[C] Opt out of GNU vector extensions for built-in SVE types

Programming / Compilers / GCC - rsandifo [138bc75d-0d04-0410-961f-82ee72b054a4] - 8 November 2019 08:35 EST

The AArch64 port defines built-in SVE types at start-up under names like __SVInt8_t. These types are represented in the front end and gimple as normal VECTOR_TYPEs and are code-generated as normal vectors. However, we'd like to stop the frontends from treating them in the same way as GNU-style ("vector_size") vectors, for several reasons:

(1) We allowed the GNU vector extensions to be mixed with Advanced SIMD
vector types and it ended up causing a lot of confusion on big-endian targets. Although SVE handles big-endian vectors differently from Advanced SIMD, there are still potential surprises; see the block comment near the head of aarch64-sve.md for details.

(2) One of the SVE vectors is a packed one-bit-per-element boolean vector. That isn't a combination the GNU vector extensions have supported before. E.g. it means that vectors can no longer decompose to arrays for indexing, and that not all elements are individually addressable. It also makes it less clear which order the initialiser should be in (lsb first, or bitfield ordering?). We could define all that of course, but it seems a bit weird to go to the effort for this case when, given all the other reasons, we don't want the extensions anyway.

(3) The GNU vector extensions only provide full-vector operations, which is a very artifical limitation on a predicated architecture like SVE.

(4) The set of operations provided by the GNU vector extensions is relatively small, whereas the SVE intrinsics provide many more.

(5) It makes it easier to ensure that (with default options) code is portable between compilers without the GNU vector extensions having to become an official part of the SVE intrinsics spec.

(6) The length of the SVE types is usually not fixed at compile time, whereas the GNU vector extension is geared around fixed-length
vectors.

It's possible to specify the length of an SVE vector using the command-line option -msve-vector-bits=N, but in principle it should be possible to have functions compiled for different N in the same translation unit. This isn't supported yet but would be very useful for implementing ifuncs. Once mixing lengths in a translation unit is supported, the SVE types should represent the same type throughout the translation unit, just as GNU vector types do.

However, when -msve-vector-bits=N is in effect, we do allow conversions between explicit GNU vector types of N bits and the corresponding SVE types. This doesn't undermine the intent of (5) because in this case the use of GNU vector types is explicit and intentional. It also doesn't undermine the intent of (6) because converting between the types is just a conditionally-supported operation. In other words, the types still represent the same types throughout the translation unit, it's just that conversions between them are valid in cases where a certain precondition is known to hold. It's similar to the way that the SVE vector types are defined throughout the translation unit but can only be used in functions for which SVE is enabled.

The patch adds a new flag to tree_type_common to select this behaviour. (We currently have 17 bits free.) To avoid making the flag too specific to vectors, I called it TYPE_INDIVISIBLE_P, to mean that the frontend should not allow the components of the type to be accessed directly. This could perhaps be useful in future for hiding the fact that a type is an array, or for hiding the fields of a record or union.

The actual frontend changes are very simple, mostly just replacing
VECTOR_TYPE_P with gnu_vector_type_p in selected places.

One interesting case is:

/* Need to convert condition operand into a vector mask. */ if (VECTOR_TYPE_P (TREE_TYPE (ifexp))) { tree vectype = TREE_TYPE (ifexp); tree elem_type = TREE_TYPE (vectype); tree zero = build_int_cst (elem_type, 0); tree zero_vec = build_vector_from_val (vectype, zero); tree cmp_type = build_same_sized_truth_vector_type (vectype); ifexp = build2 (NE_EXPR, cmp_type, ifexp, zero_vec); }

in build_conditional_expr. This appears to be trying to support elementwise conditions like "vec1 ? vec2 : vec3", which is something the C++ frontend supports. However, this code can never trigger AFAICT, because "vec1" does not survive c_objc_common_truthvalue_conversion:

case VECTOR_TYPE: error_at (location, "used vector type where scalar is required"); return error_mark_node;

Even if it did, the operation should be a VEC_COND_EXPR rather than a COND_EXPR.

I've therefore left that condition as-is, but added tests for the "vec1 ? vec2 : vec3" case to make sure that we don't accidentally allow it for SVE vectors in future.

2019-11-08 Richard Sandiford

gcc/
- tree-core.h (tree_type_common::indivisible_p): New member variable.
- tree.h (TYPE_INDIVISIBLE_P): New macro.
- config/aarch64/aarch64-sve-builtins.cc (register_builtin_types): Treat the vector types as indivisible.

gcc/c-family/
- c-common.h (gnu_vector_type_p): New function.
- c-common.c (c_build_vec_perm_expr): Require __builtin_shuffle
vectors to satisfy gnu_vector_type_p. (c_build_vec_convert): Likewise __builtin_convertvector. (convert_vector_to_array_for_subscript): Likewise when applying implicit vector to array conversion. (scalar_to_vector): Likewise when converting vector-scalar operations to vector-vector operations.

gcc/c/
- c-convert.c (convert): Only handle vector conversions if one of the types satisfies gnu_vector_type_p or if -flax-vector-conversions allows it.
- c-typeck.c (build_array_ref): Only allow vector indexing if the
vectors satisfy gnu_vector_type_p. (build_unary_op): Only allow unary operators to be applied to
vectors if they satisfy gnu_vector_type_p. (digest_init): Only allow by-element initialization of vectors if they satisfy gnu_vector_type_p. (really_start_incremental_init): Likewise. (push_init_level): Likewise. (pop_init_level): Likewise. (process_init_element): Likewise. (build_binary_op): Only allow binary operators to be applied to
vectors if they satisfy gnu_vector_type_p.

gcc/testsuite/
- gcc.target/aarch64/sve/acle/general-c/gnu_vectors_1.c: New test.
- gcc.target/aarch64/sve/acle/general-c/gnu_vectors_2.c: Likewise.

029c65a2326 [C] Opt out of GNU vector extensions for built-in SVE types
gcc/ChangeLog | 7 +
gcc/c-family/ChangeLog | 11 +
gcc/c-family/c-common.c | 21 +-
gcc/c-family/c-common.h | 10 +
gcc/c/ChangeLog | 18 +
gcc/c/c-convert.c | 16 +-
gcc/c/c-typeck.c | 71 ++--
gcc/config/aarch64/aarch64-sve-builtins.cc | 1 +
gcc/testsuite/ChangeLog | 5 +
.../aarch64/sve/acle/general-c/gnu_vectors_1.c | 415 +++++++++++++++++++++
.../aarch64/sve/acle/general-c/gnu_vectors_2.c | 415 +++++++++++++++++++++
gcc/tree-core.h | 3 +-
gcc/tree.h | 5 +
13 files changed, 957 insertions(+), 41 deletions(-)

Upstream: gcc.gnu.org


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