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Compiling CUDA with clang

Introduction

This document describes how to compile CUDA code with clang, and gives some details about LLVM and clang's CUDA implementations.

This document assumes a basic familiarity with CUDA. Information about CUDA programming can be found in the CUDA programming guide.

Compiling CUDA Code

Prerequisites

CUDA is supported since llvm 3.9. Current release of clang (7.0.0) supports CUDA 7.0 through 9.2. If you need support for CUDA 10, you will need to use clang built from r342924 or newer.

Before you build CUDA code, you'll need to have installed the appropriate driver for your nvidia GPU and the CUDA SDK. See NVIDIA's CUDA installation guide for details. Note that clang does not support the CUDA toolkit as installed by many Linux package managers; you probably need to install CUDA in a single directory from NVIDIA's package.

CUDA compilation is supported on Linux. Compilation on MacOS and Windows may or may not work and currently have no maintainers. Compilation with CUDA-9.x is currently broken on Windows.

Invoking clang

Invoking clang for CUDA compilation works similarly to compiling regular C++. You just need to be aware of a few additional flags.

You can use this program as a toy example. Save it as axpy.cu. (Clang detects that you're compiling CUDA code by noticing that your filename ends with .cu. Alternatively, you can pass -x cuda.)

To build and run, run the following commands, filling in the parts in angle brackets as described below:

$ clang++ axpy.cu -o axpy --cuda-gpu-arch=<GPU arch> \
    -L<CUDA install path>/<lib64 or lib>             \
    -lcudart_static -ldl -lrt -pthread
$ ./axpy
y[0] = 2
y[1] = 4
y[2] = 6
y[3] = 8

On MacOS, replace -lcudart_static with -lcudart; otherwise, you may get "CUDA driver version is insufficient for CUDA runtime version" errors when you run your program.

The -L and -l flags only need to be passed when linking. When compiling, you may also need to pass --cuda-path=/path/to/cuda if you didn't install the CUDA SDK into /usr/local/cuda or /usr/local/cuda-X.Y.

Flags that control numerical code

If you're using GPUs, you probably care about making numerical code run fast. GPU hardware allows for more control over numerical operations than most CPUs, but this results in more compiler options for you to juggle.

Flags you may wish to tweak include:

Standard library support

In clang and nvcc, most of the C++ standard library is not supported on the device side.

<math.h> and <cmath>

In clang, math.h and cmath are available and pass tests adapted from libc++'s test suite.

In nvcc math.h and cmath are mostly available. Versions of ::foof in namespace std (e.g. std::sinf) are not available, and where the standard calls for overloads that take integral arguments, these are usually not available.

#include <math.h>
#include <cmath.h>

// clang is OK with everything in this function.
__device__ void test() {
  std::sin(0.); // nvcc - ok
  std::sin(0);  // nvcc - error, because no std::sin(int) override is available.
  sin(0);       // nvcc - same as above.

  sinf(0.);       // nvcc - ok
  std::sinf(0.);  // nvcc - no such function
}

<std::complex>

nvcc does not officially support std::complex. It's an error to use std::complex in __device__ code, but it often works in __host__ __device__ code due to nvcc's interpretation of the "wrong-side rule" (see below). However, we have heard from implementers that it's possible to get into situations where nvcc will omit a call to an std::complex function, especially when compiling without optimizations.

As of 2016-11-16, clang supports std::complex without these caveats. It is tested with libstdc++ 4.8.5 and newer, but is known to work only with libc++ newer than 2016-11-16.

<algorithm>

In C++14, many useful functions from <algorithm> (notably, std::min and std::max) become constexpr. You can therefore use these in device code, when compiling with clang.

Detecting clang vs NVCC from code

Although clang's CUDA implementation is largely compatible with NVCC's, you may still want to detect when you're compiling CUDA code specifically with clang.

This is tricky, because NVCC may invoke clang as part of its own compilation process! For example, NVCC uses the host compiler's preprocessor when compiling for device code, and that host compiler may in fact be clang.

When clang is actually compiling CUDA code -- rather than being used as a subtool of NVCC's -- it defines the __CUDA__ macro. __CUDA_ARCH__ is defined only in device mode (but will be defined if NVCC is using clang as a preprocessor). So you can use the following incantations to detect clang CUDA compilation, in host and device modes:

#if defined(__clang__) && defined(__CUDA__) && !defined(__CUDA_ARCH__)
// clang compiling CUDA code, host mode.
#endif

#if defined(__clang__) && defined(__CUDA__) && defined(__CUDA_ARCH__)
// clang compiling CUDA code, device mode.
#endif

Both clang and nvcc define __CUDACC__ during CUDA compilation. You can detect NVCC specifically by looking for __NVCC__.

Dialect Differences Between clang and nvcc

There is no formal CUDA spec, and clang and nvcc speak slightly different dialects of the language. Below, we describe some of the differences.

This section is painful; hopefully you can skip this section and live your life blissfully unaware.

Compilation Models

Most of the differences between clang and nvcc stem from the different compilation models used by clang and nvcc. nvcc uses split compilation, which works roughly as follows:

clang uses merged parsing. This is similar to split compilation, except all of the host and device code is present and must be semantically-correct in both compilation steps.

(You may ask at this point, why does clang need to parse the input file multiple times? Why not parse it just once, and then use the AST to generate code for the host and each device architecture?

Unfortunately this can't work because we have to define different macros during host compilation and during device compilation for each GPU architecture.)

clang's approach allows it to be highly robust to C++ edge cases, as it doesn't need to decide at an early stage which declarations to keep and which to throw away. But it has some consequences you should be aware of.

