diff options
author | 2019-10-23 19:35:39 -0700 | |
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committer | 2019-10-24 10:29:13 -0400 | |
commit | 078b89b3c046d230ef3ad39494e5852184eb528b (patch) | |
tree | 90368e0b398c3e43196c823504b88ab78a8408b3 /absl/base/internal/exponential_biased.cc | |
parent | 19b021cb3ff23048dfbe236a4e611925d8930831 (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')
-rw-r--r-- | absl/base/internal/exponential_biased.cc | 84 |
1 files changed, 84 insertions, 0 deletions
diff --git a/absl/base/internal/exponential_biased.cc b/absl/base/internal/exponential_biased.cc new file mode 100644 index 00000000..d7ffd184 --- /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 <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 |