- Starting the port on a CUDA machine is often the easiest approach, since you can incrementally port pieces of the code to HIP while leaving the rest in CUDA. (Recall that on CUDA machines HIP is just a thin layer over CUDA, so the two code types can interoperate on nvcc platforms.) Also, the HIP port can be compared with the original CUDA code for function and performance.
- Once the CUDA code is ported to HIP and is running on the CUDA machine, compile the HIP code using the HIP compiler on an AMD machine.
- HIP ports can replace CUDA versions: HIP can deliver the same performance as a native CUDA implementation, with the benefit of portability to both Nvidia and AMD architectures as well as a path to future C++ standard support. You can handle platform-specific features through conditional compilation or by adding them to the open-source HIP infrastructure.
- Use **[bin/hipconvertinplace-perl.sh](https://github.com/ROCm-Developer-Tools/HIP/blob/master/bin/hipconvertinplace-perl.sh)** to hipify all code files in the CUDA source directory.
The hipexamine-perl.sh tool will scan a source directory to determine which files contain CUDA code and how much of that code can be automatically hipified.
* hipexamine-perl also presents a summary at the end of the process for the statistics collected across all files. This has similar format to the per-file reporting, and also includes a list of all kernels which have been called. An example from above:
The [hipconvertinplace-perl.sh](https://github.com/ROCm-Developer-Tools/HIP/blob/master/bin/hipconvertinplace-perl.sh) script will perform inplace conversion for all code files in the specified directory.
All HIP projects target either AMD or NVIDIA platform. The platform affects which headers are included and which libraries are used for linking.
-`HIP_PLATFORM_HCC` is defined if the HIP platform targets AMD
-`HIP_PLATFORM_NVCC` is defined if the HIP platform targets NVIDIA
On AMD platform, the compiler was hcc, but is deprecated in ROCM v3.5 release, and HIP-Clang compiler is introduced for compiling HIP programs.
For most HIP applications, the transition from hcc to HIP-Clang is transparent.
HIPCC and HIP cmake files automatically choose compilation options for HIP-Clang and hide the difference between the hcc and hip-clang code.
However, minor changes may be required as HIP-Clang has stricter syntax and semantic checks compared to hcc.
Many projects use a mixture of an accelerator compiler (AMD or NVIDIA) and a standard compiler (e.g. g++). These defines are set for both accelerator and standard compilers and thus are often the best option when writing code that uses conditional compilation.
Often, it's useful to know whether the underlying compiler is hcc, HIP-Clang or nvcc. This knowledge can guard platform-specific code or aid in platform-specific performance tuning.
Compiler directly generates the host code (using the Clang x86 target) and passes the code to another host compiler. Thus, they have no equivalent of the \__CUDA_ACC define.
nvcc makes two passes over the code: one for host code and one for device code.
HIP-Clang will have multiple passes over the code: one for the host code, and one for each architecture on the device code.
`__HIP_DEVICE_COMPILE__` is set to a nonzero value when the compiler (hcc, HIP-Clang or nvcc) is compiling code for a device inside a `__global__` kernel or for a device function. `__HIP_DEVICE_COMPILE__` can replace #ifdef checks on the `__CUDA_ARCH__` define.
|`__HIP_DEVICE_COMPILE__` | 1 if compiling for device; undefined if compiling for host | 1 if compiling for device; undefined if compiling for host |1 if compiling for device; undefined if compiling for host | Undefined
|`__HIP_ARCH_*` | 0 or 1 depending on feature support (see below) |0 or 1 depending on feature support (see below) | 0 or 1 depending on feature support (see below) | 0
This type of code requires special attention, since hcc/AMD and nvcc/CUDA devices have different architectural capabilities. Moreover, you can't determine the presence of a feature using a simple comparison against an architecture's version number. HIP provides a set of defines and device properties to query whether a specific architectural feature is supported.
