[RISCV] Allow YAML file to control multilib selection (llvm#98856)

This changes the bare-metal driver logic such that it _always_ tries
multilib.yaml if it exists, and it falls back to the hardwired/default
RISC-V multilib selection only if a multilib.yaml doesn't exist. In
contrast, the current behavior is that RISC-V can never use
multilib.yaml, but other targets will try it if it exists.

The flags `-march=` and `-mabi=` are exposed for multilib.yaml to match
on. There is no attempt to help YAML file creators to duplicate the
existing hard-wired multilib reuse logic -- they will have to implement
it using `Mappings`.

This should be backwards-compatible with existing sysroots, as
multilib.yaml was previously never used for RISC-V, and the behavior
doesn't change after this PR if the file doesn't exist.

Author: ArcaneNibble

clang/lib/Driver/ToolChain.cpp

@@ -9,6 +9,7 @@
 #include "clang/Driver/ToolChain.h"
 #include "ToolChains/Arch/AArch64.h"
 #include "ToolChains/Arch/ARM.h"
+#include "ToolChains/Arch/RISCV.h"
 #include "ToolChains/Clang.h"
 #include "ToolChains/CommonArgs.h"
 #include "ToolChains/Flang.h"
@@ -44,6 +45,7 @@
 #include "llvm/Support/VersionTuple.h"
 #include "llvm/Support/VirtualFileSystem.h"
 #include "llvm/TargetParser/AArch64TargetParser.h"
+#include "llvm/TargetParser/RISCVISAInfo.h"
 #include "llvm/TargetParser/TargetParser.h"
 #include "llvm/TargetParser/Triple.h"
 #include <cassert>
@@ -268,6 +270,19 @@ static void getARMMultilibFlags(const Driver &D,
   }
 }
 
+static void getRISCVMultilibFlags(const Driver &D, const llvm::Triple &Triple,
+                                  const llvm::opt::ArgList &Args,
+                                  Multilib::flags_list &Result) {
+  std::string Arch = riscv::getRISCVArch(Args, Triple);
+  // Canonicalize arch for easier matching
+  auto ISAInfo = llvm::RISCVISAInfo::parseArchString(
+      Arch, /*EnableExperimentalExtensions*/ true);
+  if (!llvm::errorToBool(ISAInfo.takeError()))
+    Result.push_back("-march=" + (*ISAInfo)->toString());
+
+  Result.push_back(("-mabi=" + riscv::getRISCVABI(Args, Triple)).str());
+}
+
 Multilib::flags_list
 ToolChain::getMultilibFlags(const llvm::opt::ArgList &Args) const {
   using namespace clang::driver::options;
@@ -288,6 +303,10 @@ ToolChain::getMultilibFlags(const llvm::opt::ArgList &Args) const {
   case llvm::Triple::thumbeb:
     getARMMultilibFlags(D, Triple, Args, Result);
     break;
+  case llvm::Triple::riscv32:
+  case llvm::Triple::riscv64:
+    getRISCVMultilibFlags(D, Triple, Args, Result);
+    break;
   default:
     break;
   }

clang/lib/Driver/ToolChains/BareMetal.cpp

@@ -218,17 +218,19 @@ static std::string computeBaseSysRoot(const Driver &D,
 void BareMetal::findMultilibs(const Driver &D, const llvm::Triple &Triple,
                               const ArgList &Args) {
   DetectedMultilibs Result;
-  if (isRISCVBareMetal(Triple)) {
+  // Look for a multilib.yaml before trying target-specific hardwired logic.
+  // If it exists, always do what it specifies.
+  llvm::SmallString<128> MultilibPath(computeBaseSysRoot(D, Triple));
+  llvm::sys::path::append(MultilibPath, MultilibFilename);
+  if (D.getVFS().exists(MultilibPath)) {
+    findMultilibsFromYAML(*this, D, MultilibPath, Args, Result);
+    SelectedMultilibs = Result.SelectedMultilibs;
+    Multilibs = Result.Multilibs;
+  } else if (isRISCVBareMetal(Triple)) {
     if (findRISCVMultilibs(D, Triple, Args, Result)) {
       SelectedMultilibs = Result.SelectedMultilibs;
       Multilibs = Result.Multilibs;
     }
-  } else {
-    llvm::SmallString<128> MultilibPath(computeBaseSysRoot(D, Triple));
-    llvm::sys::path::append(MultilibPath, MultilibFilename);
-    findMultilibsFromYAML(*this, D, MultilibPath, Args, Result);
-    SelectedMultilibs = Result.SelectedMultilibs;
-    Multilibs = Result.Multilibs;
   }
 }
 

clang/test/Driver/print-multi-selection-flags.c

@@ -58,3 +58,18 @@
 // RUN: %clang -print-multi-flags-experimental --target=aarch64-none-elf -march=armv9-a            | FileCheck --check-prefix=CHECK-SVE2 %s
 // CHECK-SVE2: --target=aarch64-unknown-none-elf
 // CHECK-SVE2: -march=armv{{.*}}-a{{.*}}+simd{{.*}}+sve{{.*}}+sve2{{.*}}
+
+// RUN: %clang -print-multi-flags-experimental --target=riscv32-none-elf -march=rv32g | FileCheck --check-prefix=CHECK-RV32 %s
+// CHECK-RV32: --target=riscv32-unknown-none-elf
+// CHECK-RV32: -mabi=ilp32d
+// CHECK-RV32: -march=rv32i{{[0-9]+p[0-9]+}}_m{{[0-9]+p[0-9]+}}_a{{[0-9]+p[0-9]+}}_f{{[0-9]+p[0-9]+}}_d{{[0-9]+p[0-9]+}}_zicsr{{[0-9]+p[0-9]+}}_zifencei{{[0-9]+p[0-9]+}}_zmmul{{[0-9]+p[0-9]+}}
+
+// RUN: %clang -print-multi-flags-experimental --target=riscv64-none-elf -march=rv64g | FileCheck --check-prefix=CHECK-RV64 %s
+// CHECK-RV64: --target=riscv64-unknown-none-elf
+// CHECK-RV64: -mabi=lp64d
+// CHECK-RV64: -march=rv64i{{[0-9]+p[0-9]+}}_m{{[0-9]+p[0-9]+}}_a{{[0-9]+p[0-9]+}}_f{{[0-9]+p[0-9]+}}_d{{[0-9]+p[0-9]+}}_zicsr{{[0-9]+p[0-9]+}}_zifencei{{[0-9]+p[0-9]+}}_zmmul{{[0-9]+p[0-9]+}}
+
+// RUN: %clang -print-multi-flags-experimental --target=riscv32-none-elf -march=rv32e_zicsr_c | FileCheck --check-prefix=CHECK-RV32E-ORDER %s
+// CHECK-RV32E-ORDER: --target=riscv32-unknown-none-elf
+// CHECK-RV32E-ORDER: -mabi=ilp32e
+// CHECK-RV32E-ORDER: -march=rv32e{{[0-9]+p[0-9]+}}_c{{[0-9]+p[0-9]+}}_zicsr{{[0-9]+p[0-9]+}}