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DeviceDefs.cuh
1 /**
2 * Copyright (c) 2015-present, Facebook, Inc.
3 * All rights reserved.
4 *
5 * This source code is licensed under the CC-by-NC license found in the
6 * LICENSE file in the root directory of this source tree.
7 */
8
9 // Copyright 2004-present Facebook. All Rights Reserved.
10
11 #pragma once
12
13 namespace faiss { namespace gpu {
14
15 #ifdef __CUDA_ARCH__
16 #if __CUDA_ARCH__ <= 620
17 constexpr int kWarpSize = 32;
18 #else
19 #error Unknown __CUDA_ARCH__; please define parameters for compute capability
20 #endif // __CUDA_ARCH__ types
21 #endif // __CUDA_ARCH__
22
23 #ifndef __CUDA_ARCH__
24 // dummy value for host compiler
25 constexpr int kWarpSize = 32;
26 #endif // !__CUDA_ARCH__
27
28 __forceinline__ __device__ void warpFence() {
29 // Technically, memory barriers are required via the CUDA
30 // programming model, since warp synchronous programming no longer
31 // is guaranteed.
32 //
33 // There are two components to it:
34 // -a barrier known to the compiler such that the compiler will not
35 // schedule loads and stores across the barrier;
36 // -a HW-level barrier that guarantees that writes are seen in the
37 // proper order
38 //
39 // However, __threadfence_block() is a stronger constraint than what
40 // we really want out of the hardware: a warp-wide barrier.
41 //
42 // In current hardware, it appears that warp synchronous programming
43 // is a reality; by all tests it appears safe and race-free.
44 //
45 // However, understandably it may not be in the future (based on
46 // what Nvidia says in the Kepler guide, it may change depending
47 // upon compiler/toolchain issues or future hardware).
48 //
49 // Removing the fence results in 10%+ faster performance.
50 // However, we are judicious as to where we insert the fence, so if
51 // this reality ever changes, uncommenting this will result in CUDA
52 // programming model-safe ordering again.
53 //
54 // FIXME: we should probably qualify as volatile as well, since the
55 // compiler could technically preserve values across loops? This
56 // seems very impractical for the compiler to do, however.
57
58 // __threadfence_block();
59 }
60
61 } } // namespace