From: Martin Kronbichler Date: Wed, 27 Dec 2017 15:16:00 +0000 (+0100) Subject: Add VectorizedArray::streaming_store. X-Git-Tag: v9.0.0-rc1~616^2~2 X-Git-Url: https://gitweb.dealii.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=31a7a4bd478cb29cc531fba57662a1cfec420465;p=dealii.git Add VectorizedArray::streaming_store. --- diff --git a/include/deal.II/base/vectorization.h b/include/deal.II/base/vectorization.h index d5154d85f8..02582f3107 100644 --- a/include/deal.II/base/vectorization.h +++ b/include/deal.II/base/vectorization.h @@ -262,6 +262,56 @@ public: *ptr = data; } + /** + * Write the content of the calling class into memory in form of + * @p n_array_elements to the given address using non-temporal stores that + * bypass the processor's caches, using @p _mm_stream_pd store intrinsics on + * supported CPUs. The destination of the store @p ptr must be aligned by + * the amount of bytes in the vectorized array. + * + * This store operation can be faster than usual store operations in case + * the store is streaming because it avoids the read-for-ownership transfer + * typically invoked in standard stores. This approximately works as follows + * (see the literature on computer architecture for details): When an + * algorithm stores some results to a memory address, a processor typically + * wants to move it into some of its caches as it expects the data to be + * re-used again at some point. Since caches are organized in lines of sizes + * either 64 byte or 128 byte but writes are usually smaller, a processor + * must first load in the destination cache line upon a write because only + * part of the cache line is overwritten initially. If a series of stores + * write data in a chunk bigger than any of its caches could handle, the + * data finally has to be moved out from the caches to main memory. But + * since all addressed have first been read, this doubles the load on main + * memory, which can incur a performance penalty. Furthermore, the + * organization of caches in a multicore context also requires reading an + * address before something can be written to cache to that address, see + * e.g. the Wikipedia + * article on the MESI protocol for details. The instruction underlying + * this function call signals to the processor that these two prerequisites + * on a store are relaxed: Firstly, one expects the whole cache line to be + * overwritten (that the memory subsystem then handles appropriately), so no + * need to first read the "remainder" of the cache line. Secondly, the data + * behind that particular memory will not be subject to cache coherency + * protocol as it will be in main memory both when the same processor wants + * to access it again as well as any other processors in a multicore + * chip. Due to this particular setup, any subsequent access to the data + * written by this function will need to query main memory, which is slower + * than an access from a cache both latency-wise and throughput-wise. Thus, + * this command should only be used for large stores that will collectively + * not fit into caches, as performance will be degraded otherwise. For a + * typical use case, see also this blog article. + * + * Note that streaming stores are only available in the specialized SSE/AVX + * classes of VectorizedArray of type @p double or @p float, not in the + * generic base class. + */ + DEAL_II_ALWAYS_INLINE + void streaming_store (Number *ptr) const + { + *ptr = data; + } + /** * Load @p n_array_elements from memory into the calling class, starting at * the given address and with given offsets, each entry from the offset @@ -628,6 +678,16 @@ public: _mm512_storeu_pd (ptr, data); } + /** @copydoc VectorizedArray::streaming_store() + * @note Memory must be aligned by 64 bytes. + */ + DEAL_II_ALWAYS_INLINE + void streaming_store (double *ptr) const + { + Assert(reinterpret_cast(ptr)%64==0, ExcMessage("Memory not aligned")); + _mm512_stream_pd(ptr,data); + } + /** * Load @p n_array_elements from memory into the calling class, starting at * the given address and with given offsets, each entry from the offset @@ -1008,6 +1068,16 @@ public: _mm512_storeu_ps (ptr, data); } + /** @copydoc VectorizedArray::streaming_store() + * @note Memory must be aligned by 64 bytes. + */ + DEAL_II_ALWAYS_INLINE + void streaming_store (float *ptr) const + { + Assert(reinterpret_cast(ptr)%64==0, ExcMessage("Memory not aligned")); + _mm512_stream_ps(ptr,data); + } + /** * Load @p n_array_elements from memory into the calling class, starting at * the given address and with given offsets, each entry from the offset @@ -1414,6 +1484,16 @@ public: _mm256_storeu_pd (ptr, data); } + /** @copydoc VectorizedArray::streaming_store() + * @note Memory must be aligned by 32 bytes. + */ + DEAL_II_ALWAYS_INLINE + void streaming_store (double *ptr) const + { + Assert(reinterpret_cast(ptr)%32==0, ExcMessage("Memory not aligned")); + _mm256_stream_pd(ptr,data); + } + /** * Load @p n_array_elements from memory into the calling class, starting at * the given address and with given offsets, each entry from the offset @@ -1779,6 +1859,16 @@ public: _mm256_storeu_ps (ptr, data); } + /** @copydoc VectorizedArray::streaming_store() + * @note Memory must be aligned by 32 bytes. + */ + DEAL_II_ALWAYS_INLINE + void streaming_store (float *ptr) const + { + Assert(reinterpret_cast(ptr)%32==0, ExcMessage("Memory not aligned")); + _mm256_stream_ps(ptr,data); + } + /** * Load @p n_array_elements from memory into the calling class, starting at * the given address and with given offsets, each entry from the offset @@ -2172,6 +2262,16 @@ public: _mm_storeu_pd (ptr, data); } + /** @copydoc VectorizedArray::streaming_store() + * @note Memory must be aligned by 16 bytes. + */ + DEAL_II_ALWAYS_INLINE + void streaming_store (double *ptr) const + { + Assert(reinterpret_cast(ptr)%16==0, ExcMessage("Memory not aligned")); + _mm_stream_pd(ptr,data); + } + /** * Load @p n_array_elements from memory into the calling class, starting at * the given address and with given offsets, each entry from the offset @@ -2492,6 +2592,16 @@ public: _mm_storeu_ps (ptr, data); } + /** @copydoc VectorizedArray::streaming_store() + * @note Memory must be aligned by 16 bytes. + */ + DEAL_II_ALWAYS_INLINE + void streaming_store (float *ptr) const + { + Assert(reinterpret_cast(ptr)%16==0, ExcMessage("Memory not aligned")); + _mm_stream_ps(ptr,data); + } + /** * Load @p n_array_elements from memory into the calling class, starting at * the given address and with given offsets, each entry from the offset