3c64a98180
Summary: The `errno` variable is expected to be `thread_local` by the standard. However, the GPU targets do not support `thread_local` and implementing that would be a large endeavor. Because of that, we previously didn't provide the `errno` symbol at all. However, to build some programs we at least need to be able to link against `errno`. Many things that would normally set `errno` completely ignore it currently (i.e. stdio) but some programs still need to be able to link against correct C programs. For this purpose this patch exports the `errno` symbol as a simple global. Internally, this will be updated atomically so it's at least not racy. Externally, this will be on the user. I've updated the documentation to state as such. This is required to get `libc++` to build.
52 lines
2.3 KiB
ReStructuredText
52 lines
2.3 KiB
ReStructuredText
.. _libc_gpu_motivation:
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==========================
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Motivation and Limitations
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==========================
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.. contents:: Table of Contents
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:depth: 4
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:local:
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Motivation
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==========
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This project aims to provide a large subset of the C standard library to users
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of GPU accelerators. We deliberately choose to only implement a subset of the C
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library as some features are not expressly useful or easily implemented on the
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GPU. This will be discussed further in `Limitations <libc_gpu_limitations>`_.
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The main motivation behind this project is to provide the well understood C
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library as a firm base for GPU development.
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The main idea behind this project is that programming GPUs can be as
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straightforward as programming on CPUs. This project aims to validate the GPU as
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a more general-purpose target. The implementations here will also enable more
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complex implementations of other libraries on the GPU, such as ``libc++``.
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Host services and C library features are currently provided sparsely by the
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different GPU vendors. We wish to provide these functions more completely and
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make their implementations available in a common format. This is useful for
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targets like OpenMP offloading or SYCL which wish to unify the offloading
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toolchain. We also aim to provide these functions in a format compatible with
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offloading in ``Clang`` so that we can treat the C library for the GPU as a
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standard static library.
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A valuable use for providing C library features on the GPU is for testing. For
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this reason we build `tests on the GPU <libc_gpu_testing>`_ that can run a unit
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test as if it were being run on the CPU. This also helps users port applications
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that traditionally were run on the CPU. With this support, we can expand test
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coverage for the GPU backend to the existing LLVM C library tests.
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.. _libc_gpu_limitations:
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Limitations
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===========
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We only implement a subset of the standard C library. The GPU does not
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currently support thread local variables in all cases, so variables like
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``errno`` are atomic and global. Furthermore, the GPU under the OpenCL execution
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model cannot safely provide a mutex interface. This means that features like
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file buffering are not implemented on the GPU. We can also not easily provide
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threading features on the GPU due to the execution model so these will be
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ignored, as will features like ``locale`` or ``time``.
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