Allow taking Matrix slices without an extra allocation (#56236)

Since changing Array to use Memory as the backing, we had the option of
making non-Vector arrays more flexible, but had instead preserved the
restriction that they must be zero offset and equal in length to the
Memory. This results in extra complexity, restrictions, and allocations
however, but doesn't gain many known benefits. Nanosoldier shows a
decrease in performance on linear eachindex loops, which we theorize is
due to a minor failure to CSE before SCEV or a lack of NUW/NSW on the
length multiplication calculation.

base/Base.jl

@@ -354,14 +354,14 @@ include("set.jl")
 include("char.jl")
 function array_new_memory(mem::Memory{UInt8}, newlen::Int)
     # add an optimization to array_new_memory for StringVector
-    if (@assume_effects :total @ccall jl_genericmemory_owner(mem::Any,)::Any) isa String
+    if (@assume_effects :total @ccall jl_genericmemory_owner(mem::Any,)::Any) === mem
+        # TODO: when implemented, this should use a memory growing call
+        return typeof(mem)(undef, newlen)
+    else
         # If data is in a String, keep it that way.
         # When implemented, this could use jl_gc_expand_string(oldstr, newlen) as an optimization
         str = _string_n(newlen)
         return (@assume_effects :total !:consistent @ccall jl_string_to_genericmemory(str::Any,)::Memory{UInt8})
-    else
-        # TODO: when implemented, this should use a memory growing call
-        return typeof(mem)(undef, newlen)
     end
 end
 include("strings/basic.jl")

base/abstractarray.jl

@@ -1584,7 +1584,7 @@ their component parts.  A typical definition for an array that wraps a parent is
 `Base.dataids(C::CustomArray) = dataids(C.parent)`.
 """
 dataids(A::AbstractArray) = (UInt(objectid(A)),)
-dataids(A::Memory) = (B = ccall(:jl_genericmemory_owner, Any, (Any,), A); (UInt(pointer(B isa typeof(A) ? B : A)),))
+dataids(A::Memory) = (UInt(A.ptr),)
 dataids(A::Array) = dataids(A.ref.mem)
 dataids(::AbstractRange) = ()
 dataids(x) = ()

base/array.jl

@@ -3141,10 +3141,6 @@ function _wrap(ref::MemoryRef{T}, dims::NTuple{N, Int}) where {T, N}
     mem_len = length(mem) + 1 - memoryrefoffset(ref)
     len = Core.checked_dims(dims...)
     @boundscheck mem_len >= len || invalid_wrap_err(mem_len, dims, len)
-    if N != 1 && !(ref === GenericMemoryRef(mem) && len === mem_len)
-        mem = ccall(:jl_genericmemory_slice, Memory{T}, (Any, Ptr{Cvoid}, Int), mem, ref.ptr_or_offset, len)
-        ref = memoryref(mem)
-    end
     return ref
 end
 

base/essentials.jl

@@ -9,7 +9,8 @@ const Bottom = Union{}
 # Define minimal array interface here to help code used in macros:
 length(a::Array{T, 0}) where {T} = 1
 length(a::Array{T, 1}) where {T} = getfield(a, :size)[1]
-length(a::Array) = getfield(getfield(getfield(a, :ref), :mem), :length)
+length(a::Array{T, 2}) where {T} = (sz = getfield(a, :size); sz[1] * sz[2])
+# other sizes are handled by generic prod definition for AbstractArray
 length(a::GenericMemory) = getfield(a, :length)
 throw_boundserror(A, I) = (@noinline; throw(BoundsError(A, I)))
 

base/reshapedarray.jl

@@ -35,31 +35,33 @@ end
 length(R::ReshapedArrayIterator) = length(R.iter)
 eltype(::Type{<:ReshapedArrayIterator{I}}) where {I} = @isdefined(I) ? ReshapedIndex{eltype(I)} : Any
 
