[PATCH] D71124: [RISCV] support clang driver to select cpu

[Sam Elliott via Phabricator via cfe-commits]

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).


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