[clang] [RISCV] Implement multi-lib reuse rule for RISC-V bare-metal toolchain (PR #73765)

[Brandon Wu via cfe-commits]

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@@ -1715,6 +1716,129 @@ static void findCSKYMultilibs(const Driver &D, const llvm::Triple &TargetTriple,
     Result.Multilibs = CSKYMultilibs;
 }
 
+/// Extend the multi-lib re-use selection mechanism for RISC-V.
+/// This funciton will try to re-use multi-lib if they are compatible.
+/// Definition of compatible:
+///   - ABI must be the same.
+///   - multi-lib is a subset of current arch, e.g. multi-lib=march=rv32im
+///     is a subset of march=rv32imc.
+///   - march that contains atomic extension can't reuse multi-lib that
+///     doesn't has atomic, vice versa. e.g. multi-lib=march=rv32im and
+///     march=rv32ima are not compatible, because software and hardware
+///     atomic operation can't work together correctly.
+static bool
+RISCVMultilibSelect(const MultilibSet &RISCVMultilibSet, StringRef Arch,
+                    const Multilib::flags_list &Flags,
+                    llvm::SmallVector<Multilib> &SelectedMultilibs) {
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4vtomat wrote:

Since `RISCVMultilibSet.select` uses `SmallVector` reference, so this function just aligned with it, otherwise, we need another `reinterpret_cast` when calling `RISCVMultilibSet.select` in line 1734.

https://github.com/llvm/llvm-project/pull/73765