[core] codegen: Implement indexing for NDArray

Based on 8f9d2d82: core/ndstrides: implement ndarray indexing

The functionality for `...` and `np.newaxis` is there in IRRT, but there
is no implementation of them for @kernel Python expressions because of
M-Labs/nac3#486.

nac3core/irrt/irrt.cpp

@@ -7,3 +7,4 @@
 #include "irrt/ndarray/basic.hpp"
 #include "irrt/ndarray/def.hpp"
 #include "irrt/ndarray/iter.hpp"
+#include "irrt/ndarray/indexing.hpp"

nac3core/irrt/irrt/ndarray/indexing.hpp

@@ -0,0 +1,220 @@
+#pragma once
+
+#include "irrt/exception.hpp"
+#include "irrt/int_types.hpp"
+#include "irrt/ndarray/basic.hpp"
+#include "irrt/ndarray/def.hpp"
+#include "irrt/range.hpp"
+#include "irrt/slice.hpp"
+
+namespace {
+typedef uint8_t NDIndexType;
+
+/**
+ * @brief A single element index
+ *
+ * `data` points to a `int32_t`.
+ */
+const NDIndexType ND_INDEX_TYPE_SINGLE_ELEMENT = 0;
+
+/**
+ * @brief A slice index
+ *
+ * `data` points to a `Slice<int32_t>`.
+ */
+const NDIndexType ND_INDEX_TYPE_SLICE = 1;
+
+/**
+ * @brief `np.newaxis` / `None`
+ *
+ * `data` is unused.
+ */
+const NDIndexType ND_INDEX_TYPE_NEWAXIS = 2;
+
+/**
+ * @brief `Ellipsis` / `...`
+ *
+ * `data` is unused.
+ */
+const NDIndexType ND_INDEX_TYPE_ELLIPSIS = 3;
+
+/**
+ * @brief An index used in ndarray indexing
+ *
+ * That is:
+ * ```
+ * my_ndarray[::-1, 3, ..., np.newaxis]
+ *            ^^^^  ^  ^^^  ^^^^^^^^^^ each of these is represented by an NDIndex.
+ * ```
+ */
+struct NDIndex {
+    /**
+     * @brief Enum tag to specify the type of index.
+     *
+     * Please see the comment of each enum constant.
+     */
+    NDIndexType type;
+
+    /**
+     * @brief The accompanying data associated with `type`.
+     *
+     * Please see the comment of each enum constant.
+     */
+    uint8_t* data;
+};
+}  // namespace
+
+namespace {
+namespace ndarray {
+namespace indexing {
+/**
+ * @brief Perform ndarray "basic indexing" (https://numpy.org/doc/stable/user/basics.indexing.html#basic-indexing)
+ *
+ * This function is very similar to performing `dst_ndarray = src_ndarray[indices]` in Python.
+ *
+ * This function also does proper assertions on `indices` to check for out of bounds access and more.
+ *
+ * # Notes on `dst_ndarray`
+ * The caller is responsible for allocating space for the resulting ndarray.
+ * Here is what this function expects from `dst_ndarray` when called:
+ *   - `dst_ndarray->data` does not have to be initialized.
+ *   - `dst_ndarray->itemsize` does not have to be initialized.
+ *   - `dst_ndarray->ndims` must be initialized, and it must be equal to the expected `ndims` of the `dst_ndarray` after
+ *       indexing `src_ndarray` with `indices`.
+ *   - `dst_ndarray->shape` must be allocated, through it can contain uninitialized values.
+ *   - `dst_ndarray->strides` must be allocated, through it can contain uninitialized values.
+ * When this function call ends:
+ *   - `dst_ndarray->data` is set to `src_ndarray->data`.
+ *   - `dst_ndarray->itemsize` is set to `src_ndarray->itemsize`.
+ *   - `dst_ndarray->ndims` is unchanged.
+ *   - `dst_ndarray->shape` is updated according to how `src_ndarray` is indexed.
+ *   - `dst_ndarray->strides` is updated accordingly by how ndarray indexing works.
+ *
+ * @param indices indices to index `src_ndarray`, ordered in the same way you would write them in Python.
+ * @param src_ndarray The NDArray to be indexed.
+ * @param dst_ndarray The resulting NDArray after indexing. Further details in the comments above,
+ */
+template<typename SizeT>
+void index(SizeT num_indices, const NDIndex* indices, const NDArray<SizeT>* src_ndarray, NDArray<SizeT>* dst_ndarray) {
+    // Validate `indices`.
+
+    // Expected value of `dst_ndarray->ndims`.
+    SizeT expected_dst_ndims = src_ndarray->ndims;
+    // To check for "too many indices for array: array is ?-dimensional, but ? were indexed"
+    SizeT num_indexed = 0;
+    // There may be ellipsis `...` in `indices`. There can only be 0 or 1 ellipsis.
+    SizeT num_ellipsis = 0;
+
+    for (SizeT i = 0; i < num_indices; i++) {
+        if (indices[i].type == ND_INDEX_TYPE_SINGLE_ELEMENT) {
+            expected_dst_ndims--;
+            num_indexed++;
+        } else if (indices[i].type == ND_INDEX_TYPE_SLICE) {
+            num_indexed++;
