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

1 files changed, 17 insertions, 4 deletions

diff --git a/clang/lib/AST/ExprConstant.cpp b/clang/lib/AST/ExprConstant.cpp
index 6569626a357..083572bc3e1 100644
--- a/clang/lib/AST/ExprConstant.cpp
+++ b/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;
 }