-## reshape(::Array, ::Dims) returns an Array, except for isbitsunion eltypes (issue #28611)
+@noinline throw_dmrsa(dims, len) =
+    throw(DimensionMismatch("new dimensions $(dims) must be consistent with array length $len"))
+
+## reshape(::Array, ::Dims) returns a new Array (to avoid conditionally aliasing the structure, only the data)
 # reshaping to same # of dimensions
 @eval function reshape(a::Array{T,M}, dims::NTuple{N,Int}) where {T,N,M}
-    throw_dmrsa(dims, len) =
-        throw(DimensionMismatch("new dimensions $(dims) must be consistent with array length $len"))
     len = Core.checked_dims(dims...) # make sure prod(dims) doesn't overflow (and because of the comparison to length(a))
     if len != length(a)
         throw_dmrsa(dims, length(a))
     end
-    isbitsunion(T) && return ReshapedArray(a, dims, ())
-    if N == M && dims == size(a)
-        return a
-    end
     ref = a.ref
-    if M == 1 && N !== 1
-        mem = ref.mem::Memory{T}
-        if !(ref === memoryref(mem) && len === mem.length)
-            mem = ccall(:jl_genericmemory_slice, Memory{T}, (Any, Ptr{Cvoid}, Int), mem, ref.ptr_or_offset, len)
-            ref = memoryref(mem)::typeof(ref)
-        end
-    end
-    # or we could use `a = Array{T,N}(undef, ntuple(0, Val(N))); a.ref = ref; a.size = dims; return a` here
+    # or we could use `a = Array{T,N}(undef, ntuple(i->0, Val(N))); a.ref = ref; a.size = dims; return a` here to avoid the eval
     return $(Expr(:new, :(Array{T,N}), :ref, :dims))
 end
 
+## reshape!(::Array, ::Dims) returns the original array, but must have the same dimensions and length as the original
+# see also resize! for a similar operation that can change the length
+function reshape!(a::Array{T,N}, dims::NTuple{N,Int}) where {T,N}
+    len = Core.checked_dims(dims...) # make sure prod(dims) doesn't overflow (and because of the comparison to length(a))
+    if len != length(a)
+        throw_dmrsa(dims, length(a))
+    end
+    setfield!(a, :dims, dims)
+    return a
+end
+
+
 
 """
     reshape(A, dims...) -> AbstractArray

src/codegen.cpp

@@ -1435,18 +1435,6 @@ static const auto jl_allocgenericmemory = new JuliaFunction<TypeFnContextAndSize
                 AttributeSet::get(C, RetAttrs),
                 None); },
 };
-static const auto jlarray_data_owner_func = new JuliaFunction<>{
-    XSTR(jl_array_data_owner),
-    [](LLVMContext &C) {
-        auto T_prjlvalue = JuliaType::get_prjlvalue_ty(C);
-        return FunctionType::get(T_prjlvalue,
-            {T_prjlvalue}, false);
-    },
-    [](LLVMContext &C) { return AttributeList::get(C,
-            Attributes(C, {Attribute::ReadOnly, Attribute::NoUnwind}),
-            Attributes(C, {Attribute::NonNull}),
-            None); },
-};
 #define BOX_FUNC(ct,at,attrs,nbytes)                                                    \
 static const auto box_##ct##_func = new JuliaFunction<>{                           \
     XSTR(jl_box_##ct),                                                           \
@@ -4341,8 +4329,7 @@ static bool emit_f_opmemory(jl_codectx_t &ctx, jl_cgval_t *ret, jl_value_t *f,
         }
         Value *data_owner = NULL; // owner object against which the write barrier must check
         if (isboxed || layout->first_ptr >= 0) { // if elements are just bits, don't need a write barrier
-            Value *V = emit_memoryref_FCA(ctx, ref, layout);
-            data_owner = emit_genericmemoryowner(ctx, CreateSimplifiedExtractValue(ctx, V, 1));
+            data_owner = emit_memoryref_mem(ctx, ref, layout);
         }
         *ret = typed_store(ctx,
                     ptr,

