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

[via cfe-commits]

https://github.com/ArcaneNibble created https://github.com/llvm/llvm-project/pull/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.

Some actual use-cases that I want this change for:
* `-fno-exceptions` and `-fno-rtti` multilibs for C++ runtimes on small microcontrollers
* There's currently no support for rv32e multilibs in the hard-wired logic
* I want multilibs where vendor extensions such as the QingKe "XW" compressed opcodes are enabled/disabled. Permutations of features such as this can likely never be handled in hard-wired logic.

>From 628d665dc3acc3619511a4fc8bb807fabb520b94 Mon Sep 17 00:00:00 2001
From: R <rqou at berkeley.edu>
Date: Mon, 15 Jul 2024 04:13:07 +0100
Subject: [PATCH] [RISCV] Allow YAML file to control multilib selection

---
 clang/lib/Driver/ToolChain.cpp            | 12 ++++++++++++
 clang/lib/Driver/ToolChains/BareMetal.cpp | 16 +++++++++-------
 2 files changed, 21 insertions(+), 7 deletions(-)

diff --git a/clang/lib/Driver/ToolChain.cpp b/clang/lib/Driver/ToolChain.cpp
index 85ae4d2a26fee..7cd110a07bd98 100644
--- a/clang/lib/Driver/ToolChain.cpp
+++ b/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"
@@ -258,6 +259,13 @@ 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) {
+  Result.push_back("-march=" + riscv::getRISCVArch(Args, Triple));
+  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;
@@ -278,6 +286,10 @@ 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;
   }
diff --git a/clang/lib/Driver/ToolChains/BareMetal.cpp b/clang/lib/Driver/ToolChains/BareMetal.cpp
index 852e0442f50a2..95b4051cb74c7 100644
--- a/clang/lib/Driver/ToolChains/BareMetal.cpp
+++ b/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;
   }
 }