On Tue, 6 Oct 2020, Kito Cheng wrote: > I think this patch is kind of major change for GCC RISC-V port, so I cc all > RISC-V gcc maintainer to make sure this change is fine with you guys. > > - Motivation of this patch: > 1. Sync behavior between clang/llvm. > 2. Preparation for -mcpu option support, -mcpu will set -march > according the core default arch, however it would be awkward > if we only change arch: user need to know the default arch of > the core and then set the right ABI, of cause user still can > specify arch and abi via -march and -mabi. > > - This patch has change the behavior for default value of ABI, the ABI > will derive from -march if -mabi is not given, which is same behavior > as clang/llvm.
Just to warn you: it used to be the case with the MIPS target and the `-mips[1234]' ISA level options originating from SGI's MIPSpro toolchain and it has turned out confusing and troublesome. After many discussions we ended up with the current `-march='/`-mtune='/`-mabi=' scheme, for the instruction set, the DFA scheduling and the ABI respectively. Defaults are set with `--with-arch='/`--with-tune='/`--with-abi=' respectively, and in the absence of an override `-mtune=' is derived from `-march=', which is derived from `-mabi='. Defaults for different ABIs can be set with respective `--with-arch*=' and `--with-tune*=' options. This prevents the ABI from being changed unexpectedly, especially if different though link-compatible `-march=' options are used for individual objects in a compilation. The MIPS port used to have `-mcpu=' as well, which used to be roughly equivalent to modern `-mtune='; from your description I gather `-mcpu=' is going to be roughly equivalent to a combination of `-mtune=' and `-march=' setting DFA scheduling for a specific CPU and the instruction set to the underlying architecture (do we plan to allow vendor extensions?). In which case to compile a set of CPU-specific modules to be linked together (e.g. individual platform support in a generic piece of software like an OS kernel or a bare metal library) you'll always have to specify the ABI explicitly (though maybe you want anyway, hmm). FWIW, Maciej