src/genericmemory.c

@@ -197,7 +197,7 @@ JL_DLLEXPORT jl_value_t *jl_genericmemory_to_string(jl_genericmemory_t *m, size_
     if (how != 0) {
         jl_value_t *o = jl_genericmemory_data_owner_field(m);
         jl_genericmemory_data_owner_field(m) = NULL;
-        if (how == 3 && jl_is_string(o) &&
+        if (how == 3 && // implies jl_is_string(o)
              ((mlength + sizeof(void*) + 1 <= GC_MAX_SZCLASS) == (len + sizeof(void*) + 1 <= GC_MAX_SZCLASS))) {
             if (jl_string_data(o)[len] != '\0')
                 jl_string_data(o)[len] = '\0';
@@ -221,39 +221,6 @@ JL_DLLEXPORT jl_genericmemory_t *jl_alloc_memory_any(size_t n)
     return jl_alloc_genericmemory(jl_memory_any_type, n);
 }
 
-JL_DLLEXPORT jl_genericmemory_t *jl_genericmemory_slice(jl_genericmemory_t *mem, void *data, size_t len)
-{
-    // Given a GenericMemoryRef represented as `jl_genericmemory_ref ref = {data, mem}`,
-    // return a new GenericMemory that only accesses the slice from the given GenericMemoryRef to
-    // the given length if this is possible to return. This allows us to make
-    // `length(Array)==length(Array.ref.mem)`, for simplification of this.
-    jl_datatype_t *dt = (jl_datatype_t*)jl_typetagof(mem);
-    const jl_datatype_layout_t *layout = dt->layout;
-    // repeated checks here ensure the values cannot overflow, since we know mem->length is a reasonable value
-    if (len > mem->length)
-        jl_exceptionf(jl_argumenterror_type, "invalid GenericMemory slice"); // TODO: make a BoundsError
-    if (layout->flags.arrayelem_isunion) {
-        if (!((size_t)data == 0 && mem->length == len))
-            jl_exceptionf(jl_argumenterror_type, "invalid GenericMemory slice"); // only exact slices are supported
-        data = mem->ptr;
-    }
-    else if (layout->size == 0) {
-        if ((size_t)data > mem->length || (size_t)data + len > mem->length)
-            jl_exceptionf(jl_argumenterror_type, "invalid GenericMemory slice"); // TODO: make a BoundsError
-        data = mem->ptr;
-    }
-    else {
-        if (data < mem->ptr || (char*)data > (char*)mem->ptr + mem->length * layout->size || (char*)data + len * layout->size > (char*)mem->ptr + mem->length * layout->size)
-            jl_exceptionf(jl_argumenterror_type, "invalid GenericMemory slice"); // TODO: make a BoundsError
-    }
-    jl_task_t *ct = jl_current_task;
-    jl_genericmemory_t *newmem = (jl_genericmemory_t*)jl_gc_alloc(ct->ptls, sizeof(jl_genericmemory_t) + sizeof(void*), dt);
-    newmem->length = len;
-    newmem->ptr = data;
-    jl_genericmemory_data_owner_field(newmem) = jl_genericmemory_owner(mem);
-    return newmem;
-}
-
 JL_DLLEXPORT void jl_genericmemory_copyto(jl_genericmemory_t *dest, char* destdata,
                                           jl_genericmemory_t *src, char* srcdata,
                                           size_t n) JL_NOTSAFEPOINT

src/julia.h

@@ -1233,7 +1233,7 @@ STATIC_INLINE jl_value_t *jl_svecset(
   0 = data is inlined
   1 = owns the gc-managed data, exclusively (will free it)
   2 = malloc-allocated pointer (does not own it)
-  3 = has a pointer to the object that owns the data pointer
+  3 = has a pointer to the String object that owns the data pointer (m must be isbits)
 */
 STATIC_INLINE int jl_genericmemory_how(jl_genericmemory_t *m) JL_NOTSAFEPOINT
 {
@@ -1249,8 +1249,6 @@ STATIC_INLINE int jl_genericmemory_how(jl_genericmemory_t *m) JL_NOTSAFEPOINT
 
