lenary added a comment. In D71124#1792216 <https://reviews.llvm.org/D71124#1792216>, @khchen wrote:
> The problem is how `-mcpu` interact with explicitly specified `-march` (or > target features). > > 1. in GCC, -mcpu is only used to chose the pipeline model, I think you mean "in GCC, `-mtune` is only used to choose the pipeline model" (`-mcpu` is not documented in the RISC-V specific GCC options documentation <https://gcc.gnu.org/onlinedocs/gcc/RISC-V-Options.html>). Clang should attempt to maintain compatibility with GCC flags, but if they only implement `-mtune`, then we have a little more freedom to do something ergonomic with `-mcpu`. I'll note that clang already has a large TODO around implementing `-mtune` in general, though the AArch64 backend seems to support it for some option choices. > > > 2. I also read this > <https://community.arm.com/developer/tools-software/tools/b/tools-software-ides-blog/posts/compiler-flags-across-architectures-march-mtune-and-mcpu> > article talking about the X86 and ARM to handle those options. > - -march=X: Tells the compiler that X is the minimal architecture the > binary must run on. The compiler is free to use architecture-specific > instructions. This flag behaves differently on Arm and x86. On Arm, -march > does not override -mtune, but on x86 -march will override both -mtune and > -mcpu. > - -mtune=X: Tells the compiler to optimize for microarchitecture X but does > not allow the compiler to change the ABI or make assumptions about available > instructions. This flag has the more-or-less the same meaning on Arm and x86. > - -mcpu=X: On Arm, this flag is a combination of -march and -mtune. It > simultaneously specifies the target architecture and optimizes for a given > microarchitecture. On x86 this flag is a deprecated synonym for -mtune. > > So maybe it makes sense to treat those flags behaves differently on > different target . > 3. I also tried llc to specific -mcpu and -attr (similar to -march, > target-feature) in ARM, -attr will over write the default target-feature in > -mcpu. > > on RISC-V, in sometime (or most?) we have same pipeline model but support > different extension combination, I don't believe this to be correct. lowRISC's Ibex has a completely different pipeline model to rocket, and there are countless other RISC-V cores with different pipeline characteristics, including out-of-order pipeline implementations like BOOM. I don't think we can favour one particular scheduling model (beyond the generic ones we already default to). > so I think maybe distinguishing the purpose of -mcpu and -march and make them > with no interaction is a good idea. (behavior is equal to GCC) In LLVM, if you add `target-cpu` metadata to a function (which is added by clang, based on `-mcpu`), that function will have all the features of that CPU automatically added to it (as if you had used `-mattr` with all the features in the model). If you don't add that metadata, a generic scheduling model will be chosen. This suggests at the moment there can be no separation between `-mtune` and `-march` as there is in GCC (without changes to the target-independent parts of LLVM). Repository: rG LLVM Github Monorepo CHANGES SINCE LAST ACTION https://reviews.llvm.org/D71124/new/ https://reviews.llvm.org/D71124 _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
