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

[Yeting Kuo 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) {
+  // Try to find the perfect matching multi-lib first.
+  if (RISCVMultilibSet.select(Flags, SelectedMultilibs))
+    return true;
+
+  llvm::StringMap<bool> FlagSet;
+  Multilib::flags_list NewFlags;
+  std::vector<MultilibBuilder> NewMultilibs;
+
+  auto ParseResult = llvm::RISCVISAInfo::parseArchString(
+      Arch, /*EnableExperimentalExtension=*/true,
+      /*ExperimentalExtensionVersionCheck=*/false);
+  if (!ParseResult) {
+    // Ignore any error here, we assume it will handled in another place.
+    consumeError(ParseResult.takeError());
+    return false;
+  }
+  auto &ISAInfo = *ParseResult;
+
+  auto CurrentExts = ISAInfo->getExtensions();
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yetingk wrote:

Maybe use accurate type instead of `auto`. LLVM only uses `auto` if it makes the code more readable or easier to maintain.

Ref: https://llvm.org/docs/CodingStandards.html#use-auto-type-deduction-to-make-code-more-readable

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