A language and compiler for the Raspberry Pi GPU
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README.md

QPULib

Version 0.1.0.

QPULib is a programming language and compiler for the Raspberry Pi's Quad Processing Units (QPUs). It is implemented as a C++ library that runs on the Pi's ARM CPU, generating and offloading programs to the QPUs at runtime. This page introduces and documents QPULib. For build instructions, see the Getting Started Guide.

Contents

Background

The QPU is a vector processor developed by Broadcom with instructions that operate on 16-element vectors of 32-bit integer or floating point values. For example, given two 16-element vectors

10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

and

20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35

the QPU's integer-add instruction computes a third vector

30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60

where each element in the output is the sum of the corresponding two elements in the inputs.

Each 16-element vector is comprised of four quads. This is where the name "Quad Processing Unit" comes from: a QPU processes one quad per clock cycle, and a QPU instruction takes four consecutive clock cycles to deliver a full 16-element result vector.

The Pi contains 12 QPUs in total, each running at 250MHz. That's a max throughput of 750M vector instructions per second (250M cycles divided by 4 cycles-per-instruction times 12 QPUs). Or: 12B operations per second (750M instructions times 16 vector elements). QPU instructions can in some cases deliver two results at a time, so the Pi's QPUs are often advertised at 24 GFLOPS.

The QPUs are part of the Raspberry Pi's graphics pipeline. If you're interested in doing efficient graphics on the Pi then you probably want OpenGL ES. But if you'd like to try accellerating a non-graphics part of your Pi project then QPULib is worth a look. (And so too are these references.)

Example 1: Euclid's Algorithm

Following tradition, let's start by implementing Euclid's algorithm. Given a pair of positive integers larger then zero, Euclid's algorithm computes the largest integer that divides into both without a remainder, also known as the greatest common divisor, or GCD for short.

We present two versions of the algorithm:

  1. a scalar version that runs on the ARM CPU and computes a single GCD; and

  2. a vector version that runs on a single QPU and computes 16 different GCDs in parallel.

Scalar version

In plain C++, we can express the algorithm as follows.

void gcd(int* p, int* q, int* r)
{
  int a = *p;
  int b = *q;
  while (a != b) {
    if (a > b) 
      a = a-b;
    else
      b = b-a;
  }
  *r = a;
}

Admittedly, it's slightly odd to write gcd in this way, operating on pointers to integers rather than integers directly. However, it prepares the way for the vector version which operates on arrays of inputs and outputs.

Vector version 1

Using QPULib, the algorithm looks as follows.

#include <QPULib.h>

void gcd(Ptr<Int> p, Ptr<Int> q, Ptr<Int> r)
{
  Int a = *p;
  Int b = *q;
  While (any(a != b))
    Where (a > b)
      a = a-b;
    End
    Where (a < b)
      b = b-a;
    End
  End
  *r = a;
}

Even this simple example introduces a number of concepts:

  • the Int type denotes a 16-element vector of 32-bit integers;

  • the Ptr<Int> type denotes a 16-element vector of addresses of Int vectors;

  • the expression *p denotes the Int vector in memory starting at address p0, i.e. starting at the first address in the vector p;

  • the expression a != b computes a vector of booleans via a pointwise comparison of vectors a and b;

  • the condition any(a != b) is true when any of the booleans in the vector a != b are true;

  • the statement Where (a > b) a = a-b; End is a conditional assigment: only elements in vector a for which a > b holds will be modified.

It's worth reiterating that QPULib is just standard C++ code: there are no pre-processors being used other than the standard C pre-processor. All the QPULib language constructs are simply classes, functions, and macros exported by QPULib. This kind of language is somtimes known as a Domain Specific Embedded Language.

Invoking the QPUs

Now, to compute 16 GCDs on a single QPU, we write the following program.

int main()
{
  // Compile the gcd function to a QPU kernel k
  auto k = compile(gcd);

  // Allocate and initialise arrays shared between CPU and QPUs
  SharedArray<int> a(16), b(16), r(16);

  // Initialise inputs to random values in range 100..199
  srand(0);
  for (int i = 0; i < 16; i++) {
    a[i] = 100 + rand()%100;
    b[i] = 100 + rand()%100;
  }

  // Set the number of QPUs to use
  k.setNumQPUs(1);

  // Invoke the kernel
  k(&a, &b, &r);

  // Display the result
  for (int i = 0; i < 16; i++)
    printf("gcd(%i, %i) = %i\n", a[i], b[i], r[i]);
  
  return 0;
}

Unpacking this a bit:

