From 078b89b3c046d230ef3ad39494e5852184eb528b Mon Sep 17 00:00:00 2001 From: Abseil Team Date: Wed, 23 Oct 2019 19:35:39 -0700 Subject: Export of internal Abseil changes -- e54b9c7bbb0c58475676c268e2e19c69f4bce48a by Jorg Brown : Tweak ABSL_PREDICT_TRUE slightly, for better code on some platforms and/or optimization levels. "false || (x)" is more verbose than "!!(x)", but ultimately more efficient. For example, given this code: void InitIfNecessary() { if (ABSL_PREDICT_TRUE(NeedsInit())) { SlowInitIfNecessary(); } } Clang with default optimization level will produce: Before this CL After this CL InitIfNecessary: InitIfNecessary: push rbp push rbp mov rbp, rsp mov rbp, rsp call NeedsInit call NeedsInit xor al, -1 xor al, -1 test al, 1 test al, 1 jne .LBB2_1 jne .LBB3_1 jmp .LBB2_2 jmp .LBB3_2 .LBB2_1: .LBB3_1: call SlowInitIfNecessary call SlowInitIfNecessary .LBB2_2: .LBB3_2: pop rbp pop rbp ret ret PiperOrigin-RevId: 276401386 -- 0a3c4dfd8342bf2b1b11a87f1c662c883f73cab7 by Abseil Team : Fix comment nit: sem_open => sem_init. The code calls sem_init, not sem_open, to initialize an unnamed semaphore. (sem_open creates or opens a named semaphore.) PiperOrigin-RevId: 276344072 -- b36a664e9459057509a90e83d3482e1d3a4c44c7 by Abseil Team : Fix typo in flat_hash_map.h: exchaged -> exchanged PiperOrigin-RevId: 276295792 -- 7bbd8d18276eb110c8335743e35fceb662ddf3d6 by Samuel Benzaquen : Add assertions to verify use of iterators. PiperOrigin-RevId: 276283300 -- 677398a8ffcb1f59182cffe57a4fe7ff147a0404 by Laramie Leavitt : Migrate distribution_impl.h/cc to generate_real.h/cc. Combine the methods RandU64To into a single method: GenerateRealFromBits(). Remove rejection sampling from absl::uniform_real_distribution. PiperOrigin-RevId: 276158675 -- c60c9d11d24b0c546329d998e78e15a84b3153f5 by Abseil Team : Internal change PiperOrigin-RevId: 276126962 -- 4c840cab6a8d86efa29b397cafaf7520eece68cc by Andy Soffer : Update CMakeLists.txt to address https://github.com/abseil/abseil-cpp/issues/365. This does not cover every platform, but it does at least address the first-order issue of assuming gcc implies x86. PiperOrigin-RevId: 276116253 -- 98da366e6b5d51afe5d7ac6722126aca23d85ee6 by Abseil Team : Internal change PiperOrigin-RevId: 276097452 GitOrigin-RevId: e54b9c7bbb0c58475676c268e2e19c69f4bce48a Change-Id: I02d84454bb71ab21ad3d39650acf6cc6e36f58d7 --- absl/base/internal/exponential_biased.cc | 84 ++++++++++++++++++++++++++++++++ 1 file changed, 84 insertions(+) create mode 100644 absl/base/internal/exponential_biased.cc (limited to 'absl/base/internal/exponential_biased.cc') diff --git a/absl/base/internal/exponential_biased.cc b/absl/base/internal/exponential_biased.cc new file mode 100644 index 000000000000..d7ffd184e968 --- /dev/null +++ b/absl/base/internal/exponential_biased.cc @@ -0,0 +1,84 @@ +// Copyright 2019 The Abseil Authors. +// +// Licensed under the Apache License, Version 2.0 (the "License"); +// you may not use this file except in compliance with the License. +// You may obtain a copy of the License at +// +// https://www.apache.org/licenses/LICENSE-2.0 +// +// Unless required by applicable law or agreed to in writing, software +// distributed under the License is distributed on an "AS IS" BASIS, +// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. +// See the License for the specific language governing permissions and +// limitations under the License. + +#include "absl/base/internal/exponential_biased.h" + +#include + +#include +#include +#include + +#include "absl/base/attributes.h" +#include "absl/base/optimization.h" + +namespace absl { +namespace base_internal { + +// The algorithm generates a random number between 0 and 1 and applies the +// inverse cumulative distribution function for an exponential. Specifically: +// Let m be the inverse of the sample period, then the probability +// distribution function is m*exp(-mx) so the CDF is +// p = 1 - exp(-mx), so +// q = 1 - p = exp(-mx) +// log_e(q) = -mx +// -log_e(q)/m = x +// log_2(q) * (-log_e(2) * 1/m) = x +// In the code, q is actually in the range 1 to 2**26, hence the -26 below +int64_t ExponentialBiased::Get(int64_t mean) { + if (ABSL_PREDICT_FALSE(!initialized_)) { + Initialize(); + } + + uint64_t rng = NextRandom(rng_); + rng_ = rng; + + // Take the top 26 bits as the random number + // (This plus the 1<<58 sampling bound give a max possible step of + // 5194297183973780480 bytes.) + // The uint32_t cast is to prevent a (hard-to-reproduce) NAN + // under piii debug for some binaries. + double q = static_cast(rng >> (kPrngNumBits - 26)) + 1.0; + // Put the computed p-value through the CDF of a geometric. + double interval = (std::log2(q) - 26) * (-std::log(2.0) * mean); + // Very large values of interval overflow int64_t. To avoid that, we will cheat + // and clamp any huge values to (int64_t max)/2. This is a potential source of + // bias, but the mean would need to be such a large value that it's not likely + // to come up. For example, with a mean of 1e18, the probability of hitting + // this condition is about 1/1000. For a mean of 1e17, standard calculators + // claim that this event won't happen. + if (interval > static_cast(std::numeric_limits::max() / 2)) { + return std::numeric_limits::max() / 2; + } + + return static_cast(interval); +} + +void ExponentialBiased::Initialize() { + // We don't get well distributed numbers from `this` so we call NextRandom() a + // bunch to mush the bits around. We use a global_rand to handle the case + // where the same thread (by memory address) gets created and destroyed + // repeatedly. + ABSL_CONST_INIT static std::atomic global_rand(0); + uint64_t r = reinterpret_cast(this) + + global_rand.fetch_add(1, std::memory_order_relaxed); + for (int i = 0; i < 20; ++i) { + r = NextRandom(r); + } + rng_ = r; + initialized_ = true; +} + +} // namespace base_internal +} // namespace absl -- cgit 1.4.1