Index: llvm/trunk/lib/Transforms/InstCombine/InstructionCombining.cpp
===================================================================
--- llvm/trunk/lib/Transforms/InstCombine/InstructionCombining.cpp
+++ llvm/trunk/lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -3099,13 +3099,35 @@
   ++NumSunkInst;
 
   // Also sink all related debug uses from the source basic block. Otherwise we
-  // get debug use before the def.
-  SmallVector<DbgVariableIntrinsic *, 1> DbgUsers;
+  // get debug use before the def. Attempt to salvage debug uses first, to
+  // maximise the range variables have location for. If we cannot salvage, then
+  // mark the location undef: we know it was supposed to receive a new location
+  // here, but that computation has been sunk.
+  SmallVector<DbgVariableIntrinsic *, 2> DbgUsers;
   findDbgUsers(DbgUsers, I);
-  for (auto *DII : DbgUsers) {
+  for (auto *DII : reverse(DbgUsers)) {
     if (DII->getParent() == SrcBlock) {
-      DII->moveBefore(&*InsertPos);
-      LLVM_DEBUG(dbgs() << "SINK: " << *DII << '\n');
+      // dbg.value is in the same basic block as the sunk inst, see if we can
+      // salvage it. Clone a new copy of the instruction: on success we need
+      // both salvaged and unsalvaged copies.
+      SmallVector<DbgVariableIntrinsic *, 1> TmpUser{
+          cast<DbgVariableIntrinsic>(DII->clone())};
+
+      if (!salvageDebugInfoForDbgValues(*I, TmpUser)) {
+        // We are unable to salvage: sink the cloned dbg.value, and mark the
+        // original as undef, terminating any earlier variable location.
+        LLVM_DEBUG(dbgs() << "SINK: " << *DII << '\n');
+        TmpUser[0]->insertBefore(&*InsertPos);
+        Value *Undef = UndefValue::get(I->getType());
+        DII->setOperand(0, MetadataAsValue::get(DII->getContext(),
+                                                ValueAsMetadata::get(Undef)));
+      } else {
+        // We successfully salvaged: place the salvaged dbg.value in the
+        // original location, and move the unmodified dbg.value to sink with
+        // the sunk inst.
+        TmpUser[0]->insertBefore(DII);
+        DII->moveBefore(&*InsertPos);
+      }
     }
   }
   return true;
Index: llvm/trunk/test/Transforms/InstCombine/debuginfo-sink.ll
===================================================================
--- llvm/trunk/test/Transforms/InstCombine/debuginfo-sink.ll
+++ llvm/trunk/test/Transforms/InstCombine/debuginfo-sink.ll
@@ -0,0 +1,78 @@
+; RUN: opt  %s -instcombine -S | FileCheck %s
+
+; Test sinking of dbg.values when instcombine sinks associated instructions.
+
+declare void @llvm.dbg.value(metadata, metadata, metadata)
+
+; This GEP is sunk, but can be folded into a DIExpression. Check that it
+; gets folded. The dbg.value should be duplicated in the block its sunk
+; into, to maximise liveness.
+;
+; CHECK-LABEL: define i32 @foo(i32*
+; CHECK:       call void @llvm.dbg.value(metadata i32* %a, metadata !{{[0-9]+}},
+; CHECK-SAME:  metadata !DIExpression(DW_OP_plus_uconst, 4, DW_OP_stack_value))
+; CHECK-NEXT:  br label %sink1
+
+define i32 @foo(i32 *%a) !dbg !7 {
+entry:
+  %gep = getelementptr i32, i32 *%a, i32 1
+  call void @llvm.dbg.value(metadata i32 *%gep, metadata !16, metadata !12), !dbg !15
+  br label %sink1
+
+sink1:
+; CHECK-LABEL: sink1:
+; CHECK:       call void @llvm.dbg.value(metadata i32* %gep,
+; CHECK-SAME:                    metadata !{{[0-9]+}}, metadata !DIExpression())
+; CHECK-NEXT:  load
+  %0 = load i32, i32* %gep, align 4, !dbg !15
+  ret i32 %0, !dbg !15
+}
+
+; In this example the GEP cannot (yet) be salvaged. Check that not only is the
+; dbg.value sunk, but an undef dbg.value is left to terminate any earlier
+; value range.
+
+; CHECK-LABEL: define i32 @bar(
+; CHECK:       call void @llvm.dbg.value(metadata i32* undef,
+; CHECK-NEXT:  br label %sink2
+
+define i32 @bar(i32 *%a, i32 %b) !dbg !70 {
+entry:
+  %gep = getelementptr i32, i32 *%a, i32 %b
+  call void @llvm.dbg.value(metadata i32* %gep, metadata !73, metadata !12), !dbg !74
+  br label %sink2
+
+sink2:
+; CHECK-LABEL: sink2:
+; CHECK:       call void @llvm.dbg.value(metadata i32* %gep,
+; CHECK-SAME:                    metadata !{{[0-9]+}}, metadata !DIExpression())
+; CHECK-NEXT:  load
+; CHECK-NEXT:  ret
+  %0 = load i32, i32* %gep
+  ret i32 %0
+}
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!3, !4, !5}
+!llvm.ident = !{!6}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "clang", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
+!1 = !DIFile(filename: "a.c", directory: ".")
+!2 = !{}
+!3 = !{i32 2, !"Dwarf Version", i32 4}
+!4 = !{i32 2, !"Debug Info Version", i32 3}
+!5 = !{i32 1, !"PIC Level", i32 2}
+!6 = !{!"clang"}
+!7 = distinct !DISubprogram(name: "foo", scope: !1, file: !1, line: 2, type: !8, isLocal: false, isDefinition: true, scopeLine: 3, flags: DIFlagPrototyped, isOptimized: false, unit: !0, retainedNodes: !2)
+!8 = !DISubroutineType(types: !9)
+!9 = !{!10, !10}
+!10 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!11 = !DILocalVariable(name: "j", scope: !7, file: !1, line: 2, type: !10)
+!12 = !DIExpression()
+!15 = !DILocation(line: 5, column: 3, scope: !7)
+!16 = !DILocalVariable(name: "h", scope: !7, file: !1, line: 4, type: !10)
+!70 = distinct !DISubprogram(name: "bar", scope: !1, file: !1, line: 2, type: !71, isLocal: false, isDefinition: true, scopeLine: 3, flags: DIFlagPrototyped, isOptimized: false, unit: !0, retainedNodes: !2)
+!71 = !DISubroutineType(types: !72)
+!72 = !{!10, !10, !10}
+!73 = !DILocalVariable(name: "k", scope: !70, file: !1, line: 2, type: !10)
+!74 = !DILocation(line: 5, column: 3, scope: !70)