5. Vertex vector types

The fields of a Vertex can include Vector<T> or VectorList<T> types, or a combination of those such as Vector<Input<Vector<T>>>, in its state fields. These are similar to std::vector but can have different layouts in memory, optimised for the tile architecture.

These types are documented in the runtime API section of the Poplar and PopLibs API Reference.

5.1. Parameters

As well as the data type, the Vector and VectorList templates also have parameters to specify minimum alignment of elements, and whether or not they need to be stored in interleaved memory, for example:

template <typename T, VectorLayout L, unsigned MinAlign, bool Interleaved>
class Input<Vector<T, L, MinAlign, Interleaved>>
  ...

5.1.1. Types

The vector data type (T) can be any of the supported Poplar types defined in Types.hpp.

5.1.2. Layout

The template parameter L defines the type of memory layout to use. The valid layouts for a Vector are shown in Table 5.1. Some of these layouts use compressed pointer formats. These are not supported on all platforms.

In C++ vertex code, the use of these different layouts is completely transparent: elements can be accessed using the usual square bracket syntax and the compiler will insert the appropriate instructions to access them. The details of the layouts are useful to understand how their memory usage differs, and how to write vertex code (in Assembly) that uses them.

See Section 5.2.1, Pointer compression for more information.

Table 5.1 Vector memory layouts

Name	Description	Platform support
SPAN (default)	A pointer to the start of the vector, and a count of the number of elements (not bytes) the vector contains. This means that the .size() member and iterators are available. The count is a 32-bit integer.	All
SHORT_SPAN	A pointer to the start of the vector, and a count of the number of elements (not bytes) the vector contains. The count is limited to 11 bits. This means that the .size() member and iterators are available.	Mk2
ONE_PTR	The same as SPAN but the count is not stored, so this is a single pointer to the start of the vector. The vector does not know its size, which must be found by some other means. The .size() member and iterators are not available.	All
SCALED_PTR128	The same as ONE_PTR, but using a compressed 16-bit pointer containing bits 4-19 of a full 32-bit pointer. Since the lower three bits are not stored it can only point to 128-bit aligned data.	Mk2

These layouts are described in more detail in Section 5.2, Memory layout for vectors.

Only SPAN and SHORT_SPAN provide a .size() method.

5.1.3. Minimum alignment

The MinAlign template parameter specifies the required alignment, in bytes, of the data in the Vector or VectorList.

• The default value for this is 1 byte for SPAN, SHORT_SPAN or ONE_PTR layouts.

• For SCALED_PTR128, the default alignment is 16.

However, the alignment is never less than the size of the data type. Values are always naturally aligned.

5.1.4. Interleaved memory

The final template parameter, Interleaved, tells the compiler that the data must be placed in interleaved memory (see Section 10.2, Memory architecture).

5.2. Memory layout for vectors

This section describes the ways in which Vector types can be arranged in memory.

5.2.1. Pointer compression

In order to reduce memory usage, the size of pointers to the vector data can be compressed, based on the tile memory size.

Note

Future implementations of the IPU may have memory with different sizes and base addresses. You should not hard-code any assumptions about the memory system. The Poplar library includes functions that provide information about the memory system that the code is running on (see Section 10.2, Memory architecture for more information).

Not all of these compressed pointer formats are available on all platforms. The header file AvailableVTypes.h provides macros that define which formats are supported. For example:

#include "poplar/AvailableVTypes.h"

#if defined(VECTOR_AVAIL_SCALED_PTR128)
  Input<Vector<char, VectorLayout::SCALED_PTR128, 4>> desc;
#else
  Input<Vector<char, VectorLayout::ONE_PTR, 4>> desc;
#endif

SCALED_PTR128

A 32-bit pointer can be compressed to 16 bits by enforcing 128-bit data alignment. In this case, bits [3:0] are always 0 and the compressed pointer contains bits [19:4] of the address.

5.2.2. Vector<T> layout

Vector is the simplest array type. It always stores a pointer to the start of the data array, and can optionally store the number of elements. If the number of elements is present, a .size() method is available.

The supported memory layouts are shown in Table 5.1.

Fig. 5.1 shows the memory layout for ONE_PTR and SPAN. SCALED_PTR_128 is similar to ONE_PTR but their begin pointers are 16 bits instead of 32.

Fig. 5.1 Vector<T> memory layout

The SPAN layout can be represented as:

T* begin; // 32-bit pointer
uint32_t size;

Whereas SHORT_SPAN has a layout like this:

T* begin; // Truncated 20-bit pointer
// 1 bit reserved for the future
uint11_t size;

Which means it can only store up to 2,047 elements.

5.2.3. Vector<Input<Vector<T>>> layout

It is possible to nest Vectors, and at each level the memory layout can be different. We use Vector<Input<Vector<T>>> to illustrate how these are implemented, but Input could also be Output or InOut.

