diff --git a/lib/Target/X86/X86ISelLowering.cpp b/lib/Target/X86/X86ISelLowering.cpp
index 5404c4c..de11f09 100644
--- a/lib/Target/X86/X86ISelLowering.cpp
+++ b/lib/Target/X86/X86ISelLowering.cpp
@@ -42477,14 +42477,17 @@
   if (!Res.second) {
     // Map st(0) -> st(7) -> ST0
     if (Constraint.size() == 7 && Constraint[0] == '{' &&
-        tolower(Constraint[1]) == 's' &&
-        tolower(Constraint[2]) == 't' &&
+        tolower(Constraint[1]) == 's' && tolower(Constraint[2]) == 't' &&
         Constraint[3] == '(' &&
         (Constraint[4] >= '0' && Constraint[4] <= '7') &&
-        Constraint[5] == ')' &&
-        Constraint[6] == '}')
+        Constraint[5] == ')' && Constraint[6] == '}') {
+      // st(7) is not allocatable and thus not a member of RFP80. Return
+      // singleton class in cases where we have a reference to it.
+      if (Constraint[4] == '7')
+        return std::make_pair(X86::FP7, &X86::RFP80_7RegClass);
       return std::make_pair(X86::FP0 + Constraint[4] - '0',
                             &X86::RFP80RegClass);
+    }
 
     // GCC allows "st(0)" to be called just plain "st".
     if (StringRef("{st}").equals_lower(Constraint))
diff --git a/lib/Target/X86/X86RegisterInfo.td b/lib/Target/X86/X86RegisterInfo.td
index 0c1b05f..aa20273 100644
--- a/lib/Target/X86/X86RegisterInfo.td
+++ b/lib/Target/X86/X86RegisterInfo.td
@@ -522,10 +522,16 @@
 // faster on common hardware.  In reality, this should be controlled by a
 // command line option or something.
 
+
 def RFP32 : RegisterClass<"X86",[f32], 32, (sequence "FP%u", 0, 6)>;
 def RFP64 : RegisterClass<"X86",[f64], 32, (add RFP32)>;
 def RFP80 : RegisterClass<"X86",[f80], 32, (add RFP32)>;
 
+// st(7) may be is not allocatable.
+def RFP80_7 : RegisterClass<"X86",[f80], 32, (add FP7)> {
+  let isAllocatable = 0;
+}
+
 // Floating point stack registers (these are not allocatable by the
 // register allocator - the floating point stackifier is responsible
 // for transforming FPn allocations to STn registers)