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authorGravatar Abseil Team <absl-team@google.com>2019-10-23 19:35:39 -0700
committerGravatar Derek Mauro <dmauro@google.com>2019-10-24 10:29:13 -0400
commit078b89b3c046d230ef3ad39494e5852184eb528b (patch)
tree90368e0b398c3e43196c823504b88ab78a8408b3 /absl/base/internal/exponential_biased.cc
parent19b021cb3ff23048dfbe236a4e611925d8930831 (diff)
Export of internal Abseil changes
-- e54b9c7bbb0c58475676c268e2e19c69f4bce48a by Jorg Brown <jorg@google.com>: 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 <absl-team@google.com>: 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 <absl-team@google.com>: Fix typo in flat_hash_map.h: exchaged -> exchanged PiperOrigin-RevId: 276295792 -- 7bbd8d18276eb110c8335743e35fceb662ddf3d6 by Samuel Benzaquen <sbenza@google.com>: Add assertions to verify use of iterators. PiperOrigin-RevId: 276283300 -- 677398a8ffcb1f59182cffe57a4fe7ff147a0404 by Laramie Leavitt <lar@google.com>: Migrate distribution_impl.h/cc to generate_real.h/cc. Combine the methods RandU64To<Float,Double> into a single method: GenerateRealFromBits(). Remove rejection sampling from absl::uniform_real_distribution. PiperOrigin-RevId: 276158675 -- c60c9d11d24b0c546329d998e78e15a84b3153f5 by Abseil Team <absl-team@google.com>: Internal change PiperOrigin-RevId: 276126962 -- 4c840cab6a8d86efa29b397cafaf7520eece68cc by Andy Soffer <asoffer@google.com>: 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 <absl-team@google.com>: Internal change PiperOrigin-RevId: 276097452 GitOrigin-RevId: e54b9c7bbb0c58475676c268e2e19c69f4bce48a Change-Id: I02d84454bb71ab21ad3d39650acf6cc6e36f58d7
Diffstat (limited to 'absl/base/internal/exponential_biased.cc')
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diff --git a/absl/base/internal/exponential_biased.cc b/absl/base/internal/exponential_biased.cc
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+// 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 <stdint.h>
+
+#include <atomic>
+#include <cmath>
+#include <limits>
+
+#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<uint32_t>(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<double>(std::numeric_limits<int64_t>::max() / 2)) {
+ return std::numeric_limits<int64_t>::max() / 2;
+ }
+
+ return static_cast<int64_t>(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<uint32_t> global_rand(0);
+ uint64_t r = reinterpret_cast<uint64_t>(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