+        } else if (indices[i].type == ND_INDEX_TYPE_NEWAXIS) {
+            expected_dst_ndims++;
+        } else if (indices[i].type == ND_INDEX_TYPE_ELLIPSIS) {
+            num_ellipsis++;
+            if (num_ellipsis > 1) {
+                raise_exception(SizeT, EXN_INDEX_ERROR, "an index can only have a single ellipsis ('...')", NO_PARAM,
+                                NO_PARAM, NO_PARAM);
+            }
+        } else {
+            __builtin_unreachable();
+        }
+    }
+
+    debug_assert_eq(SizeT, expected_dst_ndims, dst_ndarray->ndims);
+
+    if (src_ndarray->ndims - num_indexed < 0) {
+        raise_exception(SizeT, EXN_INDEX_ERROR,
+                        "too many indices for array: array is {0}-dimensional, "
+                        "but {1} were indexed",
+                        src_ndarray->ndims, num_indices, NO_PARAM);
+    }
+
+    dst_ndarray->data = src_ndarray->data;
+    dst_ndarray->itemsize = src_ndarray->itemsize;
+
+    // Reference code:
+    // https://github.com/wadetb/tinynumpy/blob/0d23d22e07062ffab2afa287374c7b366eebdda1/tinynumpy/tinynumpy.py#L652
+    SizeT src_axis = 0;
+    SizeT dst_axis = 0;
+
+    for (int32_t i = 0; i < num_indices; i++) {
+        const NDIndex* index = &indices[i];
+        if (index->type == ND_INDEX_TYPE_SINGLE_ELEMENT) {
+            SizeT input = (SizeT) * ((int32_t*)index->data);
+
+            SizeT k = slice::resolve_index_in_length(src_ndarray->shape[src_axis], input);
+            if (k == -1) {
+                raise_exception(SizeT, EXN_INDEX_ERROR,
+                                "index {0} is out of bounds for axis {1} "
+                                "with size {2}",
+                                input, src_axis, src_ndarray->shape[src_axis]);
+            }
+
+            dst_ndarray->data = static_cast<uint8_t*>(dst_ndarray->data) + k * src_ndarray->strides[src_axis];
+
+            src_axis++;
+        } else if (index->type == ND_INDEX_TYPE_SLICE) {
+            Slice<int32_t>* slice = (Slice<int32_t>*)index->data;
+
+            Range<int32_t> range = slice->indices_checked<SizeT>(src_ndarray->shape[src_axis]);
+
+            dst_ndarray->data = static_cast<uint8_t*>(dst_ndarray->data) + (SizeT)range.start * src_ndarray->strides[src_axis];
+            dst_ndarray->strides[dst_axis] = ((SizeT)range.step) * src_ndarray->strides[src_axis];
+            dst_ndarray->shape[dst_axis] = (SizeT)range.len<SizeT>();
+
+            dst_axis++;
+            src_axis++;
+        } else if (index->type == ND_INDEX_TYPE_NEWAXIS) {
+            dst_ndarray->strides[dst_axis] = 0;
+            dst_ndarray->shape[dst_axis] = 1;
+
+            dst_axis++;
+        } else if (index->type == ND_INDEX_TYPE_ELLIPSIS) {
+            // The number of ':' entries this '...' implies.
+            SizeT ellipsis_size = src_ndarray->ndims - num_indexed;
+
+            for (SizeT j = 0; j < ellipsis_size; j++) {
+                dst_ndarray->strides[dst_axis] = src_ndarray->strides[src_axis];
+                dst_ndarray->shape[dst_axis] = src_ndarray->shape[src_axis];
+
+                dst_axis++;
+                src_axis++;
+            }
+        } else {
+            __builtin_unreachable();
+        }
+    }
+
+    for (; dst_axis < dst_ndarray->ndims; dst_axis++, src_axis++) {
+        dst_ndarray->shape[dst_axis] = src_ndarray->shape[src_axis];
+        dst_ndarray->strides[dst_axis] = src_ndarray->strides[src_axis];
+    }
+
+    debug_assert_eq(SizeT, src_ndarray->ndims, src_axis);
+    debug_assert_eq(SizeT, dst_ndarray->ndims, dst_axis);
+}
+}  // namespace indexing
+}  // namespace ndarray
+}  // namespace
+
+extern "C" {
+using namespace ndarray::indexing;
+
+void __nac3_ndarray_index(int32_t num_indices,
+                          NDIndex* indices,
+                          NDArray<int32_t>* src_ndarray,
+                          NDArray<int32_t>* dst_ndarray) {
+    index(num_indices, indices, src_ndarray, dst_ndarray);
+}
+
+void __nac3_ndarray_index64(int64_t num_indices,
+                            NDIndex* indices,
+                            NDArray<int64_t>* src_ndarray,
+                            NDArray<int64_t>* dst_ndarray) {
+    index(num_indices, indices, src_ndarray, dst_ndarray);
+}
+}

