3 files changed, 82 insertions, 2 deletions
diff --git a/lib/CodeGen/CGExpr.cpp b/lib/CodeGen/CGExpr.cpp
index f4c552b085..62be62b79e 100644
--- a/lib/CodeGen/CGExpr.cpp
+++ b/lib/CodeGen/CGExpr.cpp
@@ -18,6 +18,7 @@
 #include "CGDebugInfo.h"
 #include "CGRecordLayout.h"
 #include "CGObjCRuntime.h"
+#include "TargetInfo.h"
 #include "clang/AST/ASTContext.h"
 #include "clang/AST/DeclObjC.h"
 #include "clang/Frontend/CodeGenOptions.h"
@@ -2414,8 +2415,35 @@ RValue CodeGenFunction::EmitCall(QualType CalleeType, llvm::Value *Callee,
   CallArgList Args;
   EmitCallArgs(Args, dyn_cast<FunctionProtoType>(FnType), ArgBeg, ArgEnd);
 
-  return EmitCall(CGM.getTypes().getFunctionInfo(Args, FnType),
-                  Callee, ReturnValue, Args, TargetDecl);
+  const CGFunctionInfo &FnInfo = CGM.getTypes().getFunctionInfo(Args, FnType);
+
+  // C99 6.5.2.2p6:
+  //   If the expression that denotes the called function has a type
+  //   that does not include a prototype, [the default argument
+  //   promotions are performed]. If the number of arguments does not
+  //   equal the number of parameters, the behavior is undefined. If
+  //   the function is defined with a type that includes a prototype,
+  //   and either the prototype ends with an ellipsis (, ...) or the
+  //   types of the arguments after promotion are not compatible with
+  //   the types of the parameters, the behavior is undefined. If the
+  //   function is defined with a type that does not include a
+  //   prototype, and the types of the arguments after promotion are
+  //   not compatible with those of the parameters after promotion,
+  //   the behavior is undefined [except in some trivial cases].
+  // That is, in the general case, we should assume that a call
+  // through an unprototyped function type works like a *non-variadic*
+  // call.  The way we make this work is to cast to the exact type
+  // of the promoted arguments.
+  if (isa<FunctionNoProtoType>(FnType) &&
+      !getTargetHooks().isNoProtoCallVariadic(FnType->getCallConv())) {
+    assert(cast<llvm::FunctionType>(Callee->getType()->getContainedType(0))
+             ->isVarArg());
+    llvm::Type *CalleeTy = getTypes().GetFunctionType(FnInfo, false);
+    CalleeTy = CalleeTy->getPointerTo();
+    Callee = Builder.CreateBitCast(Callee, CalleeTy, "callee.knr.cast");
+  }
+
+  return EmitCall(FnInfo, Callee, ReturnValue, Args, TargetDecl);
 }
 
 LValue CodeGenFunction::
diff --git a/lib/CodeGen/TargetInfo.cpp b/lib/CodeGen/TargetInfo.cpp
index 434e202f2e..bad8a1d928 100644
--- a/lib/CodeGen/TargetInfo.cpp
+++ b/lib/CodeGen/TargetInfo.cpp
@@ -98,6 +98,14 @@ unsigned TargetCodeGenInfo::getSizeOfUnwindException() const {
   return 32;
 }
 
+bool TargetCodeGenInfo::isNoProtoCallVariadic(CallingConv CC) const {
+  // The following conventions are known to require this to be false:
+  //   x86_stdcall
+  //   MIPS
+  // For everything else, we just prefer false unless we opt out.
+  return false;
+}
+
 static bool isEmptyRecord(ASTContext &Context, QualType T, bool AllowArrays);
 
 /// isEmptyField - Return true iff a the field is "empty", that is it
@@ -959,6 +967,15 @@ public:
     return X86AdjustInlineAsmType(CGF, Constraint, Ty);
   }
 
+  bool isNoProtoCallVariadic(CallingConv CC) const {
+    // The default CC on x86-64 sets %al to the number of SSA
+    // registers used, and GCC sets this when calling an unprototyped
+    // function, so we override the default behavior.
+    if (CC == CC_Default || CC == CC_C) return true;
+
+    return TargetCodeGenInfo::isNoProtoCallVariadic(CC);
+  }
+
 };
 
 class WinX86_64TargetCodeGenInfo : public TargetCodeGenInfo {
diff --git a/lib/CodeGen/TargetInfo.h b/lib/CodeGen/TargetInfo.h
index 22e6eb8383..8f90c7bdd9 100644
--- a/lib/CodeGen/TargetInfo.h
+++ b/lib/CodeGen/TargetInfo.h
@@ -16,6 +16,7 @@
 #define CLANG_CODEGEN_TARGETINFO_H
 
 #include "clang/Basic/LLVM.h"
+#include "clang/AST/Type.h"
 #include "llvm/ADT/StringRef.h"
 
 namespace llvm {
@@ -126,6 +127,40 @@ namespace clang {
     virtual StringRef getARCRetainAutoreleasedReturnValueMarker() const {
       return "";
     }
+
+    /// Determine whether a call to an unprototyped functions under
+    /// the given calling convention should use the variadic
+    /// convention or the non-variadic convention.
+    ///
+    /// There's a good reason to make a platform's variadic calling
+    /// convention be different from its non-variadic calling
+    /// convention: the non-variadic arguments can be passed in
+    /// registers (better for performance), and the variadic arguments
+    /// can be passed on the stack (also better for performance).  If
+    /// this is done, however, unprototyped functions *must* use the
+    /// non-variadic convention, because C99 states that a call
+    /// through an unprototyped function type must succeed if the
+    /// function was defined with a non-variadic prototype with
+    /// compatible parameters.  Therefore, splitting the conventions
+    /// makes it impossible to call a variadic function through an
+    /// unprototyped type.  Since function prototypes came out in the
+    /// late 1970s, this is probably an acceptable trade-off.
+    /// Nonetheless, not all platforms are willing to make it, and in
+    /// particularly x86-64 bends over backwards to make the
+    /// conventions compatible.
+    ///
+    /// The default is false.  This is correct whenever:
+    ///   - the conventions are exactly the same, because it does not
+    ///     matter and the resulting IR will be somewhat prettier in
+    ///     certain cases; or
+    ///   - the conventions are substantively different in how they pass
+    ///     arguments, because in this case using the variadic convention
+    ///     will lead to C99 violations.
+    /// It is not necessarily correct when arguments are passed in the
+    /// same way and some out-of-band information is passed for the
+    /// benefit of variadic callees, as is the case for x86-64.
+    /// In this case the ABI should be consulted.
+    virtual bool isNoProtoCallVariadic(CallingConv CC) const;
   };
 }