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Replace offsetof cache-line check with a well-defined runtime check (#1877) #2264

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Replace offsetof cache-line check with a well-defined runtime check (#1877) #2264

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49 changes: 13 additions & 36 deletions src/benchmark.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -222,42 +222,19 @@ State::State(std::string name, IterationCount max_iters,
}
}

// Note: The use of offsetof below is technically undefined until C++17
// because State is not a standard layout type. However, all compilers
// currently provide well-defined behavior as an extension (which is
// demonstrated since constexpr evaluation must diagnose all undefined
// behavior). However, GCC and Clang also warn about this use of offsetof,
// which must be suppressed.
#if defined(__INTEL_COMPILER)
#pragma warning push
#pragma warning(disable : 1875)
#elif defined(__GNUC__) || defined(__clang__)
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Winvalid-offsetof"
#endif
#if defined(__NVCC__)
#pragma nv_diagnostic push
#pragma nv_diag_suppress 1427
#endif
#if defined(__NVCOMPILER)
#pragma diagnostic push
#pragma diag_suppress offset_in_non_POD_nonstandard
#endif
// Offset tests to ensure commonly accessed data is on the first cache line.
const int cache_line_size = 64;
static_assert(
offsetof(State, skipped_) <= (cache_line_size - sizeof(skipped_)), "");
#if defined(__INTEL_COMPILER)
#pragma warning pop
#elif defined(__GNUC__) || defined(__clang__)
#pragma GCC diagnostic pop
#endif
#if defined(__NVCC__)
#pragma nv_diagnostic pop
#endif
#if defined(__NVCOMPILER)
#pragma diagnostic pop
#endif
// Ensure commonly accessed data is on the first cache line.
//
// We deliberately avoid offsetof() here: State is not a standard-layout type
// (it has non-static data members with differing access control), so
// offsetof() on it is undefined behavior and is diagnosed by
// -Winvalid-offsetof (and equivalents on other compilers), which previously
// had to be suppressed with a stack of compiler-specific pragmas. Instead we
// compute the offset from the live object, which is well defined. This runs
// only in debug builds, as BM_CHECK is a no-op under NDEBUG.
BM_CHECK_LE(reinterpret_cast<const char*>(&skipped_) -
reinterpret_cast<const char*>(this),
/*cache_line_size=*/64 -
static_cast<std::ptrdiff_t>(sizeof(skipped_)));
}

void State::PauseTiming() {
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