nac3core/src/codegen/irrt/ndarray/indexing.rs

@@ -0,0 +1,29 @@
+use crate::codegen::{
+    expr::infer_and_call_function,
+    irrt::get_usize_dependent_function_name,
+    values::{ndarray::NDArrayValue, ArrayLikeValue, ArraySliceValue, ProxyValue},
+    CodeGenContext, CodeGenerator,
+};
+
+pub fn call_nac3_ndarray_index<'ctx, G: CodeGenerator + ?Sized>(
+    generator: &G,
+    ctx: &CodeGenContext<'ctx, '_>,
+    indices: ArraySliceValue<'ctx>,
+    src_ndarray: NDArrayValue<'ctx>,
+    dst_ndarray: NDArrayValue<'ctx>,
+) {
+    let name = get_usize_dependent_function_name(generator, ctx, "__nac3_ndarray_index");
+    infer_and_call_function(
+        ctx,
+        &name,
+        None,
+        &[
+            indices.size(ctx, generator).into(),
+            indices.base_ptr(ctx, generator).into(),
+            src_ndarray.as_base_value().into(),
+            dst_ndarray.as_base_value().into(),
+        ],
+        None,
+        None,
+    );
+}

nac3core/src/codegen/irrt/ndarray/mod.rs

@@ -16,9 +16,11 @@ use crate::codegen::{
     CodeGenContext, CodeGenerator,
 };
 pub use basic::*;
+pub use indexing::*;
 pub use iter::*;
 
 mod basic;
+mod indexing;
 mod iter;
 