Host code should query the architecture feature flags in the device properties that hipGetDeviceProperties returns, rather than testing the "major" and "minor" fields directly:
`HIP_ROCclr` is defined on AMD platform that HIP use Radeon Open Compute Common Language Runtime, called ROCclr.
ROCclr is a virtual device interface that HIP runtimes interact with different backends which allows runtimes to work on Linux , as well as Windows without much efforts.
- NVIDIA platform
On Nvidia platform, HIP is just a thin layer on top of CUDA.
On non-AMD platform, HIP runtime determines if nvcc is available and can be used. If available, HIP_PLATFORM is set to nvcc and underneath CUDA path is used.
hipLaunchKernel is a variadic macro which accepts as parameters the launch configurations (grid dims, group dims, stream, dynamic shared size) followed by a variable number of kernel arguments.
While this can be a convenient single-line kernel launch syntax, the macro implementation can cause issues when nested inside other macros. For example, consider the following:
```
// Will cause compile error:
#define MY_LAUNCH(command, doTrace) \
{\
if (doTrace) printf ("TRACE: %s\n", #command); \
(command); /* The nested ( ) will cause compile error */\
hipcc is a portable compiler driver that will call nvcc or HIP-Clang (depending on the target system) and attach all required include and library options. It passes options through to the target compiler. Tools that call hipcc must ensure the compiler options are appropriate for the target compiler.
The `hipconfig` script may helpful in identifying the target platform, compiler and runtime. It can also help set options appropriately.
hipcc adds the necessary libraries for HIP as well as for the accelerator compiler (nvcc or AMD compiler). We recommend linking with hipcc since it automatically links the binary to the necessary HIP runtime libraries. It also has knowledge on how to link and to manage the GPU objects.
CUDA code often uses nvcc for accelerator code (defining and launching kernels, typically defined in .cu or .cuh files).
It also uses a standard compiler (g++) for the rest of the application. nvcc is a preprocessor that employs a standard host compiler (gcc) to generate the host code.
Code compiled using this tool can employ only the intersection of language features supported by both nvcc and the host compiler.
In some cases, you must take care to ensure the data types and alignment of the host compiler are identical to those of the device compiler. Only some host compilers are supported---for example, recent nvcc versions lack Clang host-compiler capability.
hcc generates both device and host code using the same Clang-based compiler. The code uses the same API as gcc, which allows code generated by different gcc-compatible compilers to be linked together. For example, code compiled using hcc can link with code compiled using "standard" compilers (such as gcc, ICC and Clang). Take care to ensure all compilers use the same standard C++ header and library formats.
If you pass "--stdlib=libc++" to hipcc, hipcc will use the libc++ library. Generally, libc++ provides a broader set of C++ features while libstdc++ is the standard for more compilers (notably including g++).
When cross-linking C++ code, any C++ functions that use types from the C++ standard library (including std::string, std::vector and other containers) must use the same standard-library implementation. They include the following:
Applications which are compiled entirely with hipcc, and which benefit from advanced C++ features not supported in libstdc++, and which do not require portability to nvcc, may choose to use libc++.
- hip_runtime_api.h: defines all the HIP runtime APIs (e.g., hipMalloc) and the types required to call them. A source file that is only calling HIP APIs but neither defines nor launches any kernels can include hip_runtime_api.h. hip_runtime_api.h uses no custom hc language features and can be compiled using a standard C++ compiler.
- hip_runtime.h: included in hip_runtime_api.h. It additionally provides the types and defines required to create and launch kernels. hip_runtime.h does use custom hc language features, but they are guarded by ifdef checks. It can be compiled using a standard C++ compiler but will expose a subset of the available functions.
CUDA has slightly different contents for these two files. In some cases you may need to convert hipified code to include the richer hip_runtime.h instead of hip_runtime_api.h.
You can compile hip\_runtime\_api.h using a standard C or C++ compiler (e.g., gcc or ICC). The HIP include paths and defines (`__HIP_PLATFORM_HCC__` or `__HIP_PLATFORM_NVCC__`) must pass to the standard compiler; hipconfig then returns the necessary options:
ensure that the file includes hip_runtime.h (or hip_runtime_api.h, if appropriate).
