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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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48 changes: 12 additions & 36 deletions src/benchmark.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -42,6 +42,7 @@
#include <algorithm>
#include <atomic>
#include <condition_variable>
#include <cstddef>
#include <cstdio>
#include <cstdlib>
#include <cstring>
Expand Down Expand Up @@ -222,42 +223,17 @@ 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. State is not a
// standard-layout type, so the offset is measured on the live object.
//
// The check is enabled under NDEBUG as well: a State is constructed once per
// benchmark instance and thread, never inside the iteration loop.
BM_CHECK_ALWAYS(reinterpret_cast<const char*>(&skipped_) -
reinterpret_cast<const char*>(this) <=
/*cache_line_size=*/64 -
static_cast<std::ptrdiff_t>(sizeof(skipped_)))
<< "the commonly accessed members of State must fit in the first cache "
"line";
}

void State::PauseTiming() {
Expand Down
13 changes: 9 additions & 4 deletions src/check.h
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Original file line number Diff line number Diff line change
Expand Up @@ -79,15 +79,20 @@ class CheckHandler {
} // end namespace internal
} // end namespace benchmark

// The BM_CHECK macro returns a std::ostream object that can have extra
// information written to it.
#ifndef NDEBUG
#define BM_CHECK(b) \
// Like BM_CHECK, but enabled in every build configuration. Reserved for
// invariants that are cheap enough to verify unconditionally and that must not
// be violated silently in a release build.
#define BM_CHECK_ALWAYS(b) \
(b ? ::benchmark::internal::GetNullLogInstance() \
: ::benchmark::internal::CheckHandler( \
std::string_view(#b), std::string_view(__FILE__), \
std::string_view(__func__), __LINE__) \
.GetLog())

// The BM_CHECK macro returns a std::ostream object that can have extra
// information written to it.
#ifndef NDEBUG
#define BM_CHECK(b) BM_CHECK_ALWAYS(b)
#else
#define BM_CHECK(b) ::benchmark::internal::GetNullLogInstance()
#endif
Expand Down