Index: test/DebugInfo/debugify-bogus-dbg-value.ll
===================================================================
--- /dev/null
+++ test/DebugInfo/debugify-bogus-dbg-value.ll
@@ -0,0 +1,51 @@
+; RUN: opt -check-debugify < %s 2>&1 | FileCheck %s
+
+define <2 x i64> @test-fun(<2 x i64> %A) !dbg !6 {
+  %and = and <2 x i64> %A, <i64 23, i64 42>, !dbg !14
+
+; CHECK: ERROR: dbg.value operand has size 128, but its variable has size 256
+  call void @llvm.dbg.value(metadata <2 x i64> %and, metadata !12, metadata !DIExpression()), !dbg !15
+
+; CHECK: ERROR: dbg.value operand has size 512, but its variable has size 256
+  call void @llvm.dbg.value(metadata <2 x i256> zeroinitializer, metadata !12, metadata !DIExpression()), !dbg !15
+
+; CHECK: ERROR: dbg.value operand has size 8, but its variable has size 256
+  call void @llvm.dbg.value(metadata i8 0, metadata !12, metadata !DIExpression()), !dbg !15
+
+; CHECK-NOT: ERROR: dbg.value operand has size 512, but its variable has size 256
+  call void @llvm.dbg.value(metadata i512 0, metadata !12, metadata !DIExpression()), !dbg !15
+
+  ret <2 x i64> %and, !dbg !16
+}
+; CHECK: CheckModuleDebugify: FAIL
+
+; Function Attrs: nounwind readnone speculatable
+declare void @llvm.dbg.value(metadata, metadata, metadata) #0
+
+attributes #0 = { nounwind readnone speculatable }
+
+!llvm.dbg.cu = !{!0}
+!llvm.debugify = !{!3, !4}
+!llvm.module.flags = !{!5}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C, file: !1, producer: "debugify", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !2)
+!1 = !DIFile(filename: "/Users/vsk/src/llvm.org-master/llvm/test/DebugInfo/debugify-bogus-dbg-value.ll", directory: "/")
+!2 = !{}
+!3 = !{i32 4}
+!4 = !{i32 3}
+!5 = !{i32 2, !"Debug Info Version", i32 3}
+!6 = distinct !DISubprogram(name: "test-fun", linkageName: "test-fun", scope: null, file: !1, line: 1, type: !7, isLocal: false, isDefinition: true, scopeLine: 1, isOptimized: true, unit: !0, retainedNodes: !8)
+!7 = !DISubroutineType(types: !2)
+!8 = !{!9, !11, !12}
+!9 = !DILocalVariable(name: "1", scope: !6, file: !1, line: 1, type: !10)
+!10 = !DIBasicType(name: "ty64", size: 64, encoding: DW_ATE_unsigned)
+!11 = !DILocalVariable(name: "2", scope: !6, file: !1, line: 2, type: !10)
+!12 = !DILocalVariable(name: "3", scope: !6, file: !1, line: 3, type: !13)
+
+; Set the size here to an incorrect value to check that the size diagnostic
+; triggers.
+!13 = !DIBasicType(name: "ty128", size: 256, encoding: DW_ATE_unsigned)
+
+!14 = !DILocation(line: 2, column: 1, scope: !6)
+!15 = !DILocation(line: 3, column: 1, scope: !6)
+!16 = !DILocation(line: 4, column: 1, scope: !6)
Index: tools/opt/Debugify.cpp
===================================================================
--- tools/opt/Debugify.cpp
+++ tools/opt/Debugify.cpp
@@ -40,6 +40,10 @@
 
 raw_ostream &dbg() { return Quiet ? nulls() : errs(); }
 
+uint64_t getSizeInBits(Module &M, Type *Ty) {
+  return Ty->isSized() ? M.getDataLayout().getTypeAllocSizeInBits(Ty) : 0;
+}
+
 bool isFunctionSkipped(Function &F) {
   return F.isDeclaration() || !F.hasExactDefinition();
 }
@@ -71,8 +75,7 @@
   // Get a DIType which corresponds to Ty.
   DenseMap<uint64_t, DIType *> TypeCache;
   auto getCachedDIType = [&](Type *Ty) -> DIType * {
-    uint64_t Size =
-        Ty->isSized() ? M.getDataLayout().getTypeAllocSizeInBits(Ty) : 0;
+    uint64_t Size = getSizeInBits(M, Ty);
     DIType *&DTy = TypeCache[Size];
     if (!DTy) {
       std::string Name = "ty" + utostr(Size);
@@ -208,7 +211,7 @@
       HasErrors = true;
     }
 
-    // Find missing variables.
+    // Find missing variables and mis-sized debug values.
     for (Instruction &I : instructions(F)) {
       auto *DVI = dyn_cast<DbgValueInst>(&I);
       if (!DVI)
@@ -218,6 +221,41 @@
       (void)to_integer(DVI->getVariable()->getName(), Var, 10);
       assert(Var <= OriginalNumVars && "Unexpected name for DILocalVariable");
       MissingVars.reset(Var - 1);
+
+      // The size of a dbg.value's value operand should match the size of the
+      // variable it corresponds to.
+      //
+      // TODO: This, along with a check for non-null value operands, should be
+      // promoted to verifier failures.
+      if (Value *V = DVI->getValue()) {
+        // For now, don't try to interpret DW_OP_deref, or anything more
+        // complicated than an empty DIExpression.
+        if (!DVI->getExpression()->getNumElements()) {
+          Type *Ty = V->getType();
+          uint64_t ActualSizeInBits = getSizeInBits(M, Ty);
+          uint64_t ExpectedSizeInBits = *DVI->getFragmentSizeInBits();
+
+          // If the described integer is smaller than the actual value, the
+          // debugger can display the right value using truncation. However, if
+          // the described integer is larger than the actual value, it cannot
+          // display the proper value without guessing (resorting to sign/zero
+          // extension).
+          //
+          // For all other types (vectors, floats, etc.): it's a bug if the
+          // described value doesn't have the same size as the actual value.
+          bool HasBadSize = Ty->isIntegerTy()
+                                ? (ActualSizeInBits < ExpectedSizeInBits)
+                                : (ActualSizeInBits != ExpectedSizeInBits);
+          if (HasBadSize) {
+            dbg() << "ERROR: dbg.value operand has size " << ActualSizeInBits
+                  << ", but its variable has size " << ExpectedSizeInBits
+                  << ": ";
+            DVI->print(dbg());
+            dbg() << "\n";
+            HasErrors = true;
+          }
+        }
+      }
     }
   }