Overloading Based on __host__ and __device__ Attributes

Let "H", "D", and "HD" stand for "__host__ functions", "__device__ functions", and "__host__ __device__ functions", respectively. Functions with no attributes behave the same as H.

nvcc does not allow you to create H and D functions with the same signature:

// nvcc: error - function "foo" has already been defined
__host__ void foo() {}
__device__ void foo() {}

However, nvcc allows you to "overload" H and D functions with different signatures:

// nvcc: no error
__host__ void foo(int) {}
__device__ void foo() {}

In clang, the __host__ and __device__ attributes are part of a function's signature, and so it's legal to have H and D functions with (otherwise) the same signature:

// clang: no error
__host__ void foo() {}
__device__ void foo() {}

HD functions cannot be overloaded by H or D functions with the same signature:

// nvcc: error - function "foo" has already been defined
// clang: error - redefinition of 'foo'
__host__ __device__ void foo() {}
__device__ void foo() {}

// nvcc: no error
// clang: no error
__host__ __device__ void bar(int) {}
__device__ void bar() {}

When resolving an overloaded function, clang considers the host/device attributes of the caller and callee. These are used as a tiebreaker during overload resolution. See IdentifyCUDAPreference for the full set of rules, but at a high level they are:

Some examples:

__host__ void foo();
__device__ void foo();

__host__ void bar();
__host__ __device__ void bar();

__host__ void test_host() {
  foo();  // calls H overload
  bar();  // calls H overload
}

__device__ void test_device() {
  foo();  // calls D overload
  bar();  // calls HD overload
}

__host__ __device__ void test_hd() {
  foo();  // calls H overload when compiling for host, otherwise D overload
  bar();  // always calls HD overload
}

Wrong-side rule example:

__host__ void host_only();

// We don't codegen inline functions unless they're referenced by a
// non-inline function.  inline_hd1() is called only from the host side, so
// does not generate an error.  inline_hd2() is called from the device side,
// so it generates an error.
inline __host__ __device__ void inline_hd1() { host_only(); }  // no error
inline __host__ __device__ void inline_hd2() { host_only(); }  // error

__host__ void host_fn() { inline_hd1(); }
__device__ void device_fn() { inline_hd2(); }

// This function is not inline, so it's always codegen'ed on both the host
// and the device.  Therefore, it generates an error.
__host__ __device__ void not_inline_hd() { host_only(); }

For the purposes of the wrong-side rule, templated functions also behave like inline functions: They aren't codegen'ed unless they're instantiated (usually as part of the process of invoking them).

clang's behavior with respect to the wrong-side rule matches nvcc's, except nvcc only emits a warning for not_inline_hd; device code is allowed to call not_inline_hd. In its generated code, nvcc may omit not_inline_hd's call to host_only entirely, or it may try to generate code for host_only on the device. What you get seems to depend on whether or not the compiler chooses to inline host_only.

Member functions, including constructors, may be overloaded using H and D attributes. However, destructors cannot be overloaded.

Using a Different Class on Host/Device

Occasionally you may want to have a class with different host/device versions.

If all of the class's members are the same on the host and device, you can just provide overloads for the class's member functions.

However, if you want your class to have different members on host/device, you won't be able to provide working H and D overloads in both classes. In this case, clang is likely to be unhappy with you.

#ifdef __CUDA_ARCH__
struct S {
  __device__ void foo() { /* use device_only */ }
  int device_only;
};
#else
struct S {
  __host__ void foo() { /* use host_only */ }
  double host_only;
};

__device__ void test() {
  S s;
  // clang generates an error here, because during host compilation, we
  // have ifdef'ed away the __device__ overload of S::foo().  The __device__
  // overload must be present *even during host compilation*.
  S.foo();
}
#endif

We posit that you don't really want to have classes with different members on H and D. For example, if you were to pass one of these as a parameter to a kernel, it would have a different layout on H and D, so would not work properly.

To make code like this compatible with clang, we recommend you separate it out into two classes. If you need to write code that works on both host and device, consider writing an overloaded wrapper function that returns different types on host and device.

struct HostS { ... };
struct DeviceS { ... };

__host__ HostS MakeStruct() { return HostS(); }
__device__ DeviceS MakeStruct() { return DeviceS(); }

// Now host and device code can call MakeStruct().

Unfortunately, this idiom isn't compatible with nvcc, because it doesn't allow you to overload based on the H/D attributes. Here's an idiom that works with both clang and nvcc:

struct HostS { ... };
struct DeviceS { ... };

#ifdef __NVCC__
  #ifndef __CUDA_ARCH__
    __host__ HostS MakeStruct() { return HostS(); }
  #else
    __device__ DeviceS MakeStruct() { return DeviceS(); }
  #endif
#else
  __host__ HostS MakeStruct() { return HostS(); }
  __device__ DeviceS MakeStruct() { return DeviceS(); }
#endif

// Now host and device code can call MakeStruct().

Hopefully you don't have to do this sort of thing often.

Optimizations

Modern CPUs and GPUs are architecturally quite different, so code that's fast on a CPU isn't necessarily fast on a GPU. We've made a number of changes to LLVM to make it generate good GPU code. Among these changes are:

Publication

The team at Google published a paper in CGO 2016 detailing the optimizations they'd made to clang/LLVM. Note that "gpucc" is no longer a meaningful name: The relevant tools are now just vanilla clang/LLVM.

gpucc: An Open-Source GPGPU Compiler
Jingyue Wu, Artem Belevich, Eli Bendersky, Mark Heffernan, Chris Leary, Jacques Pienaar, Bjarke Roune, Rob Springer, Xuetian Weng, Robert Hundt
Proceedings of the 2016 International Symposium on Code Generation and Optimization (CGO 2016)

Slides from the CGO talk

Tutorial given at CGO

Obtaining Help

To obtain help on LLVM in general and its CUDA support, see the LLVM community.