Add support for generating MIPS legacy NaN

Currently, the NaN values emitted for MIPS architectures do not cover
non-IEEE754-2008 compliant case. This change fixes the issue.

Patch by Vladimir Radosavljevic.

Differential Revision: http://reviews.llvm.org/D7882

llvm-svn: 230653

clang/include/clang/Basic/TargetInfo.h

@@ -638,6 +638,12 @@ public:
     return std::string(1, *Constraint);
   }
 
+  /// \brief Returns true if NaN encoding is IEEE 754-2008.
+  /// Only MIPS allows a different encoding.
+  virtual bool isNan2008() const {
+    return true;
+  }
+
   /// \brief Returns a string of target-specific clobbers, in LLVM format.
   virtual const char *getClobbers() const = 0;
 

clang/lib/AST/ExprConstant.cpp

@@ -7590,10 +7590,23 @@ static bool TryEvaluateBuiltinNaN(const ASTContext &Context,
   else if (S->getString().getAsInteger(0, fill))
     return false;
 
-  if (SNaN)
-    Result = llvm::APFloat::getSNaN(Sem, false, &fill);
-  else
-    Result = llvm::APFloat::getQNaN(Sem, false, &fill);
+  if (Context.getTargetInfo().isNan2008()) {
+    if (SNaN)
+      Result = llvm::APFloat::getSNaN(Sem, false, &fill);
+    else
+      Result = llvm::APFloat::getQNaN(Sem, false, &fill);
+  } else {
+    // Prior to IEEE 754-2008, architectures were allowed to choose whether
+    // the first bit of their significand was set for qNaN or sNaN. MIPS chose
+    // a different encoding to what became a standard in 2008, and for pre-
+    // 2008 revisions, MIPS interpreted sNaN-2008 as qNan and qNaN-2008 as
+    // sNaN. This is now known as "legacy NaN" encoding.
+    if (SNaN)
+      Result = llvm::APFloat::getQNaN(Sem, false, &fill);
+    else
+      Result = llvm::APFloat::getSNaN(Sem, false, &fill);
+  }
+
   return true;
 }
 

clang/lib/Basic/Targets.cpp

@@ -5674,6 +5674,10 @@ public:
     return CPU == "mips32r6" || ABI == "n32" || ABI == "n64" || ABI == "64";
   }
 
+  bool isNan2008() const override {
+    return IsNan2008;
+  }
+
   StringRef getABI() const override { return ABI; }
   bool setCPU(const std::string &Name) override {
     bool IsMips32 = getTriple().getArch() == llvm::Triple::mips ||

clang/test/CodeGen/builtin-nan-legacy.c

@@ -0,0 +1,13 @@
+// RUN: %clang -target mipsel-unknown-linux -mnan=legacy -emit-llvm -S %s -o - | FileCheck %s
+// CHECK: float 0x7FF4000000000000, float 0x7FF8000000000000
+// CHECK: double 0x7FF4000000000000, double 0x7FF8000000000000
+
+float f[] = {
+  __builtin_nan(""),
+  __builtin_nans(""),
+};
+
+double d[] = {
+  __builtin_nan(""),
+  __builtin_nans(""),
+};