Index: llvm/trunk/lib/CodeGen/CodeGenPrepare.cpp
===================================================================
--- llvm/trunk/lib/CodeGen/CodeGenPrepare.cpp
+++ llvm/trunk/lib/CodeGen/CodeGenPrepare.cpp
@@ -4270,6 +4270,14 @@
   TPT.commit();
   I->removeFromParent();
   I->insertAfter(LI);
+  // CGP does not check if the zext would be speculatively executed when moved
+  // to the same basic block as the load. Preserving its original location would
+  // pessimize the debugging experience, as well as negatively impact the 
+  // quality of sample pgo. We don't want to use "line 0" as that has a
+  // size cost in the line-table section and logically the zext can be seen as
+  // part of the load. Therefore we conservatively reuse the same debug location
+  // for the load and the zext.
+  I->setDebugLoc(LI->getDebugLoc());
   ++NumExtsMoved;
   return true;
 }
Index: llvm/trunk/test/DebugInfo/X86/zextload.ll
===================================================================
--- llvm/trunk/test/DebugInfo/X86/zextload.ll
+++ llvm/trunk/test/DebugInfo/X86/zextload.ll
@@ -0,0 +1,71 @@
+; RUN: opt -S -codegenprepare < %s | FileCheck %s
+;
+; This test case was generated from the following source code:
+; 
+; long long foo(int *ptr, int cond) {
+;   long long result = 3;
+;   unsigned val = *ptr; // line 3
+;   switch (cond) {
+;   case 3:
+;     result = val;      // line 6
+;     break;
+;   case 4:
+;     result += 2;
+;   }
+; 
+;   return result + val;
+; };
+;
+; When CGP moves a zext Z of a load L to the block where L lives,  Z should not
+; retain its original debug location. Instead, Z should reuse the debug location
+; associated with L. Logically the zero extend will become part of the load; the
+; code generator will attempt to fuse the two instructions into a zextload.
+
+; CHECK-LABEL: @test
+; CHECK:   [[LOADVAL:%[0-9]+]] = load i32, i32* %ptr, align 4, !dbg [[DEBUGLOC:![0-9]+]]
+; CHECK-NEXT:                    zext i32 [[LOADVAL]] to i64, !dbg [[DEBUGLOC]]
+; CHECK:   [[DEBUGLOC]] = !DILocation(line: 3
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+define i64 @test(i32* %ptr, i32 %cond) !dbg !5 {
+entry:
+  %0 = load i32, i32* %ptr, align 4, !dbg !7
+  switch i32 %cond, label %sw.epilog [
+    i32 3, label %sw.bb
+    i32 4, label %sw.bb1
+  ], !dbg !8
+
+sw.bb:                                            ; preds = %entry
+  %conv = zext i32 %0 to i64, !dbg !9
+  br label %sw.epilog, !dbg !10
+
+sw.bb1:                                           ; preds = %entry
+  br label %sw.epilog, !dbg !11
+
+sw.epilog:                                        ; preds = %sw.bb1, %entry, %sw.bb
+  %result.0 = phi i64 [ 3, %entry ], [ 5, %sw.bb1 ], [ %conv, %sw.bb ]
+  %conv2 = zext i32 %0 to i64, !dbg !12
+  %add3 = add nuw nsw i64 %result.0, %conv2, !dbg !13
+  ret i64 %add3, !dbg !14
+}
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!3, !4}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "", isOptimized: true, runtimeVersion: 0, emissionKind: LineTablesOnly, enums: !2)
+!1 = !DIFile(filename: "test.c", directory: "")
+!2 = !{}
+!3 = !{i32 2, !"Dwarf Version", i32 4}
+!4 = !{i32 2, !"Debug Info Version", i32 3}
+!5 = distinct !DISubprogram(name: "test", scope: !1, file: !1, line: 1, type: !6, isLocal: false, isDefinition: true, scopeLine: 1, flags: DIFlagPrototyped, isOptimized: true, unit: !0, variables: !2)
+!6 = !DISubroutineType(types: !2)
+!7 = !DILocation(line: 3, column: 18, scope: !5)
+!8 = !DILocation(line: 4, column: 3, scope: !5)
+!9 = !DILocation(line: 6, column: 14, scope: !5)
+!10 = !DILocation(line: 7, column: 5, scope: !5)
+!11 = !DILocation(line: 10, column: 3, scope: !5)
+!12 = !DILocation(line: 12, column: 19, scope: !5)
+!13 = !DILocation(line: 12, column: 17, scope: !5)
+!14 = !DILocation(line: 12, column: 3, scope: !5)