[Driver][BareMetal] Error if no matching multilib

[michaelplatings]

Summary

Previously if no matching multilib was found then the user would
typically see an error like "fatal error: 'stdio.h' file not found"
which gives no indication as to the underlying problem.
With this change the user will instead see an error like

clang: error: no multilib found matching flags: --target=thumbv7em-none-unknown-eabi -march=...
clang: note: available multilibs are:
--target=armv4t-none-unknown-eabi
--target=thumbv6m-none-unknown-eabi -mfpu=none
...

[simon_tatham]

LGTM.

One of the thoughts I mentioned offline before this patch was written was that maybe the error would need to be conditional, via a directive inside multilib.yaml itself – perhaps some users want to fall off the end of the multilib list without error and continue using the default sysroot, whereas others want to diagnose "sorry, no library at all is available to support your flags"? But I suppose there won't be a large user base who were already using this particular YAML technique in the former mode, or at all, because it's so new. And if necessary one could reorganise to put multilib.yaml in its own directory, and add a final catch-all entry pointing at the fallback sysroot. So it should be OK to have the error unconditional.

clang/test/Driver/baremetal-multilib.yaml

88–90

I'm not sure what the purpose of this addition is. It doesn't seem to be listed in the expected error messages (but it's presumably being printed anyway, and skipped over because the checks don't say -NEXT?). Did it sneak in here from another commit, perhaps?

[michaelplatings]

Thanks @simon_tatham. I agree there's definitely scope to improve this. The behaviour is quite specific so I kept the code in the BareMetal toolchain rather than as part of the core Multilib class. Other toolchains may require different behaviour as you say.

This change also doesn't address the layered multilib scenario. In that scenario multiple multilibs may be required to match to get a complete set of headers and libraries. But this change moves us in the right direction.

clang/test/Driver/baremetal-multilib.yaml

88–90

Sorry I forgot to explain this already. With the new error, CHECK-NO-EXECUTE-ONLY-ASM started failing because no multilib variant matched. I added this variant to fix that.

clang/include/clang/Basic/DiagnosticDriverKinds.td

 def err_drv_riscv_unsupported_with_linker_relaxation : Error<
   "%0 is unsupported with RISC-V linker relaxation (-mrelax)">;
 
 def err_drv_loongarch_invalid_mfpu_EQ : Error<
   "invalid argument '%0' to -mfpu=; must be one of: 64, 32, none, 0 (alias for none)">;
 
 def err_drv_expand_response_file : Error<
   "failed to expand response file: %0">;
+
+def err_drv_no_matching_multilib : Error<
+  "no multilib found matching flags: %0">;
+def note_drv_available_multilibs : Note<
+  "available multilibs are:%0">;
 }

clang/lib/Driver/ToolChains/BareMetal.cpp

 #include "clang/Driver/DriverDiagnostic.h"
 #include "clang/Driver/MultilibBuilder.h"
 #include "clang/Driver/Options.h"
 #include "llvm/Option/ArgList.h"
 #include "llvm/Support/Path.h"
 #include "llvm/Support/VirtualFileSystem.h"
 #include "llvm/Support/raw_ostream.h"
 
+#include <sstream>
+
 using namespace llvm::opt;
 using namespace clang;
 using namespace clang::driver;
 using namespace clang::driver::tools;
 using namespace clang::driver::toolchains;
 
 static bool findRISCVMultilibs(const Driver &D,
                                const llvm::Triple &TargetTriple,
if (Triple.getVendor() != llvm::Triple::UnknownVendor)
     return false;
 
   if (Triple.getOS() != llvm::Triple::UnknownOS)
     return false;
 
   return Triple.getEnvironmentName() == "elf";
 }
 
-static bool findMultilibsFromYAML(const ToolChain &TC, const Driver &D,
+static void findMultilibsFromYAML(const ToolChain &TC, const Driver &D,
                                   StringRef MultilibPath, const ArgList &Args,
                                   DetectedMultilibs &Result) {
   llvm::ErrorOr<std::unique_ptr<llvm::MemoryBuffer>> MB =
       D.getVFS().getBufferForFile(MultilibPath);
   if (!MB)
-    return false;
+    return;
   Multilib::flags_list Flags = TC.getMultilibFlags(Args);
   llvm::ErrorOr<MultilibSet> ErrorOrMultilibSet =
       MultilibSet::parseYaml(*MB.get());
   if (ErrorOrMultilibSet.getError())
-    return false;
+    return;
   Result.Multilibs = ErrorOrMultilibSet.get();
-  return Result.Multilibs.select(Flags, Result.SelectedMultilibs);
+  if (Result.Multilibs.select(Flags, Result.SelectedMultilibs))
+    return;
+  D.Diag(clang::diag::err_drv_no_matching_multilib) << llvm::join(Flags, " ");
+  std::stringstream ss;
+  for (const Multilib &Multilib : Result.Multilibs)
+    ss << "\n" << llvm::join(Multilib.flags(), " ");
+  D.Diag(clang::diag::note_drv_available_multilibs) << ss.str();
 }
 
