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

State is not a standard-layout type, so offsetof(State, skipped_) is only
conditionally-supported. Compilers diagnose it, and the static_assert was
kept quiet by a stack of vendor pragmas for GCC, clang, ICC, NVCC and NVHPC.
That stack does not cover every compiler: #1877 is clang-18/19 on Windows
reporting it anyway.

Measure the offset on the object being constructed instead. That is well
defined, needs no pragmas, and expresses the same invariant.

The check uses a new BM_CHECK_ALWAYS, which is BM_CHECK without the NDEBUG
opt-out, so it still holds in a release build. A State is constructed once
per benchmark instance and thread, never inside the iteration loop.

Verified: with the bound lowered to 8 bytes, a release build fails at run
time with

  src/benchmark.cc:231: State: Check `... <= 8 - ...' failed.
  the commonly accessed members of State must fit in the first cache line

and src/benchmark.cc compiles clean under -Winvalid-offsetof -Wall -Wextra
-Werror with both gcc 15 and clang 20.

Co-authored-by: dominic <510002+dmah42@users.noreply.github.com>

src/benchmark.cc

diff --git a/src/benchmark.cc b/src/benchmark.cc
index 22a395d..9388e34 100644
--- a/src/benchmark.cc
+++ b/src/benchmark.cc
@@ -42,6 +42,7 @@
 #include <algorithm>
 #include <atomic>
 #include <condition_variable>
+#include <cstddef>
 #include <cstdio>
 #include <cstdlib>
 #include <cstring>
@@ -222,42 +223,17 @@
     }
   }
 
-  // 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() {

src/check.h

diff --git a/src/check.h b/src/check.h
index aa8c78c..b6fc22f 100644
--- a/src/check.h
+++ b/src/check.h
@@ -79,15 +79,20 @@
 }  // 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