compute Matrix length from dims

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 any known benefits. This PR aims to test if
nanosoldier detects any benefit, or whether this restriction has
outlived its usefulness.

base/array.jl

@@ -3130,10 +3130,6 @@ 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/genericmemory.c

@@ -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