 static constexpr llvm::StringLiteral MultilibFilename = "multilib.yaml";
 
 // Get the sysroot, before multilib takes effect.
 static std::string computeBaseSysRoot(const Driver &D,
                                       const llvm::Triple &Triple) {
   if (!D.SysRoot.empty())

clang/test/Driver/baremetal-multilib.yaml

 # CHECK-SAME: "-internal-isystem" "[[SYSROOT]]/bin/../lib/clang-runtimes/arm-none-eabi/thumb/v8-m.main/fp/include/c++/v1"
 # CHECK-SAME: "-internal-isystem" "[[SYSROOT]]/bin/../lib/clang-runtimes/arm-none-eabi/thumb/v8-m.main/fp/include"
 # CHECK-SAME: "-x" "c++" "{{.*}}baremetal-multilib.yaml"
 # CHECK-NEXT: ld{{(.exe)?}}" "{{.*}}.o" "-Bstatic"
 # CHECK-SAME: "-L[[SYSROOT]]/bin/../lib/clang-runtimes/arm-none-eabi/thumb/v8-m.main/fp/lib"
 # CHECK-SAME: "-lc" "-lm" "-lclang_rt.builtins"
 # CHECK-SAME: "-o" "{{.*}}.tmp.out"
 
+# RUN: %T/baremetal_multilib/bin/clang -no-canonical-prefixes -x c++ %s -### -o %t.out 2>&1 \
+# RUN:     --target=thumbv7em-none-eabi -mfpu=fpv4-sp-d16 --sysroot= \
+# RUN:   | FileCheck --check-prefix=CHECK-NO-MATCH %s
+# CHECK-NO-MATCH: error: no multilib found matching flags:
+# CHECK-NO-MATCH-SAME: --target=thumbv7em-none-unknown-eabi
+# CHECK-NO-MATCH: note: available multilibs are:
+# CHECK-NO-MATCH: --target=thumbv6m-none-unknown-eabi -mfpu=none
+# CHECK-NO-MATCH: --target=thumbv7m-none-unknown-eabi -mfpu=none
+# CHECK-NO-MATCH: --target=thumbv7em-none-unknown-eabi -mfpu=none
+
 # RUN: %T/baremetal_multilib/bin/clang -no-canonical-prefixes -print-multi-directory 2>&1 \
 # RUN:     --target=thumbv8m.main-none-eabihf --sysroot= \
 # RUN:   | FileCheck --check-prefix=CHECK-PRINT-MULTI-DIRECTORY %s
 # CHECK-PRINT-MULTI-DIRECTORY: arm-none-eabi/thumb/v8-m.main/fp
 
 # RUN: %T/baremetal_multilib/bin/clang -no-canonical-prefixes -print-multi-lib 2>&1 \
 # RUN:     --target=arm-none-eabi --sysroot= \
 # RUN:   | FileCheck --check-prefix=CHECK-PRINT-MULTI-LIB %s

 # The first section of the file is the list of library variants.
 # A library is considered compatible if the are a subset of the flags derived
 # from the arguments provided by the user.
 # If multiple libraries are deemed compatible then the one that appears
 # last in the list wins. A ToolChain may instead opt to use more than one
 # multilib, layered on top of each other.
 
 Variants:
+- Dir: arm-none-eabi/arm/v4t
+  Flags: [--target=armv4t-none-unknown-eabi]
+
 - Dir: arm-none-eabi/thumb/v6-m/nofp
   Flags: [--target=thumbv6m-none-unknown-eabi, -mfpu=none]
 
 - Dir: arm-none-eabi/thumb/v7-m/nofp
   Flags: [--target=thumbv7m-none-unknown-eabi, -mfpu=none]
 
 - Dir: arm-none-eabi/thumb/v7e-m/nofp
   Flags: [--target=thumbv7em-none-unknown-eabi, -mfpu=none]