 /// Generates a call to `__nac3_ndarray_calc_size`. Returns an [`IntValue`] representing the

nac3core/src/codegen/types/ndarray/indexing.rs

@@ -0,0 +1,215 @@
+use inkwell::{
+    context::{AsContextRef, Context},
+    types::{AnyTypeEnum, BasicType, BasicTypeEnum, IntType, PointerType},
+    values::{IntValue, PointerValue},
+    AddressSpace,
+};
+use itertools::Itertools;
+
+use nac3core_derive::StructFields;
+
+use crate::codegen::{
+    types::{
+        structure::{check_struct_type_matches_fields, StructField, StructFields},
+        ProxyType,
+    },
+    values::{
+        ndarray::{NDIndexValue, RustNDIndex},
+        ArrayLikeIndexer, ArraySliceValue, ProxyValue,
+    },
+    CodeGenContext, CodeGenerator,
+};
+
+#[derive(Debug, PartialEq, Eq, Clone, Copy)]
+pub struct NDIndexType<'ctx> {
+    ty: PointerType<'ctx>,
+    llvm_usize: IntType<'ctx>,
+}
+
+#[derive(PartialEq, Eq, Clone, Copy, StructFields)]
+pub struct NDIndexStructFields<'ctx> {
+    #[value_type(i8_type())]
+    pub type_: StructField<'ctx, IntValue<'ctx>>,
+    #[value_type(i8_type().ptr_type(AddressSpace::default()))]
+    pub data: StructField<'ctx, PointerValue<'ctx>>,
+}
+
+impl<'ctx> NDIndexType<'ctx> {
+    /// Checks whether `llvm_ty` represents a `ndindex` type, returning [Err] if it does not.
+    pub fn is_representable(
+        llvm_ty: PointerType<'ctx>,
+        llvm_usize: IntType<'ctx>,
+    ) -> Result<(), String> {
+        let ctx = llvm_ty.get_context();
+
+        let llvm_ty = llvm_ty.get_element_type();
+        let AnyTypeEnum::StructType(llvm_ty) = llvm_ty else {
+            return Err(format!(
+                "Expected struct type for `ContiguousNDArray` type, got {llvm_ty}"
+            ));
+        };
+
+        let fields = NDIndexStructFields::new(ctx, llvm_usize);
+
+        check_struct_type_matches_fields(fields, llvm_ty, "NDIndex", &[])
+    }
+
+    #[must_use]
+    fn fields(
+        ctx: impl AsContextRef<'ctx>,
+        llvm_usize: IntType<'ctx>,
+    ) -> NDIndexStructFields<'ctx> {
+        NDIndexStructFields::new(ctx, llvm_usize)
+    }
+
+    #[must_use]
+    pub fn get_fields(&self) -> NDIndexStructFields<'ctx> {
+        Self::fields(self.ty.get_context(), self.llvm_usize)
+    }
+
+    #[must_use]
+    fn llvm_type(ctx: &'ctx Context, llvm_usize: IntType<'ctx>) -> PointerType<'ctx> {
+        let field_tys =
+            Self::fields(ctx, llvm_usize).into_iter().map(|field| field.1).collect_vec();
+
+        ctx.struct_type(&field_tys, false).ptr_type(AddressSpace::default())
+    }
+
+    #[must_use]
+    pub fn new<G: CodeGenerator + ?Sized>(generator: &G, ctx: &'ctx Context) -> Self {
+        let llvm_usize = generator.get_size_type(ctx);
+        let llvm_ndindex = Self::llvm_type(ctx, llvm_usize);
+
+        Self { ty: llvm_ndindex, llvm_usize }
+    }
+
+    #[must_use]
+    pub fn from_type(ptr_ty: PointerType<'ctx>, llvm_usize: IntType<'ctx>) -> Self {
+        debug_assert!(Self::is_representable(ptr_ty, llvm_usize).is_ok());
+
+        Self { ty: ptr_ty, llvm_usize }
+    }
+
+    #[must_use]
+    pub fn alloca<G: CodeGenerator + ?Sized>(
+        &self,
+        generator: &mut G,
+        ctx: &mut CodeGenContext<'ctx, '_>,
+        name: Option<&'ctx str>,
+    ) -> <Self as ProxyType<'ctx>>::Value {
+        <Self as ProxyType<'ctx>>::Value::from_pointer_value(
+            self.raw_alloca(generator, ctx, name),
+            self.llvm_usize,
+            name,
+        )
+    }
+
+    /// Serialize a list of [`RustNDIndex`] as a newly allocated LLVM array of [`NDIndexValue`].
