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Export of internal Abseil changes

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--
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

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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

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b36a664e9459057509a90e83d3482e1d3a4c44c7 by Abseil Team <absl-team@google.com>:

Fix typo in flat_hash_map.h: exchaged -> exchanged

PiperOrigin-RevId: 276295792

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7bbd8d18276eb110c8335743e35fceb662ddf3d6 by Samuel Benzaquen <sbenza@google.com>:

Add assertions to verify use of iterators.

PiperOrigin-RevId: 276283300

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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

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c60c9d11d24b0c546329d998e78e15a84b3153f5 by Abseil Team <absl-team@google.com>:

Internal change

PiperOrigin-RevId: 276126962

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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

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98da366e6b5d51afe5d7ac6722126aca23d85ee6 by Abseil Team <absl-team@google.com>:

Internal change

PiperOrigin-RevId: 276097452
GitOrigin-RevId: e54b9c7bbb0c58475676c268e2e19c69f4bce48a
Change-Id: I02d84454bb71ab21ad3d39650acf6cc6e36f58d7
This commit is contained in:
Abseil Team 2019-10-23 19:35:39 -07:00 • committed by Derek Mauro
parent 19b021cb3f
commit 078b89b3c0
28 changed files with 739 additions and 370 deletions
Download patch file Download diff file Expand all files Collapse all files

83
absl/container/internal/hashtablez_sampler.cc
View file

@ -21,6 +21,7 @@
#include <limits>
#include "absl/base/attributes.h"
#include "absl/base/internal/exponential_biased.h"
#include "absl/container/internal/have_sse.h"
#include "absl/debugging/stacktrace.h"
#include "absl/memory/memory.h"
@ -37,77 +38,13 @@ ABSL_CONST_INIT std::atomic<bool> g_hashtablez_enabled{
ABSL_CONST_INIT std::atomic<int32_t> g_hashtablez_sample_parameter{1 << 10};
ABSL_CONST_INIT std::atomic<int32_t> g_hashtablez_max_samples{1 << 20};
// Returns the next pseudo-random value.
// pRNG is: aX+b mod c with a = 0x5DEECE66D, b = 0xB, c = 1<<48
// This is the lrand64 generator.
uint64_t NextRandom(uint64_t rnd) {
const uint64_t prng_mult = uint64_t{0x5DEECE66D};
const uint64_t prng_add = 0xB;
const uint64_t prng_mod_power = 48;
const uint64_t prng_mod_mask = ~(~uint64_t{0} << prng_mod_power);
return (prng_mult * rnd + prng_add) & prng_mod_mask;
}
// Generates a geometric variable with the specified mean.
// This is done by generating a random number between 0 and 1 and applying
// 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 GetGeometricVariable(int64_t mean) {
#if ABSL_HAVE_THREAD_LOCAL
thread_local
#else // ABSL_HAVE_THREAD_LOCAL
// SampleSlow and hence GetGeometricVariable is guarded by a single mutex when
// there are not thread locals. Thus, a single global rng is acceptable for
// that case.
static
#endif // ABSL_HAVE_THREAD_LOCAL
uint64_t rng = []() {
// 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>(&rng) +
global_rand.fetch_add(1, std::memory_order_relaxed);
for (int i = 0; i < 20; ++i) {
r = NextRandom(r);
}
return r;
}();
rng = NextRandom(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.)
const uint64_t prng_mod_power = 48; // Number of bits in prng
// 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 >> (prng_mod_power - 26)) + 1.0;
// Put the computed p-value through the CDF of a geometric.
double interval = (log2(q) - 26) * (-std::log(2.0) * mean);
// Very large values of interval overflow int64_t. If we happen to
// hit such improbable condition, we simply cheat and clamp interval
// to largest supported value.
if (interval > static_cast<double>(std::numeric_limits<int64_t>::max() / 2)) {
return std::numeric_limits<int64_t>::max() / 2;
}
// Small values of interval are equivalent to just sampling next time.
if (interval < 1) {
return 1;
}
return static_cast<int64_t>(interval);
}
thread_local absl::base_internal::ExponentialBiased
g_exponential_biased_generator;
#else
ABSL_CONST_INIT static absl::base_internal::ExponentialBiased
g_exponential_biased_generator;
#endif
} // namespace
@ -253,8 +190,12 @@ HashtablezInfo* SampleSlow(int64_t* next_sample) {
}
bool first = *next_sample < 0;
*next_sample = GetGeometricVariable(
*next_sample = g_exponential_biased_generator.Get(
g_hashtablez_sample_parameter.load(std::memory_order_relaxed));
// Small values of interval are equivalent to just sampling next time.
if (*next_sample < 1) {
*next_sample = 1;
}
// g_hashtablez_enabled can be dynamically flipped, we need to set a threshold
// low enough that we will start sampling in a reasonable time, so we just use