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coder.gpu.kernel - Pragma that maps for-loops to GPU kernels - MATLAB

Pragma that maps for-loops to GPU kernels

Syntax

Description

coder.gpu.kernel() is a loop-level pragma that you must place immediately before a for-loop. This pragma generates a kernel and computes the launch parameters from the loop parameters.

The coder.gpu.kernel pragma overrides all parallel loop analysis checks. This override allows GPU Coder™ to parallelize loops in situations where parallel loop analysis cannot prove that all iterations are independent. Consider using coder.gpu.kernelfun to parallelize loops in functions that pass the parallel loop analysis check.

Note

Using the coder.gpu.kernel pragma before a for-loop that contains reductions is not recommended.

example

coder.gpu.kernel(B,T) generates a kernel with the dimensions specified by B and T. B[Bx,By,Bz] is an array that defines the number of blocks in the grid along dimensions x and y (z not used). T[Tx,Ty,Tz] is an array that defines the number of threads in the block along dimensions x, y, and z.

A value of -1 for B and T indicates that GPU Coder must infer the grid and block dimensions automatically. The coder.gpu.kernel pragma generates errors for invalid grid and block dimensions.

example

coder.gpu.kernel(B,T,M,name) specifies optional arguments M and name. M is a positive integer that specifies the minimum number of blocks per streaming multiprocessor. Increasing M can reduce the register usage within a kernel and improve kernel occupancy. A value of -1 for M indicates that GPU Coder must use the default value of 1. name is a character array that allows you to customize the name of the generated kernel.

This function is a code generation function. It has no effect in MATLAB®.

Examples

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This example shows how to use the coder.gpu.kernel pragma to generate a CUDA® kernel.

Create a function named multiplyVectors that performs element-wise multiplication on two 1-by-1024 input vectors, a and b. The function contains a for-loop that multiplies the elements of the vectors.

function out = multiplyVectors(a,b) %#codegen
out = zeros(size(a));

for i=1:size(a,2)
    out(i) = a(i)*b(i);
end
end

To generate a kernel from the for-loop, add the coder.gpu.kernel pragma before the for-loop. To compute the kernel launch parameters from the loop parameters, specify the coder.gpu.kernel pragma without input arguments.

function out = multiplyVectors(a,b) %#codegen
out = zeros(size(a));

coder.gpu.kernel();
for i=1:size(a,2)
    out(i) = a(i)*b(i);
end
end

Use the codegen command to generate code from multiplyVectors. The generated code contains a kernel named multiplyVectors_kernel1.

cfg = coder.gpuConfig("mex");
a = ones([1 1024]);
b = ones([1 1024]);
codegen -config cfg -args {a,b} -report multiplyVectors

This example shows how to use the coder.gpu.kernel pragma to generate a CUDA kernel and specify the launch parameters.

Create a function named addVectors that accepts two 1-by-4096 inputs, x and y. The function has one for-loop that adds x and y. 

function out = addVectors(x,y) %#codegen
out = zeros(size(x));
    
for i=1:size(x,2)
    out(i) = x(i)+y(i);
end
end

To create a kernel, place the coder.gpu.kernel pragma immediately before the vector addition loop. To automatically determine the number of blocks, specify the number of blocks as -1, and specify 128 threads per block. 

function out = addVectors(x,y) %#codegen
out = zeros(size(x));
    
coder.gpu.kernel(-1,128);
for i=1:size(x,2)
    out(i) = x(i)+y(i);
end
end

Use the codegen command to generate CUDA code.

cfg = coder.gpuConfig("mex");
x = ones([1 4096]);
y = ones([1 4096]);
codegen -config cfg -args {x,y} -report addVectors

The generated code contains a kernel named addVectors_kernel1. The kernel launches with 32 blocks and 128 threads per block.

addVectors_kernel1<<<dim3(32U, 1U, 1U), dim3(128U, 1U, 1U)>>>(*gpu_y, *gpu_x,
                                                                *gpu_out);

You can use variables or expressions when specifying the kernel launch parameters. For example, you can add an input argument named T to the addVectors function and specify T as the number of threads by using coder.gpu.kernel. 

function out = addVectors(x,y,T) %#codegen
out = zeros(size(x));

coder.gpu.kernel(1,T);
for i=1:size(x,2)
    out(i) = x(i)+y(i);
end
end

Use the codegen function to generate CUDA code. The generated code uses the input variable T to determine the number of threads for each block.

cfg = coder.gpuConfig("dll");
x = ones([1 4096]);
y = ones([1 4096]);
T = 512;
codegen -config cfg -args {x,y,T} -report addVectors

Version History

Introduced in R2017b

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