+    #[must_use]
+    pub fn construct_ndindices<G: CodeGenerator + ?Sized>(
+        &self,
+        generator: &mut G,
+        ctx: &mut CodeGenContext<'ctx, '_>,
+        in_ndindices: &[RustNDIndex<'ctx>],
+    ) -> ArraySliceValue<'ctx> {
+        // Allocate the LLVM ndindices.
+        let num_ndindices = self.llvm_usize.const_int(in_ndindices.len() as u64, false);
+        let ndindices = self.array_alloca(generator, ctx, num_ndindices, None);
+
+        // Initialize all of them.
+        for (i, in_ndindex) in in_ndindices.iter().enumerate() {
+            let pndindex = unsafe {
+                ndindices.ptr_offset_unchecked(
+                    ctx,
+                    generator,
+                    &ctx.ctx.i64_type().const_int(u64::try_from(i).unwrap(), false),
+                    None,
+                )
+            };
+
+            in_ndindex.write_to_ndindex(
+                generator,
+                ctx,
+                NDIndexValue::from_pointer_value(pndindex, self.llvm_usize, None),
+            );
+        }
+
+        ndindices
+    }
+
+    #[must_use]
+    pub fn map_value(
+        &self,
+        value: <<Self as ProxyType<'ctx>>::Value as ProxyValue<'ctx>>::Base,
+        name: Option<&'ctx str>,
+    ) -> <Self as ProxyType<'ctx>>::Value {
+        <Self as ProxyType<'ctx>>::Value::from_pointer_value(value, self.llvm_usize, name)
+    }
+}
+
+impl<'ctx> ProxyType<'ctx> for NDIndexType<'ctx> {
+    type Base = PointerType<'ctx>;
+    type Value = NDIndexValue<'ctx>;
+
+    fn is_type<G: CodeGenerator + ?Sized>(
+        generator: &G,
+        ctx: &'ctx Context,
+        llvm_ty: impl BasicType<'ctx>,
+    ) -> Result<(), String> {
+        if let BasicTypeEnum::PointerType(ty) = llvm_ty.as_basic_type_enum() {
+            <Self as ProxyType<'ctx>>::is_representable(generator, ctx, ty)
+        } else {
+            Err(format!("Expected pointer type, got {llvm_ty:?}"))
+        }
+    }
+
+    fn is_representable<G: CodeGenerator + ?Sized>(
+        generator: &G,
+        ctx: &'ctx Context,
+        llvm_ty: Self::Base,
+    ) -> Result<(), String> {
+        Self::is_representable(llvm_ty, generator.get_size_type(ctx))
+    }
+
+    fn raw_alloca<G: CodeGenerator + ?Sized>(
+        &self,
+        generator: &mut G,
+        ctx: &mut CodeGenContext<'ctx, '_>,
+        name: Option<&'ctx str>,
+    ) -> <Self::Value as ProxyValue<'ctx>>::Base {
+        generator
+            .gen_var_alloc(
+                ctx,
+                self.as_base_type().get_element_type().into_struct_type().into(),
+                name,
+            )
+            .unwrap()
+    }
+
+    fn array_alloca<G: CodeGenerator + ?Sized>(
+        &self,
+        generator: &mut G,
+        ctx: &mut CodeGenContext<'ctx, '_>,
+        size: IntValue<'ctx>,
+        name: Option<&'ctx str>,
+    ) -> ArraySliceValue<'ctx> {
+        generator
+            .gen_array_var_alloc(
+                ctx,
+                self.as_base_type().get_element_type().into_struct_type().into(),
+                size,
+                name,
+            )
+            .unwrap()
+    }
+
+    fn as_base_type(&self) -> Self::Base {
+        self.ty
+    }
+}
+
+impl<'ctx> From<NDIndexType<'ctx>> for PointerType<'ctx> {
+    fn from(value: NDIndexType<'ctx>) -> Self {
+        value.as_base_type()
+    }
+}

nac3core/src/codegen/types/ndarray/mod.rs

@@ -21,9 +21,11 @@ use crate::{
     typecheck::typedef::Type,
 };
 pub use contiguous::*;
+pub use indexing::*;
 pub use nditer::*;
 
 mod contiguous;
+mod indexing;
 mod nditer;
 
