diff options
Diffstat (limited to 'absl/base/internal/periodic_sampler.h')
| -rw-r--r-- | absl/base/internal/periodic_sampler.h | 62 |
1 files changed, 39 insertions, 23 deletions
diff --git a/absl/base/internal/periodic_sampler.h b/absl/base/internal/periodic_sampler.h index 2c0600f05ea4..238797424fc0 100644 --- a/absl/base/internal/periodic_sampler.h +++ b/absl/base/internal/periodic_sampler.h @@ -111,33 +111,49 @@ class PeriodicSamplerBase { // Returns the current period of this sampler. Thread-safe. virtual int period() const noexcept = 0; - int64_t stride_ = 0; + // Keep and decrement stride_ as an unsigned integer, but compare the value + // to zero casted as a signed int. clang and msvc do not create optimum code + // if we use signed for the combined decrement and sign comparison. + // + // Below 3 alternative options, all compiles generate the best code + // using the unsigned increment <---> signed int comparison option. + // + // Option 1: + // int64_t stride_; + // if (ABSL_PREDICT_TRUE(++stride_ < 0)) { ... } + // + // GCC x64 (OK) : https://gcc.godbolt.org/z/R5MzzA + // GCC ppc (OK) : https://gcc.godbolt.org/z/z7NZAt + // Clang x64 (BAD): https://gcc.godbolt.org/z/t4gPsd + // ICC x64 (OK) : https://gcc.godbolt.org/z/rE6s8W + // MSVC x64 (OK) : https://gcc.godbolt.org/z/ARMXqS + // + // Option 2: + // int64_t stride_ = 0; + // if (ABSL_PREDICT_TRUE(--stride_ >= 0)) { ... } + // + // GCC x64 (OK) : https://gcc.godbolt.org/z/jSQxYK + // GCC ppc (OK) : https://gcc.godbolt.org/z/VJdYaA + // Clang x64 (BAD): https://gcc.godbolt.org/z/Xm4NjX + // ICC x64 (OK) : https://gcc.godbolt.org/z/4snaFd + // MSVC x64 (BAD): https://gcc.godbolt.org/z/BgnEKE + // + // Option 3: + // uint64_t stride_; + // if (ABSL_PREDICT_TRUE(static_cast<int64_t>(++stride_) < 0)) { ... } + // + // GCC x64 (OK) : https://gcc.godbolt.org/z/bFbfPy + // GCC ppc (OK) : https://gcc.godbolt.org/z/S9KkUE + // Clang x64 (OK) : https://gcc.godbolt.org/z/UYzRb4 + // ICC x64 (OK) : https://gcc.godbolt.org/z/ptTNfD + // MSVC x64 (OK) : https://gcc.godbolt.org/z/76j4-5 + uint64_t stride_ = 0; ExponentialBiased rng_; }; inline bool PeriodicSamplerBase::SubtleMaybeSample() noexcept { - // We explicitly count up and not down, as the compiler does not generate - // ideal code for counting down. See also https://gcc.godbolt.org/z/FTPDSM - // - // With `if (ABSL_PREDICT_FALSE(++stride_ < 0))` - // add QWORD PTR fs:sampler@tpoff+8, 1 - // jns .L15 - // ret - // - // With `if (ABSL_PREDICT_FALSE(--stride_ > 0))` - // mov rax, QWORD PTR fs:sampler@tpoff+8 - // sub rax, 1 - // mov QWORD PTR fs:sampler@tpoff+8, rax - // test rax, rax - // jle .L15 - // ret - // add QWORD PTR fs:sampler@tpoff+8, 1 - // jns .L15 - // ret - // - // --stride >= 0 does work, but makes our logic slightly harder as in that - // case we have less convenient zero-init and over-run values. - if (ABSL_PREDICT_FALSE(++stride_ < 0)) { + // See comments on `stride_` for the unsigned increment / signed compare. + if (ABSL_PREDICT_TRUE(static_cast<int64_t>(++stride_) < 0)) { return false; } return true; |