide selector element by arg len";
> > >           return NULL_TREE;
> > >         }
> > Ah right, thanks for the clarification!
> > I am currently away on vacation and will return next Thursday, and
> > will post a follow up patch based on your patch.
> > Sorry for the delay.
> Hi,
> Sorry for slow response, I have rebased your patch and added couple of tests.
> The attached patch resulted in fallout for aarch64/sve/slp_3.c and
> aarch64/sve/slp_4.c.
>
> Specifically for slp_3.c, we didn't fold following case:
> arg0, arg1 are dup vectors.
> sel = { 0, len, 1, len + 1, 2, len + 2, ... } // (npatterns = 2,
> nelts_per_pattern = 3)
> because res_nelts_per_pattern was set to 3, and upon encountering 2,
> fold_vec_perm_cst returned false.
>
> With patch, we set res_nelts_per_pattern = 2 (since input vectors are
> dup), and thus gets folded to:
> res = { arg0[0], arg1[0], ... } // (2, 1)
>
> Which results in using ldrqd for loading the result instead of doing
> the permutation at runtime with mov and zip1.
> I have adjusted the tests for new code-gen.
> Does it look OK ?
>
> There's also this strange failure observed on x86_64, as well as on aarch64:
> New tests that FAIL (1 tests):
> libitm.c++/dropref.C -B
> /home/prathamesh.kulkarni/gnu-toolchain/gcc/gnu-964-5/bootstrap-build-after/aarch64-unknown-linux-gnu/./libitm/../libstdc++-v3/src/.libs
> execution test
>
> Looking at dropref.C:
> /* { dg-xfail-run-if "unsupported" { *-*-* } } */
> #include <libitm.h>
>
> char *pp;
>
> int main()
> {
>   __transaction_atomic {
>     _ITM_dropReferences (pp, 555);
>   }
>   return 0;
> }
>
> doesn't seem relevant to VEC_PERM_EXPR folding ?
> The patch otherwise passes bootstrap+test on aarch64-linux-gnu with
> and without SVE, and on x86_64-linux-gnu.
Hi,
ping: https://gcc.gnu.org/pipermail/gcc-patches/2023-November/635728.html

Thanks,
Prathamesh
>
> Thanks,
> Prathamesh
> >
> > Thanks,
> > Prathamesh
> > >
> > > Thanks,
> > > Richard