 /// Proxy type for a `ndarray` type in LLVM.

nac3core/src/codegen/values/ndarray/indexing.rs

@@ -0,0 +1,262 @@
+use inkwell::{
+    types::IntType,
+    values::{IntValue, PointerValue},
+    AddressSpace,
+};
+use itertools::Itertools;
+
+use nac3parser::ast::{Expr, ExprKind};
+
+use crate::{
+    codegen::{
+        irrt,
+        types::{
+            ndarray::{NDArrayType, NDIndexType},
+            structure::StructField,
+            utils::SliceType,
+        },
+        values::{ndarray::NDArrayValue, utils::RustSlice, ProxyValue},
+        CodeGenContext, CodeGenerator,
+    },
+    typecheck::typedef::Type,
+};
+
+/// An IRRT representation of an ndarray subscript index.
+#[derive(Copy, Clone)]
+pub struct NDIndexValue<'ctx> {
+    value: PointerValue<'ctx>,
+    llvm_usize: IntType<'ctx>,
+    name: Option<&'ctx str>,
+}
+
+impl<'ctx> NDIndexValue<'ctx> {
+    /// Checks whether `value` is an instance of `ndindex`, returning [Err] if `value` is not an
+    /// instance.
+    pub fn is_representable(
+        value: PointerValue<'ctx>,
+        llvm_usize: IntType<'ctx>,
+    ) -> Result<(), String> {
+        <Self as ProxyValue<'ctx>>::Type::is_representable(value.get_type(), llvm_usize)
+    }
+
+    /// Creates an [`NDIndexValue`] from a [`PointerValue`].
+    #[must_use]
+    pub fn from_pointer_value(
+        ptr: PointerValue<'ctx>,
+        llvm_usize: IntType<'ctx>,
+        name: Option<&'ctx str>,
+    ) -> Self {
+        debug_assert!(Self::is_representable(ptr, llvm_usize).is_ok());
+
+        Self { value: ptr, llvm_usize, name }
+    }
+
+    fn type_field(&self) -> StructField<'ctx, IntValue<'ctx>> {
+        self.get_type().get_fields().type_
+    }
+
+    pub fn load_type(&self, ctx: &CodeGenContext<'ctx, '_>) -> IntValue<'ctx> {
+        self.type_field().get(ctx, self.value, self.name)
+    }
+
+    pub fn store_type(&self, ctx: &CodeGenContext<'ctx, '_>, value: IntValue<'ctx>) {
+        self.type_field().set(ctx, self.value, value, self.name);
+    }
+
+    fn data_field(&self) -> StructField<'ctx, PointerValue<'ctx>> {
+        self.get_type().get_fields().data
+    }
+
+    pub fn load_data(&self, ctx: &CodeGenContext<'ctx, '_>) -> PointerValue<'ctx> {
+        self.data_field().get(ctx, self.value, self.name)
+    }
+
+    pub fn store_data(&self, ctx: &CodeGenContext<'ctx, '_>, value: PointerValue<'ctx>) {
+        self.data_field().set(ctx, self.value, value, self.name);
+    }
+}
+
+impl<'ctx> ProxyValue<'ctx> for NDIndexValue<'ctx> {
+    type Base = PointerValue<'ctx>;
+    type Type = NDIndexType<'ctx>;
+
+    fn get_type(&self) -> Self::Type {
+        Self::Type::from_type(self.value.get_type(), self.llvm_usize)
+    }
+
+    fn as_base_value(&self) -> Self::Base {
+        self.value
+    }
+}
+
+impl<'ctx> From<NDIndexValue<'ctx>> for PointerValue<'ctx> {
+    fn from(value: NDIndexValue<'ctx>) -> Self {
+        value.as_base_value()
+    }
+}
+
+impl<'ctx> NDArrayValue<'ctx> {
+    /// Get the expected `ndims` after indexing with `indices`.
+    #[must_use]
+    fn deduce_ndims_after_indexing_with(&self, indices: &[RustNDIndex<'ctx>]) -> Option<u64> {
+        let mut ndims = self.ndims?;
+
+        for index in indices {
+            match index {
+                RustNDIndex::SingleElement(_) => {