For example, if both levels use ONE_PTR you would have the layout shown in Fig. 5.2.

Fig. 5.2 Vector<Input<Vector<T>>> memory layout using ONEPTR

Or if both levels used SPAN the layout would be as shown in Fig. 5.3.

Fig. 5.3 Vector<Input<Vector<T>>> memory layout with SPAN

Note that this produces a “jagged” 2D vector. In other words, the length of each sub-vector is not guaranteed to be identical (although it might be).

You can use different layouts for each level, for example: Vector<Input<Vector<T, ONE_PTR>>, SPAN>.

5.2.4. VectorList layout

As shown in Fig. 5.2 and Fig. 5.3, nested vectors such as Vector<Input<Vector<T>>> require the use of more memory, beyond the data itself, to store the pointers and sizes of the sub-vectors. This additional memory can become significant, especially if the data consists of many short sub-vectors. To reduce the additional memory usage, Poplar provides a more memory-efficient 2D vector type called VectorList.

This reduced memory usage comes at the cost of a slightly slower access to the start of each sub-vector because of the added complexity of the data structures. There is also a limit on the total number of sub-vectors (65,535) and more stringent limits on the sizes of each sub-vector which depend on the specific data type stored in VectorList (see Table 5.2).

For Mk2 and newer IPU architectures, there is one available layout for VectorList, named DELTANELEMENTS.

Note

Using PopVision Graph Analyser to analyse memory usage

PopVision Graph Analyser lists the amount of additional memory used by Vector and VectorList under Vertex Data, using different Variable Types:

Type of nested vector	Graph Analyser Variable Type
Vector<Input<Vector<T>>	Vector Field Data
VectorList<T, DELTANELEMENTS>>	Vector List Descriptor

In C++ vertex code, the use of VectorList is completely transparent: sub-vector elements can be accessed using the same square bracket syntax used for nested Vector data. The compiler will insert the appropriate code to extract pointers and sizes. For instance, the code below will compile and work as expected:

Output<VectorList<float, VectorListLayout::DELTANELEMENTS>> out;
 . . .
out[3][7] = 3.141592;

When a Vertex object containing a VectorList field is instantiated in C++ host code, the connection from graph variables to that field is also done in the same way as for a nested Vector.

The detailed memory layout described below will only be needed to understand the details of the memory usage, and how to access the data from vertex code written in Assembly.

DELTANELEMENTS layout

This has a base structure which contains the base address of the data, the size of the vector (that is, the number of sub-vectors) and a pointer to an array of structures describing the sub-vectors.

Each of the sub-vector structures contain a pointer to its data (as an offset from the base address specified in the base structure) and the number of data elements. Each sub-vector can be a different size. The base address points to the start of the vector data and so one of the offsets is always zero.

Fig. 5.4 shows this implementation, using C-like types to represent the pointer and count sizes.

Fig. 5.4 DELTANELEMENTS memory layout

The top-level structure contains a pointer to the base of the vector data, a count of the number of sub-vectors and a pointer to an array of DeltaNElement structures for the sub-vectors. Both pointers are 21 bits so that the full architectural memory space of the tile can be addressed.

These values are packed into two 32-bit words, as shown in Fig. 5.5. The reserved bits should not be assumed to be zero and should be masked off when extracting the address fields.

Fig. 5.5 DELTANELEMENTS base structure bit packing

Each DeltaNElement structure represents a sub-vector. It has a pointer to the data, which is an element-sized offset from the base address (so, for example, for naturally-aligned float data, this will be an offset in 32-bit words). It also has a count of the number of data elements in this sub-vector.

The offset and count are packed into a 32-bit word. The number of bits for each depends on the data alignment. For byte-aligned data, the offset is 21 bits and the count is 11 bits. For larger alignments, fewer bits are required for the offset and more bits are available for the count. For example, for 32-bit aligned data, only 19 bits are required for the offset, so 13 bits are available for the sub-vector size.

The number of bits available for the offset and the count for various data types are summarised in Table 5.2 and illustrated in Fig. 5.6.

Table 5.2 Offset and count sizes for DELTANELEMENTS

Data Type Size	Offset Field Size	Count Field Size	Maximum Count Value	Offset Unpacking
1 byte	uint21_t	unit11_t	2,047	<< 0
2 bytes (e.g. half)	uint20_t	unit12_t	4,095	<< 1
4 bytes (e.g. float)	uint19_t	unit13_t	8,191	<< 2
8 bytes	uint18_t	unit14_t	16,383	<< 3
16 bytes	uint17_t	unit15_t	32,767	<< 4

Fig. 5.6 DELTANELEMENTS sub-vector structure bit packing

Note that the alignment must be a multiple of the data type size.