  • compile takes function defining a QPU computation and returns a CPU-side handle that can be used to invoke it;

  • the handle k is of type Kernel<Ptr<Int>, Ptr<Int>, Ptr<Int>>, capturing the types of gcd's parameters, but we use the auto keyword to avoid clutter;

  • when the kernel is invoked by writing k(&a, &b, &r), QPULib knows how to automatically convert CPU values of type SharedArray<int>* into QPU values of type Ptr<Int>;

  • the SharedArray<α> type is used to allocate memory that is accessed by both the CPU and the QPUs: memory allocated with new and malloc() will not be accessible from the QPUs.

Running this program, we get:

gcd(183, 186) = 3
gcd(177, 115) = 1
gcd(193, 135) = 1
gcd(186, 192) = 6
gcd(149, 121) = 1
gcd(162, 127) = 1
gcd(190, 159) = 1
gcd(163, 126) = 1
gcd(140, 126) = 14
gcd(172, 136) = 4
gcd(111, 168) = 3
gcd(167, 129) = 1
gcd(182, 130) = 26
gcd(162, 123) = 3
gcd(167, 135) = 1
gcd(129, 102) = 3

Vector version 2: loop unrolling

Loop unrolling is a technique for improving performance by reducing the number of costly branch instructions executed.

The QPU's branch instruction can indeed be costly: it requires three delay slots (that's 12 clock cycles), and QPULib currently makes no attempt to fill these slots with useful work. Although QPULib doesn't do loop unrolling for you, it does make it easy to express: we can simply use a C++ loop to generate multiple QPU statements.

void gcd(Ptr<Int> p, Ptr<Int> q, Ptr<Int> r)
{
  Int a = *p;
  Int b = *q;
  While (any(a != b))
    // Unroll the loop body 32 times
    for (int i = 0; i < 32; i++) {
      Where (a > b)
        a = a-b;
      End
      Where (a < b)
        b = b-a;
      End
    }
  End
  *r = a;
}

Using C++ as a meta-language in this way is one of the attractions of QPULib. We will see lots more examples of this later!

Example 2: 3D Rotation

Let's move to another simple example that helps to introduce ideas: a routine to rotate 3D objects.

(Of course, OpenGL ES would be a much better path for doing efficient graphics; this is just for illustration purposes.)

Scalar version

The following function will rotate n vertices about the Z axis by θ degrees.

void rot3D(int n, float cosTheta, float sinTheta, float* x, float* y)
{
  for (int i = 0; i < n; i++) {
    float xOld = x[i];
    float yOld = y[i];
    x[i] = xOld * cosTheta - yOld * sinTheta;
    y[i] = yOld * cosTheta + xOld * sinTheta;
  }
}

If we apply this to the vertices in Newell's teapot (rendered using Richard Hull's wireframes tool)

Newell's teapot

with θ = 180 degrees, then we get

Newell's teapot

Vector version 1

Our first vector version is almost identical to the scalar version above: the only difference is that each loop iteration now processes 16 vertices at a time rather than a single vertex.

void rot3D(Int n, Float cosTheta, Float sinTheta, Ptr<Float> x, Ptr<Float> y)
{
  For (Int i = 0, i < n, i = i+16)
    Float xOld = x[i];
    Float yOld = y[i];
    x[i] = xOld * cosTheta - yOld * sinTheta;
    y[i] = yOld * cosTheta + xOld * sinTheta;
  End
}

Unfortunately, this simple solution is not the most efficient: it will spend a lot of time blocked on the memory subsystem, waiting for vector loads and stores to complete. To get good performance on a QPU, it is desirable to overlap memory access with computation, and the current QPULib compiler is not clever enough to do this automatically. We can however solve the problem manually, using non-blocking load and store operations.

Vector version 2: non-blocking loads and stores

QPULib supports non-blocking loads through two functions:

  • Given a vector of addresses p, the statement gather(p) will request the value at each address in p.

  • A subsequent a call to receive(x), where x is vector, will block until the value at each address in p has been loaded into x.

Unlike the statement x = *p, the statement gather(p) will request the value at each address in p, not the vector beginning at the first address in p. In addition, gather(p) does not block until the loads have completed: between gather(p) and receive(x) the program is free to perform computation in parallel with the slow memory accesses.