+                    ndims -= 1; // Single elements decrements ndims
+                }
+                RustNDIndex::NewAxis => {
+                    ndims += 1; // `np.newaxis` / `none` adds a new axis
+                }
+                RustNDIndex::Ellipsis | RustNDIndex::Slice(_) => {}
+            }
+        }
+
+        Some(ndims)
+    }
+
+    /// Index into the ndarray, and return a newly-allocated view on this ndarray.
+    ///
+    /// This function behaves like NumPy's ndarray indexing, but if the indices index
+    /// into a single element, an unsized ndarray is returned.
+    #[must_use]
+    pub fn index<G: CodeGenerator + ?Sized>(
+        &self,
+        generator: &mut G,
+        ctx: &mut CodeGenContext<'ctx, '_>,
+        indices: &[RustNDIndex<'ctx>],
+    ) -> Self {
+        assert!(self.ndims.is_some(), "NDArrayValue::index is only supported for instances with compile-time known ndims (self.ndims = Some(...))");
+
+        let dst_ndims = self.deduce_ndims_after_indexing_with(indices);
+        let dst_ndarray = NDArrayType::new(generator, ctx.ctx, self.dtype, dst_ndims)
+            .construct_uninitialized(generator, ctx, None);
+
+        let indices =
+            NDIndexType::new(generator, ctx.ctx).construct_ndindices(generator, ctx, indices);
+        irrt::ndarray::call_nac3_ndarray_index(generator, ctx, indices, *self, dst_ndarray);
+
+        dst_ndarray
+    }
+}
+
+/// A convenience enum representing a [`NDIndexValue`].
+// TODO: Rename to CTConstNDIndex
+#[derive(Debug, Clone)]
+pub enum RustNDIndex<'ctx> {
+    SingleElement(IntValue<'ctx>),
+    Slice(RustSlice<'ctx>),
+    NewAxis,
+    Ellipsis,
+}
+
+impl<'ctx> RustNDIndex<'ctx> {
+    /// Generate LLVM code to transform an ndarray subscript expression to
+    /// its list of [`RustNDIndex`]
+    ///
+    /// i.e.,
+    /// ```python
+    /// my_ndarray[::3, 1, :2:]
+    ///            ^^^^^^^^^^^ Then these into a three `RustNDIndex`es
+    /// ```
+    pub fn from_subscript_expr<G: CodeGenerator>(
+        generator: &mut G,
+        ctx: &mut CodeGenContext<'ctx, '_>,
+        subscript: &Expr<Option<Type>>,
+    ) -> Result<Vec<RustNDIndex<'ctx>>, String> {
+        // Annoying notes about `slice`
+        //  - `my_array[5]`
+        //    - slice is a `Constant`
+        //  - `my_array[:5]`
+        //    - slice is a `Slice`
+        //  - `my_array[:]`
+        //    - slice is a `Slice`, but lower upper step would all be `Option::None`
+        //  - `my_array[:, :]`
+        //    - slice is now a `Tuple` of two `Slice`-s
+        //
+        // In summary:
+        //  - when there is a comma "," within [], `slice` will be a `Tuple` of the entries.
+        //  - when there is not comma "," within [] (i.e., just a single entry), `slice` will be that entry itself.
+        //
+        // So we first "flatten" out the slice expression
+        let index_exprs = match &subscript.node {
+            ExprKind::Tuple { elts, .. } => elts.iter().collect_vec(),
+            _ => vec![subscript],
+        };
+
+        // Process all index expressions
+        let mut rust_ndindices: Vec<RustNDIndex> = Vec::with_capacity(index_exprs.len()); // Not using iterators here because `?` is used here.
+        for index_expr in index_exprs {
