On Thu, Sep 18, 2014 at 1:41 PM, Alan Lawrence <alan.lawre...@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). Richard. > --Alan >