 STATIC_INLINE jl_value_t *jl_genericmemory_owner(jl_genericmemory_t *m JL_PROPAGATES_ROOT) JL_NOTSAFEPOINT
 {
-    if (jl_genericmemory_how(m) == 3)
-        return jl_genericmemory_data_owner_field(m);
     return (jl_value_t*)m;
 }
 
@@ -1280,8 +1278,6 @@ STATIC_INLINE jl_value_t *jl_genericmemory_ptr_set(
     assert(i < m_->length);
     jl_atomic_store_release(((_Atomic(jl_value_t*)*)(m_->ptr)) + i, (jl_value_t*)x);
     if (x) {
-        if (jl_genericmemory_how(m_) == 3)
-            m = (void*)jl_genericmemory_data_owner_field(m_);
         jl_gc_wb(m, x);
     }
     return (jl_value_t*)x;

src/staticdata.c

@@ -932,7 +932,7 @@ static void jl_insert_into_serialization_queue(jl_serializer_state *s, jl_value_
         jl_genericmemory_t *m = (jl_genericmemory_t*)v;
         const char *data = (const char*)m->ptr;
         if (jl_genericmemory_how(m) == 3) {
-            jl_queue_for_serialization_(s, jl_genericmemory_data_owner_field(v), 1, immediate);
+            assert(jl_is_string(jl_genericmemory_data_owner_field(m)));
         }
         else if (layout->flags.arrayelem_isboxed) {
             size_t i, l = m->length;
@@ -1472,17 +1472,7 @@ static void jl_write_values(jl_serializer_state *s) JL_GC_DISABLED
             jl_genericmemory_t *m = (jl_genericmemory_t*)v;
             const jl_datatype_layout_t *layout = t->layout;
             size_t len = m->length;
-            if (jl_genericmemory_how(m) == 3 && jl_is_genericmemory(jl_genericmemory_data_owner_field(m))) {
-                jl_genericmemory_t *owner = (jl_genericmemory_t*)jl_genericmemory_data_owner_field(m);
-                size_t data = ((char*)m->ptr - (char*)owner->ptr); // relocation offset (bytes)
-                write_uint(f, len);
-                write_uint(f, data);
-                write_pointerfield(s, (jl_value_t*)owner);
-                // similar to record_memoryref, but the field is always an (offset) pointer
-                arraylist_push(&s->memowner_list, (void*)(reloc_offset + offsetof(jl_genericmemory_t, ptr))); // relocation location
-                arraylist_push(&s->memowner_list, NULL); // relocation target (ignored)
-            }
-            // else if (jl_genericmemory_how(m) == 3) {
+            // if (jl_genericmemory_how(m) == 3) {
             //     jl_value_t *owner = jl_genericmemory_data_owner_field(m);
             //     write_uint(f, len);
             //     write_pointerfield(s, owner);
@@ -1491,7 +1481,8 @@ static void jl_write_values(jl_serializer_state *s) JL_GC_DISABLED
             //     assert(new_mem->ptr == NULL);
             //     new_mem->ptr = (void*)((char*)m->ptr - (char*)owner); // relocation offset
             // }
-            else {
+            // else
+            {
                 size_t datasize = len * layout->size;
                 size_t tot = datasize;
                 int isbitsunion = layout->flags.arrayelem_isunion;
@@ -1538,10 +1529,13 @@ static void jl_write_values(jl_serializer_state *s) JL_GC_DISABLED
                             ios_write(s->const_data, (char*)m->ptr, tot);
                         }
                     }
-                    if (len == 0) // TODO: should we have a zero-page, instead of writing each type's fragment separately?
+                    if (len == 0) { // TODO: should we have a zero-page, instead of writing each type's fragment separately?
                         write_padding(s->const_data, layout->size ? layout->size : isbitsunion);
-                    else if (jl_genericmemory_how(m) == 3 && jl_is_string(jl_genericmemory_data_owner_field(m)))
+                    }
+                    else if (jl_genericmemory_how(m) == 3) {
+                        assert(jl_is_string(jl_genericmemory_data_owner_field(m)));
                         write_padding(s->const_data, 1);
+                    }
                 }
                 else {
                     // Pointer eltypes are encoded in the mutable data section