Index: lib/Target/X86/X86InstrCompiler.td
===================================================================
--- lib/Target/X86/X86InstrCompiler.td
+++ lib/Target/X86/X86InstrCompiler.td
@@ -1191,6 +1191,7 @@
   return N->getOpcode() != ISD::TRUNCATE &&
          N->getOpcode() != TargetOpcode::EXTRACT_SUBREG &&
          N->getOpcode() != ISD::CopyFromReg &&
+         N->getOpcode() != ISD::AssertSext &&
          N->getOpcode() != X86ISD::CMOV;
 }]>;
 
Index: test/CodeGen/X86/TruncAssertZext.ll
===================================================================
--- test/CodeGen/X86/TruncAssertZext.ll
+++ test/CodeGen/X86/TruncAssertZext.ll
@@ -0,0 +1,15 @@
+; RUN: llc < %s -O2 -march=x86-64 | FileCheck %s
+; Checks that a zeroing mov is inserted for the trunc/zext pair even when
+; the source of the zext is an AssertSext node
+; PR20494
+
+define i64 @main(i64 %a) { 
+; CHECK-LABEL: main
+; CHECK: movl %ecx, %eax
+  %or = or i64 %a, -2
+  %trunc = trunc i64 %or to i32
+  br label %l
+l:
+  %ext = zext i32 %trunc to i64
+  ret i64 %ext
+}
\ No newline at end of file