Inside the QPU, an 4-element FIFO is used to hold gather requests: each call to gather will enqueue the FIFO, and each call to receive will dequeue it. This means that a maximum of four gather calls may be issued before a receive must be called.

Non-blocking stores are not as powerfull, but they are still useful:

  • Given vector of addresses p and a vector x, the statement store(x, p) will write vector x to memory beginning at the first address in p.

Unlike the statement *p = x, the statement store(p, x) will not wait until x has been written. However, any subsequent call to store will wait until the previous store has completed. (Future improvements to QPULib could allow several outstanding stores instead of just one.)

We are now ready to implement a vectorised rotation routine that overlaps memory access with computation:

void rot3D(Int n, Float cosTheta, Float sinTheta, Ptr<Float> x, Ptr<Float> y)
{
  // Function index() returns vector <0 1 2 ... 14 15>
  Ptr<Float> p = x + index();
  Ptr<Float> q = y + index();
  // Pre-fetch first two vectors
  gather(p); gather(q);

  Float xOld, yOld;
  For (Int i = 0, i < n, i = i+16)
    // Pre-fetch two vectors for the *next* iteration
    gather(p+16); gather(q+16);
    // Receive vectors for *this* iteration
    receive(xOld); receive(yOld);
    // Store results
    store(xOld * cosTheta - yOld * sinTheta, p);
    store(yOld * cosTheta + xOld * sinTheta, q);
    p = p+16; q = q+16;
  End

  // Discard pre-fetched vectors from final iteration
  receive(xOld); receive(yOld);
}

While the outputs from one iteration are being computed and written to memory, the inputs for the next iteration are being loaded in parallel.

Vector version 3: multiple QPUs

QPULib provides a simple mechanism to execute the same kernel on multiple QPUs in parallel: before invoking a kernel k, call k.setNumQPUs(n) to use n QPUs. For this to be useful the programmer needs a way to tell each QPU to compute a different part of the overall result. Accordingly, QPULib provides the me() function which returns the unique id of the QPU that called it. More specifically, me() returns a vector of type Int with all elements holding the QPU id. In addition, the numQPUs() function returns the number of QPUs that are executing the kernel. A QPU id will always lie in the range 0 to numQPUs()-1.

Now, to spread the rot3D computation accross multiple QPUs we will use a loop increment of 16*numQPUs() instead of 16, and offset the initial pointers x and y by 16*me().

void rot3D(Int n, Float cosTheta, Float sinTheta, Ptr<Float> x, Ptr<Float> y)
{
  Int inc = numQPUs() << 4;
  Ptr<Float> p = x + index() + (me() << 4);
  Ptr<Float> q = y + index() + (me() << 4);
  gather(p); gather(q);

  Float xOld, yOld;
  For (Int i = 0, i < n, i = i+inc)
    gather(p+inc); gather(q+inc);
    receive(xOld); receive(yOld);
    store(xOld * cosTheta - yOld * sinTheta, p);
    store(yOld * cosTheta + xOld * sinTheta, q);
    p = p+inc; q = q+inc;
  End

  // Discard pre-fetched vectors from final iteration
  receive(xOld); receive(yOld);
}

Performance

Times taken to rotate an object with 192,000 vertices:

Version Number of QPUs Run-time (s)
Scalar 0 0.018
Vector 1 1 0.040
Vector 2 1 0.018
Vector 3 1 0.018
Vector 3 2 0.016

Non-blocking loads and stores (vector version 2) give a significant performance boost: in this case a factor of 2.

Unforunately, the program does not scale well to multiple QPUs. I'm not entirely sure why, but my suspicion is that the compute-to-memory ratio is too low: we do only 2 arithmetic operations for every memory access, perhaps overwhelming the memory subsystem. If there are possibilities for QPULib to generate better code here, hopefully they will be discovered in due course. (Do let me know if you have any suggestions.)

Example 3: 2D Convolution (Heat Transfer)

Let's move to a somewhat more substantial example: modelling the heat flow across a 2D surface. Newton's law of cooling states that an object cools at a rate proportional to the difference between its temperature T and the temperature of its environment (or ambient temperature) A:

dT/dt = −k(T − A)

When simulating this equation below, we will consider each point on our 2D surface to be a seperate object, and the ambient temperature of each object to be the average of the temperatures of the 8 surrounding objects. This is very similar to 2D convolution using a mean filter.

