1 files changed, 15 insertions, 9 deletions
diff --git a/lib/Target/X86/X86InstructionSelector.cpp b/lib/Target/X86/X86InstructionSelector.cpp
index 5eb5ad52840a..61956f741820 100644
--- a/lib/Target/X86/X86InstructionSelector.cpp
+++ b/lib/Target/X86/X86InstructionSelector.cpp
@@ -449,24 +449,30 @@ bool X86InstructionSelector::selectTrunc(MachineInstr &I,
   if (!SrcRC)
     return false;
 
-  if (!RBI.constrainGenericRegister(SrcReg, *SrcRC, MRI) ||
-      !RBI.constrainGenericRegister(DstReg, *DstRC, MRI)) {
-    DEBUG(dbgs() << "Failed to constrain G_TRUNC\n");
-    return false;
-  }
-
+  unsigned SubIdx;
   if (DstRC == SrcRC) {
     // Nothing to be done
+    SubIdx = X86::NoSubRegister;
   } else if (DstRC == &X86::GR32RegClass) {
-    I.getOperand(1).setSubReg(X86::sub_32bit);
+    SubIdx = X86::sub_32bit;
   } else if (DstRC == &X86::GR16RegClass) {
-    I.getOperand(1).setSubReg(X86::sub_16bit);
+    SubIdx = X86::sub_16bit;
   } else if (DstRC == &X86::GR8RegClass) {
-    I.getOperand(1).setSubReg(X86::sub_8bit);
+    SubIdx = X86::sub_8bit;
   } else {
     return false;
   }
 
+  SrcRC = TRI.getSubClassWithSubReg(SrcRC, SubIdx);
+
+  if (!RBI.constrainGenericRegister(SrcReg, *SrcRC, MRI) ||
+      !RBI.constrainGenericRegister(DstReg, *DstRC, MRI)) {
+    DEBUG(dbgs() << "Failed to constrain G_TRUNC\n");
+    return false;
+  }
+
+  I.getOperand(1).setSubReg(SubIdx);
+
   I.setDesc(TII.get(X86::COPY));
   return true;
 }