[PATCH 0/14+2][Vectorizer] Made reductions endianness-neutral, fixes PR/61114

Richard Biener richard.guenther@gmail.com
Mon Sep 22 11:21:00 GMT 2014

On Thu, Sep 18, 2014 at 1:41 PM, Alan Lawrence <alan.lawrence@arm.com> wrote:
> The end goal here is to remove this code from tree-vect-loop.c
> (vect_create_epilog_for_reduction):
>       if (BYTES_BIG_ENDIAN)
>         bitpos = size_binop (MULT_EXPR,
>                              bitsize_int (TYPE_VECTOR_SUBPARTS (vectype) -
> 1),
>                              TYPE_SIZE (scalar_type));
>       else
> as this is the root cause of PR/61114 (see testcase there, failing on all
> bigendian targets supporting reduc_[us]plus_optab). Quoting Richard Biener,
> "all code conditional on BYTES/WORDS_BIG_ENDIAN in tree-vect* is
> suspicious". The code snippet above is used on two paths:
> (Path 1) (patches 1-6) Reductions using REDUC_(PLUS|MIN|MAX)_EXPR =
> reduc_[us](plus|min|max)_optab.
> The optab is documented as "the scalar result is stored in the least
> significant bits of operand 0", but the tree code as "the first element in
> the vector holding the result of the reduction of all elements of the
> operand". This mismatch means that when the tree code is folded, the code
> snippet above reads the result from the wrong end of the vector.
> The strategy (as per
> https://gcc.gnu.org/ml/gcc-patches/2014-08/msg00041.html) is to define new
> tree codes and optabs that produce scalar results directly; this seems
> better than tying (the element of the vector into which the result is
> placed) to (the endianness of the target), and avoids generating extra moves
> on current bigendian targets. However, the previous optabs are retained for
> now as a migration strategy so as not to break existing backends; moving
> individual platforms over will follow.
> A complication here is on AArch64, where we directly generate
> REDUC_PLUS_EXPRs from intrinsics in gimple_fold_builtin; I temporarily
> remove this folding in order to decouple the midend and AArch64 backend.

Sounds fine.  I hope we can transition all backends for 5.0 and remove
the vector variant optabs (maybe renaming the scalar ones).

> (Path 2) (patches 7-13) Reductions using whole-vector-shifts, i.e.
> VEC_RSHIFT_EXPR and vec_shr_optab. Here the tree code as well as the optab
> is defined in an endianness-dependent way, leading to significant
> complication in fold-const.c. (Moreover, the "equivalent" vec_shl_optab is
> never used!). Few platforms appear to handle vec_shr_optab (and fewer
> bigendian - I see only PowerPC and MIPS), so it seems pertinent to change
> the existing optab to be endianness-neutral.
> Patch 10 defines vec_shr for AArch64, for the old specification; patch 13
> updates that implementation to fit the new endianness-neutral specification,
> serving as a guide for other existing backends. Patches/RFCs 15 and 16 are
> equivalents for MIPS and PowerPC; I haven't tested these but hope they act
> as useful pointers for the port maintainers.
> Finally patch 14 cleans up the affected part of tree-vect-loop.c
> (vect_create_epilog_for_reduction).

As said during the individual patches review I'd like the vectorizer to
use a VEC_PERM_EXPR instead of VEC_RSHIFT_EXPR (with
only whole-element amounts).  This means we can remove
VEC_RSHIFT_EXPR.  It also means that if the backend defines
vec_perm_const (which it really should) it can handle the special
permutes that boil down to a possibly more efficient vector shift
there (a good optimization anyway).  Until it does that all backends
would at least create correct code (with the endian dependent
vec_shr removed).


> --Alan

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