Index: llvm/trunk/lib/Transforms/Utils/Local.cpp
===================================================================
--- llvm/trunk/lib/Transforms/Utils/Local.cpp
+++ llvm/trunk/lib/Transforms/Utils/Local.cpp
@@ -1052,9 +1052,31 @@
     ExtendedArg = dyn_cast<Argument>(ZExt->getOperand(0));
   if (SExtInst *SExt = dyn_cast<SExtInst>(SI->getOperand(0)))
     ExtendedArg = dyn_cast<Argument>(SExt->getOperand(0));
-  if (ExtendedArg)
-    Builder.insertDbgValueIntrinsic(ExtendedArg, 0, DIVar, DIExpr,
+  if (ExtendedArg) {
+    // We're now only describing a subset of the variable. The piece we're
+    // describing will always be smaller than the variable size, because
+    // VariableSize == Size of Alloca described by DDI. Since SI stores
+    // to the alloca described by DDI, if it's first operand is an extend,
+    // we're guaranteed that before extension, the value was narrower than
+    // the size of the alloca, hence the size of the described variable.
+    SmallVector<uint64_t, 3> NewDIExpr;
+    unsigned PieceOffset = 0;
+    // If this already is a bit piece, we drop the bit piece from the expression
+    // and record the offset.
+    if (DIExpr->isBitPiece()) {
+      NewDIExpr.append(DIExpr->elements_begin(), DIExpr->elements_end()-3);
+      PieceOffset = DIExpr->getBitPieceOffset();
+    } else {
+      NewDIExpr.append(DIExpr->elements_begin(), DIExpr->elements_end());
+    }
+    NewDIExpr.push_back(dwarf::DW_OP_bit_piece);
+    NewDIExpr.push_back(PieceOffset); //Offset
+    const DataLayout &DL = DDI->getModule()->getDataLayout();
+    NewDIExpr.push_back(DL.getTypeSizeInBits(ExtendedArg->getType())); // Size
+    Builder.insertDbgValueIntrinsic(ExtendedArg, 0, DIVar,
+                                    Builder.createExpression(NewDIExpr),
                                     DDI->getDebugLoc(), SI);
+  }
   else
     Builder.insertDbgValueIntrinsic(SI->getOperand(0), 0, DIVar, DIExpr,
                                     DDI->getDebugLoc(), SI);
Index: llvm/trunk/test/Transforms/Util/split-bit-piece.ll
===================================================================
--- llvm/trunk/test/Transforms/Util/split-bit-piece.ll
+++ llvm/trunk/test/Transforms/Util/split-bit-piece.ll
@@ -0,0 +1,45 @@
+; Checks that llvm.dbg.declare -> llvm.dbg.value conversion utility
+; (here exposed through the SROA) pass, properly inserts bit_piece expressions
+; if it only describes part of the variable.
+; RUN: opt -S -sroa %s | FileCheck %s
+
+; Function Attrs: nounwind readnone
+declare void @llvm.dbg.declare(metadata, metadata, metadata) #0
+
+; Function Attrs: nounwind uwtable
+define hidden void @_ZN6__tsan9FastState14SetHistorySizeEi(i32 %hs) #1 align 2 {
+entry:
+  %hs.addr = alloca i32, align 4
+  %v1 = alloca i64, align 8
+  %v2 = alloca i64, align 8
+  store i32 %hs, i32* %hs.addr, align 4
+; CHECK: call void @llvm.dbg.value(metadata i32 %hs, i64 0, metadata !{{[0-9]+}}, metadata ![[EXPR:[0-9]+]])
+; CHECK: ![[EXPR]] = !DIExpression(DW_OP_bit_piece, 0
+  call void @llvm.dbg.declare(metadata i64* %v1, metadata !9, metadata !12), !dbg !13
+  %0 = load i32, i32* %hs.addr, align 4
+  %conv = sext i32 %0 to i64
+  store i64 %conv, i64* %v1, align 8
+  %1 = load i64, i64* %v2, align 8
+  unreachable
+}
+
+attributes #0 = { nounwind readnone }
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!7}
+!llvm.ident = !{!8}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus, file: !1, producer: "clang version 3.8.0 (trunk 256979) (llvm/trunk 257107)", isOptimized: false, runtimeVersion: 0, emissionKind: 1, retainedTypes: !2)
+!1 = !DIFile(filename: "tsan_shadow_test.cc", directory: "/tmp")
+!2 = !{!3, !5}
+!3 = !DICompositeType(tag: DW_TAG_class_type, name: "FastState", file: !4, line: 91, size: 64, align: 64, identifier: "_ZTSN6__tsan9FastStateE")
+!4 = !DIFile(filename: "/mnt/extra/llvm/projects/compiler-rt/lib/tsan/rtl/tsan_rtl.h", directory: "/tmp")
+!5 = distinct !DIDerivedType(tag: DW_TAG_typedef, name: "u64", line: 78, baseType: !6)
+!6 = !DIBasicType(name: "long long unsigned int", size: 64, align: 64, encoding: DW_ATE_unsigned)
+!7 = !{i32 2, !"Debug Info Version", i32 3}
+!8 = !{!"clang version 3.8.0 (trunk 256979) (llvm/trunk 257107)"}
+!9 = !DILocalVariable(name: "v1", scope: !10, file: !4, line: 136, type: !5)
+!10 = distinct !DILexicalBlock(scope: !11, file: !4, line: 136, column: 5)
+!11 = distinct !DISubprogram(name: "SetHistorySize", linkageName: "_ZN6__tsan9FastState14SetHistorySizeEi", scope: !"_ZTSN6__tsan9FastStateE", file: !4, line: 135, isLocal: false, isDefinition: true, scopeLine: 135, flags: DIFlagPrototyped, isOptimized: false)
+!12 = !DIExpression()
+!13 = !DILocation(line: 136, column: 5, scope: !10)