ldionne wrote: > > > > For SystemZ the correct value is 256. > > > > > > > > > Thanks! Double-checking: for both constructive and destructive? > > > > > > Yes, for both. Every SystemZ model (supported by GCC/LLVM) has a L1 cache > > line size of 256 bytes. > > Excellent, thank you! > > > > > In general I agree it makes sense to look at the GCC implementation as > > > > a source of reasonable values. Also, I think there probably should be > > > > no generic default value at all - it there is no platform-specific > > > > value known, it seems better to not define those values rather than > > > > define them to some incorrect value ... > > > > > > > > > On the one hand, I agree. But then libc++ will still have to pick default > > > values to expose (these are `constexpr size_t` variables in libc++ rather > > > than macros or `std::optional` values), so that just moves the problem > > > elsewhere. Also, as best I can tell, that doesn't seem to be how GCC > > > behaves (at least, from trying various GCC flavors on Compiler Explorer). > > > > > > Looks like GCC's libstdc++ will simply not provide those variables either > > if GCC does not know those values for the current target: > > ``` > > #ifdef __cpp_lib_hardware_interference_size // C++ >= 17 && > > defined(gcc_dest_sz) > > inline constexpr size_t hardware_destructive_interference_size = > > __GCC_DESTRUCTIVE_SIZE; > > inline constexpr size_t hardware_constructive_interference_size = > > __GCC_CONSTRUCTIVE_SIZE; > > #endif // __cpp_lib_hardware_interference_size > > ``` > > @ldionne do you have a preference? I can always skip defining the predefined > macros unless a target has opted into providing that information.
Not defining these variables when we don't have a good value for them would make us non-conforming, so I agree with @cor3ntin : I think we should define these macros all the time and simply document that it's a best effort thing. This whole feature is a best effort thing anyways. > I think it makes sense to go with this patch and let target maintainers tweak > the value subsequently. It seems that trying to not provide a value unless > explicitly specified by the target would be worse for library vendors as > `hardware_destructive_interference_size` is specified to exist > unconditionally. Exactly. It's arguable whether this was a good decision on WG21's end or not, but it was specified as provided unconditionally by a conforming implementation, therefore that's what we need to do (IMO). https://github.com/llvm/llvm-project/pull/89446 _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