The hipify-perl script automatically converts "cuda_runtime.h" to "hip_runtime.h," and it converts "cuda_runtime_api.h" to "hip_runtime_api.h", but it may miss nested headers or macros.
For quick HIP ports, leaving these file extensions unchanged is often easier, as it minimizes the work required to change file names in the directory and #include statements in the files.
Code should not assume a warp size of 32 or 64. See [Warp Cross-Lane Functions](hip_kernel_language.md#warp-cross-lane-functions) for information on how to write portable wave-aware code.
Note that the symbol name needs to be encased in the HIP_SYMBOL macro, as shown in the code example below. This also applies to hipMemcpyFromSymbol, hipGetSymbolAddress, and hipGetSymbolSize.
Threadfence_system makes all device memory writes, all writes to mapped host memory, and all writes to peer memory visible to CPU and other GPU devices.
Some implementations can provide this behavior by flushing the GPU L2 cache.
HIP/HCC does not provide this functionality. As a workaround, users can set the environment variable `HSA_DISABLE_CACHE=1` to
disable the GPU L2 cache. This will affect all accesses and for all kernels and so may have
Compute programs sometimes use textures either to access dedicated texture caches or to use the texture-sampling hardware for interpolation and clamping. The former approach uses simple point samplers with linear interpolation, essentially only reading a single point. The latter approach uses the sampler hardware to interpolate and combine multiple
point samples. AMD hardware, as well as recent competing hardware,
has a unified texture/L1 cache, so it no longer has a dedicated texture cache. But the nvcc path often caches global loads in the L2 cache, and some programs may benefit from explicit control of the L1 cache contents. We recommend the __ldg instruction for this purpose.
On an hcc/AMD platform, set the HIP_TRACE_API environment variable to see a textural API trace. Use the following bit mask:
- 0x1 = trace APIs
- 0x2 = trace synchronization operations
- 0x4 = trace memory allocation / deallocation
### Environment Variables
On hcc/AMD platforms, set the HIP_PRINT_ENV environment variable to 1 and run an application that calls a HIP API to see all HIP-supported environment variables and their current values:
- HIP_PRINT_ENV = 1: print HIP environment variables
- HIP_LAUNCH_BLOCKING = 0: make HIP APIs host-synchronous so they are blocked until any kernel launches or data-copy commands are complete (an alias is CUDA_LAUNCH_BLOCKING)
To see the detailed commands that hipcc issues, set the environment variable HIPCC_VERBOSE to 1. Doing so will print to stderr the hcc (or nvcc) commands that hipcc generates.
If you pass a ".cu" file, hcc will attempt to compile it as a CUDA language file. You must tell hcc that it's in fact a C++ file: use the "-x c++" option.
On the HCC path, HIP provides a number of environment variables that control the behavior of HIP. Some of these are useful for application development (for example HIP_VISIBLE_DEVICES, HIP_LAUNCH_BLOCKING),
some are useful for performance tuning or experimentation (for example HIP_STAGING*), and some are useful for debugging (HIP_DB). You can see the environment variables supported by HIP as well as
their current values and usage with the environment var "HIP_PRINT_ENV" - set this and then run any HIP application. For example:
```
$ HIP_PRINT_ENV=1 ./myhipapp
HIP_PRINT_ENV = 1 : Print HIP environment variables.
HIP_LAUNCH_BLOCKING = 0 : Make HIP APIs 'host-synchronous', so they block until any kernel launches or data copy commands complete. Alias: CUDA_LAUNCH_BLOCKING.
HIP_DB = 0 : Print various debug info. Bitmask, see hip_hcc.cpp for more information.
HIP_TRACE_API = 0 : Trace each HIP API call. Print function name and return code to stderr as program executes.
HIP_VISIBLE_DEVICES = 0 : Only devices whose index is present in the secquence are visible to HIP applications and they are enumerated in the order of secquence