Scalar version

The following function simulates a single time-step of the differential equation, applied to each object in the 2D grid.

void step(float** grid, float** gridOut, int width, int height)
{
  for (int y = 1; y < height-1; y++) {
    for (int x = 1; x < width-1; x++) {
      float surroundings =
        grid[y-1][x-1] + grid[y-1][x]   + grid[y-1][x+1] +
        grid[y][x-1]   +                  grid[y][x+1]   +
        grid[y+1][x-1] + grid[y+1][x]   + grid[y+1][x+1];
      surroundings *= 0.125;
      gridOut[y][x] = grid[y][x] - (K * (grid[y][x] - surroundings));
    }
  }
}

If we apply heat at the north and east edges of our 2D surface, and cold at the south and west edges, then after of several simulation steps we get:

Heat flow across 2D surface

Vector version

Before vectorising the simulation routine, we will introduce the idea of a cursor which is useful for implementing sliding window algorithms. A cursor points to a window of three continguous vectors in memory: prev, current and next.

  cursor  ------>  +---------+---------+---------+
                   |  prev   | current |  next   |
                   +---------+---------+---------+
                 +0:      +16:      +32:      +48:

and supports three main operations:

  1. advance the cursor by one vector, i.e. slide the window right by one vector;

  2. shift-left the current vector by one element, using the value of the next vector;

  3. shift-right the current vector by one element, using the value of the prev vector.

Here is a QPULib implementation of a cursor, using a C++ class.

class Cursor {
  Ptr<Float> cursor;
  Float prev, current, next;

 public:

  // Initialise to cursor to a given pointer
  // and fetch the first vector.
  void init(Ptr<Float> p) {
    gather(p);
    current = 0;
    cursor = p+16;
  }

  // Receive the first vector and fetch the second.
  // (prime the software pipeline)
  void prime() {
    receive(next);
    gather(cursor);
  }

  // Receive the next vector and fetch another.
  void advance() {
    cursor = cursor+16;
    prev = current;
    gather(cursor);
    current = next;
    receive(next);
  }

  // Receive final vector and don't fetch any more.
  void finish() {
    receive(next);
  }

  // Shift the current vector left one element
  void shiftLeft(Float& result) {
    result = rotate(current, 15);
    Float nextRot = rotate(next, 15);
    Where (index() == 15)
      result = nextRot;
    End
  }

  // Shift the current vector right one element
  void shiftRight(Float& result) {
    result = rotate(current, 1);
    Float prevRot = rotate(prev, 1);
    Where (index() == 0)
      result = prevRot;
    End
  }
};

Given a vector x, the QPULib operation rotate(x, n) will rotate x right by n places where n is a integer in the range 0 to 15. Notice that rotating right by 15 is the same as rotating left by 1.

Now, using cursors the vectorised simulation step is expressed below. A slight structural difference from the scalar version is that we no longer treat the grid as a 2D array: it is now 1D array with a pitch parameter that gives the increment needed to get from the start of one row to the start of the next.

void step(Ptr<Float> grid, Ptr<Float> gridOut, Int pitch, Int width, Int height)
{
  Cursor row[3];
  grid = grid + pitch*me() + index();

  // Skip first row of output grid
  gridOut = gridOut + pitch;

  For (Int y = me(), y < height, y=y+numQPUs())
    // Point p to the output row
    Ptr<Float> p = gridOut + y*pitch;

    // Initilaise three cursors for the three input rows
    for (int i = 0; i < 3; i++) row[i].init(grid + i*pitch);
    for (int i = 0; i < 3; i++) row[i].prime();

    // Compute one output row
    For (Int x = 0, x < width, x=x+16)

      for (int i = 0; i < 3; i++) row[i].advance();

      Float left[3], right[3];
      for (int i = 0; i < 3; i++) {
        row[i].shiftLeft(right[i]);
        row[i].shiftRight(left[i]);
      }

      Float sum = left[0] + row[0].current + right[0] +
                  left[1] +                  right[1] +
                  left[2] + row[2].current + right[2];

      store(row[1].current - K * (row[1].current - sum * 0.125), p);
      p = p + 16;

    End

    // Cursors are finished for this row
    for (int i = 0; i < 3; i++) row[i].finish();

    // Move to the next input rows
    grid = grid + pitch*numQPUs();
  End
}

Performance

Times taken to simulate a 512x512 surface for 2000 steps:

Version Number of QPUs Run-time (s)
Scalar 0 431.46
Vector 1 49.34
Vector 2 24.91
Vector 4 20.36

References

The following works were very helpful in the development of QPULib.