[PATCH] D140693: [Driver][RISCV] Adjust the priority between -mcpu, -mtune and -march

[Kito Cheng via Phabricator via cfe-commits]

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RISC-V supports `-march`, `-mtune`, and `-mcpu`: `-march` provides the
architecture extension information, `-mtune` provide the pipeline model, and
`-mcpu` provides both.

What's the priority among those options for now(w/o this patch)?

Pipeline model:

- Take from `-mtune` if present.
- Take from `-mcpu` if present
- Use the default pipeline model: `generic-rv32` or `generic-rv64`

Architecture extension:

- Take from `-mcpu` if present.
- Take from `-march` if present.
- Use the default architecture depending on the target triple

We treat `-mcpu`/`-mtune` and `-mcpu`/`-march` differently, and it's
kind of counterintuitive: -march is explicitly specified but ignored.

This patch adjusts the priority between `-mcpu`/`-march`, letting it use
architecture extension information from `-march` if it's present.

So the priority of architecture extension information becomes:

- Take from `-march` if present.
- Take from `-mcpu` if present.
- Use the default architecture depending on the target triple

And this also match what we implement in RISC-V GCC too.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D140693

Files:
  clang/lib/Driver/ToolChains/Arch/RISCV.cpp
  clang/lib/Driver/ToolChains/Arch/RISCV.h
  clang/lib/Driver/ToolChains/Clang.cpp
  clang/test/Driver/riscv-cpus.c
  clang/test/Driver/riscv-march-mcpu-mtune.c

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