+            // NOTE: Currently nac3core's slices do not have an object representation,
+            // so the code/implementation looks awkward - we have to do pattern matching on the expression
+            let ndindex = if let ExprKind::Slice { lower, upper, step } = &index_expr.node {
+                // Handle slices
+                let slice = RustSlice::from_slice_expr(generator, ctx, lower, upper, step)?;
+                RustNDIndex::Slice(slice)
+            } else {
+                // Treat and handle everything else as a single element index.
+                let index = generator.gen_expr(ctx, index_expr)?.unwrap().to_basic_value_enum(
+                    ctx,
+                    generator,
+                    ctx.primitives.int32, // Must be int32, this checks for illegal values
+                )?;
+                let index = index.into_int_value();
+
+                RustNDIndex::SingleElement(index)
+            };
+            rust_ndindices.push(ndindex);
+        }
+        Ok(rust_ndindices)
+    }
+
+    /// Get the value to set `NDIndex::type` for this variant.
+    #[must_use]
+    pub fn get_type_id(&self) -> u64 {
+        // Defined in IRRT, must be in sync
+        match self {
+            RustNDIndex::SingleElement(_) => 0,
+            RustNDIndex::Slice(_) => 1,
+            RustNDIndex::NewAxis => 2,
+            RustNDIndex::Ellipsis => 3,
+        }
+    }
+
+    /// Serialize this [`RustNDIndex`] by writing it into an LLVM [`NDIndexValue`].
+    pub fn write_to_ndindex<G: CodeGenerator + ?Sized>(
+        &self,
+        generator: &mut G,
+        ctx: &mut CodeGenContext<'ctx, '_>,
+        dst_ndindex: NDIndexValue<'ctx>,
+    ) {
+        let llvm_pi8 = ctx.ctx.i8_type().ptr_type(AddressSpace::default());
+
+        // Set `dst_ndindex.type`
+        dst_ndindex.store_type(ctx, ctx.ctx.i8_type().const_int(self.get_type_id(), false));
+
+        // Set `dst_ndindex_ptr->data`
+        match self {
+            RustNDIndex::SingleElement(in_index) => {
+                let index_ptr = ctx.builder.build_alloca(ctx.ctx.i32_type(), "").unwrap();
+                ctx.builder.build_store(index_ptr, *in_index).unwrap();
+
+                dst_ndindex.store_data(
+                    ctx,
+                    ctx.builder.build_pointer_cast(index_ptr, llvm_pi8, "").unwrap(),
+                );
+            }
+            RustNDIndex::Slice(in_rust_slice) => {
+                let user_slice_ptr =
+                    SliceType::new(ctx.ctx, ctx.ctx.i32_type(), generator.get_size_type(ctx.ctx))
+                        .alloca(generator, ctx, None);
+                in_rust_slice.write_to_slice(ctx, user_slice_ptr);
+
+                dst_ndindex.store_data(
+                    ctx,
+                    ctx.builder.build_pointer_cast(user_slice_ptr.into(), llvm_pi8, "").unwrap(),
+                );
+            }
+            RustNDIndex::NewAxis | RustNDIndex::Ellipsis => {}
+        }
+    }
+}

nac3core/src/codegen/values/ndarray/mod.rs

@@ -17,9 +17,11 @@ use crate::codegen::{
     CodeGenContext, CodeGenerator,
 };
 pub use contiguous::*;
+pub use indexing::*;
 pub use nditer::*;
 
 mod contiguous;
+mod indexing;
 mod nditer;
 
 /// Proxy type for accessing an `NDArray` value in LLVM.