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| -rw-r--r-- | llvm/docs/CompileCudaWithLLVM.rst | 40 |
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diff --git a/llvm/docs/CompileCudaWithLLVM.rst b/llvm/docs/CompileCudaWithLLVM.rst index 5ed3f14005a..f57839cec96 100644 --- a/llvm/docs/CompileCudaWithLLVM.rst +++ b/llvm/docs/CompileCudaWithLLVM.rst @@ -148,6 +148,46 @@ compilation, in host and device modes: Both clang and nvcc define ``__CUDACC__`` during CUDA compilation. You can detect NVCC specifically by looking for ``__NVCC__``. +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: + +* ``-ffp-contract={on,off,fast}`` (defaults to ``fast`` on host and device when + compiling CUDA) Controls whether the compiler emits fused multiply-add + operations. + + * ``off``: never emit fma operations, and prevent ptxas from fusing multiply + and add instructions. + * ``on``: fuse multiplies and adds within a single statement, but never + across statements (C11 semantics). Prevent ptxas from fusing other + multiplies and adds. + * ``fast``: fuse multiplies and adds wherever profitable, even across + statements. Doesn't prevent ptxas from fusing additional multiplies and + adds. + + Fused multiply-add instructions can be much faster than the unfused + equivalents, but because the intermediate result in an fma is not rounded, + this flag can affect numerical code. + +* ``-fcuda-flush-denormals-to-zero`` (default: off) When this is enabled, + floating point operations may flush `denormal + <https://en.wikipedia.org/wiki/Denormal_number>`_ inputs and/or outputs to 0. + Operations on denormal numbers are often much slower than the same operations + on normal numbers. + +* ``-fcuda-approx-transcendentals`` (default: off) When this is enabled, the + compiler may emit calls to faster, approximate versions of transcendental + functions, instead of using the slower, fully IEEE-compliant versions. For + example, this flag allows clang to emit the ptx ``sin.approx.f32`` + instruction. + + This is implied by ``-ffast-math``. + Optimizations ============= |
