libexpr: Replace hardcoded cache line size with std::hardware_destructive_interference_size

This expands to __GCC_DESTRUCTIVE_SIZE, which is also 64 (at least in the x86_64 stdenv).
Let the compiler decide what's the appropriate cache line size is. Also, on aarch64-darwin
the cache line size 128 bytes, so the previous fix didn't actually get rid of false sharing
reliably. Clang does this [1] [2], so it overestimates the sizes somewhat, but that's still enough
for avoiding false sharing on darwin.

[1]: https://github.com/llvm/llvm-project/blob/a289341dedf4d73955faefbb0b3c13881e13dd06/clang/lib/Frontend/InitPreprocessor.cpp#L1331-L1339
[2]: https://github.com/llvm/llvm-project/blob/6f51f8e0f93e6ff2baf0ed2f60021308920a557f/clang/lib/Basic/Targets/AArch64.h#L262-L264

nix-meson-build-support/common/meson.build

@@ -25,6 +25,7 @@ add_project_arguments(
   '-Wignored-qualifiers',
   '-Wimplicit-fallthrough',
   '-Wno-deprecated-declarations',
+  '-Wno-interference-size', # Used for C++ ABI only. We don't provide any guarantees about different march tunings.
   language : 'cpp',
 )
 

src/libexpr/eval.cc

@@ -318,7 +318,6 @@ EvalState::EvalState(
     countCalls = getEnv("NIX_COUNT_CALLS").value_or("0") != "0";
 
     static_assert(sizeof(Env) <= 16, "environment must be <= 16 bytes");
-    static_assert(sizeof(Counter) == 64, "counters must be 64 bytes");
 
     /* Construct the Nix expression search path. */
     assert(lookupPath.elements.empty());

src/libexpr/include/nix/expr/counter.hh

@@ -11,7 +11,7 @@ namespace nix {
  * variable is set. This is to prevent contention on these counters
  * when multi-threaded evaluation is enabled.
  */
-struct alignas(64) Counter
+struct alignas(std::hardware_destructive_interference_size) Counter
 {
     using value_type = uint64_t;