Fix pr18235.

The cpp backend is not a reasonable fallback for a missing target. It is a
very special backend, so it is reasonable to use it only if explicitly
requested.

While at it, simplify the interface a bit.

llvm-svn: 197241

include/llvm/Support/TargetRegistry.h

@@ -79,7 +79,7 @@ namespace llvm {
   public:
     friend struct TargetRegistry;
 
-    typedef unsigned (*TripleMatchQualityFnTy)(const std::string &TT);
+    typedef bool (*ArchMatchFnTy)(Triple::ArchType Arch);
 
     typedef MCAsmInfo *(*MCAsmInfoCtorFnTy)(const MCRegisterInfo &MRI,
                                             StringRef TT);
@@ -154,9 +154,8 @@ namespace llvm {
     /// TargetRegistry.
     Target *Next;
 
-    /// TripleMatchQualityFn - The target function for rating the match quality
-    /// of a triple.
-    TripleMatchQualityFnTy TripleMatchQualityFn;
+    /// The target function for checking if an architecture is supported.
+    ArchMatchFnTy ArchMatchFn;
 
     /// Name - The target name.
     const char *Name;
@@ -578,14 +577,13 @@ namespace llvm {
     /// @param Name - The target name. This should be a static string.
     /// @param ShortDesc - A short target description. This should be a static
     /// string.
-    /// @param TQualityFn - The triple match quality computation function for
-    /// this target.
+    /// @param ArchMatchFn - The arch match checking function for this target.
     /// @param HasJIT - Whether the target supports JIT code
     /// generation.
     static void RegisterTarget(Target &T,
                                const char *Name,
                                const char *ShortDesc,
-                               Target::TripleMatchQualityFnTy TQualityFn,
+                               Target::ArchMatchFnTy ArchMatchFn,
                                bool HasJIT = false);
 
     /// RegisterMCAsmInfo - Register a MCAsmInfo implementation for the
@@ -831,15 +829,11 @@ namespace llvm {
            bool HasJIT = false>
   struct RegisterTarget {
     RegisterTarget(Target &T, const char *Name, const char *Desc) {
-      TargetRegistry::RegisterTarget(T, Name, Desc,
-                                     &getTripleMatchQuality,
-                                     HasJIT);
+      TargetRegistry::RegisterTarget(T, Name, Desc, &getArchMatch, HasJIT);
     }
 
-    static unsigned getTripleMatchQuality(const std::string &TT) {
-      if (Triple(TT).getArch() == TargetArchType)
-        return 20;
-      return 0;
+    static bool getArchMatch(Triple::ArchType Arch) {
+      return Arch == TargetArchType;
     }
   };
 

lib/Support/TargetRegistry.cpp

@@ -71,42 +71,34 @@ const Target *TargetRegistry::lookupTarget(const std::string &TT,
     Error = "Unable to find target for this triple (no targets are registered)";
     return 0;
   }
-  const Target *Best = 0, *EquallyBest = 0;
-  unsigned BestQuality = 0;
+  const Target *Matching = 0;
+  Triple::ArchType Arch =  Triple(TT).getArch();
   for (iterator it = begin(), ie = end(); it != ie; ++it) {
-    if (unsigned Qual = it->TripleMatchQualityFn(TT)) {
-      if (!Best || Qual > BestQuality) {
-        Best = &*it;
-        EquallyBest = 0;
-        BestQuality = Qual;
-      } else if (Qual == BestQuality)
-        EquallyBest = &*it;
+    if (it->ArchMatchFn(Arch)) {
+      if (Matching) {
+        Error = std::string("Cannot choose between targets \"") +
+          Matching->Name  + "\" and \"" + it->Name + "\"";
+        return 0;
+      }
+      Matching = &*it;
     }
   }
 
-  if (!Best) {
+  if (!Matching) {
     Error = "No available targets are compatible with this triple, "
       "see -version for the available targets.";
     return 0;
   }
 
-  // Otherwise, take the best target, but make sure we don't have two equally
-  // good best targets.
-  if (EquallyBest) {
-    Error = std::string("Cannot choose between targets \"") +
-      Best->Name  + "\" and \"" + EquallyBest->Name + "\"";
-    return 0;
-  }
-
-  return Best;
+  return Matching;
 }
 
 void TargetRegistry::RegisterTarget(Target &T,
                                     const char *Name,
                                     const char *ShortDesc,
-                                    Target::TripleMatchQualityFnTy TQualityFn,
+                                    Target::ArchMatchFnTy ArchMatchFn,
                                     bool HasJIT) {
-  assert(Name && ShortDesc && TQualityFn &&
+  assert(Name && ShortDesc && ArchMatchFn &&
          "Missing required target information!");
 
   // Check if this target has already been initialized, we allow this as a
@@ -120,7 +112,7 @@ void TargetRegistry::RegisterTarget(Target &T,
 
   T.Name = Name;
   T.ShortDesc = ShortDesc;
-  T.TripleMatchQualityFn = TQualityFn;
+  T.ArchMatchFn = ArchMatchFn;
   T.HasJIT = HasJIT;
 }
 

lib/Target/CppBackend/TargetInfo/CppBackendTargetInfo.cpp

@@ -14,9 +14,10 @@ using namespace llvm;
 
 Target llvm::TheCppBackendTarget;
 
-static unsigned CppBackend_TripleMatchQuality(const std::string &TT) {
-  // This class always works, but shouldn't be the default in most cases.
-  return 1;
+static bool CppBackend_TripleMatchQuality(Triple::ArchType Arch) {
+  // This backend doesn't correspond to any architecture. It must be explicitly
+  // selected with -march.
+  return false;
 }
 
 extern "C" void LLVMInitializeCppBackendTargetInfo() { 

test/CodeGen/Generic/no-target.ll

@@ -0,0 +1,3 @@
+; RUN: not llc -mtriple le32-unknown-nacl %s -o - 2>&1 | FileCheck %s
+
+; CHECK: error: unable to get target for 'le32-unknown-nacl'