Index: llvm/lib/Transforms/Scalar/LoopIdiomRecognize.cpp
===================================================================
--- llvm/lib/Transforms/Scalar/LoopIdiomRecognize.cpp
+++ llvm/lib/Transforms/Scalar/LoopIdiomRecognize.cpp
@@ -1460,9 +1460,15 @@
   const Value *Args[] =
       {InitX, ZeroCheck ? ConstantInt::getTrue(InitX->getContext())
                         : ConstantInt::getFalse(InitX->getContext())};
-  if (CurLoop->getHeader()->size() != IdiomCanonicalSize &&
+
+  // @llvm.dbg doesn't count as they have no semantic effect.
+  auto InstWithoutDebugIt = CurLoop->getHeader()->instructionsWithoutDebug();
+  uint32_t HeaderSize =
+      std::distance(InstWithoutDebugIt.begin(), InstWithoutDebugIt.end());
+
+  if (HeaderSize != IdiomCanonicalSize &&
       TTI->getIntrinsicCost(IntrinID, InitX->getType(), Args) >
-        TargetTransformInfo::TCC_Basic)
+          TargetTransformInfo::TCC_Basic)
     return false;
 
   transformLoopToCountable(IntrinID, PH, CntInst, CntPhi, InitX, DefX,
Index: llvm/test/Transforms/LoopIdiom/dbginfo-cost.ll
===================================================================
--- /dev/null
+++ llvm/test/Transforms/LoopIdiom/dbginfo-cost.ll
@@ -0,0 +1,68 @@
+; RUN: opt -S -loop-idiom -mtriple=systemz-unknown -mcpu=z13 %s | FileCheck %s
+
+; CHECK: @llvm.ctlz.i32
+
+declare void @llvm.dbg.declare(metadata, metadata, metadata) #0
+define dso_local i32 @CeilLog2(i32 %arg) local_unnamed_addr #1 !dbg !38 {
+bb:
+  %tmp4 = add i32 %arg, -1, !dbg !45
+  call void @llvm.dbg.value(metadata i32 0, metadata !44, metadata !DIExpression()), !dbg !45
+  %tmp71 = icmp eq i32 %tmp4, 0, !dbg !45
+  br i1 %tmp71, label %bb13, label %bb8.preheader, !dbg !48
+
+bb8.preheader:                                    ; preds = %bb
+  br label %bb8, !dbg !49
+
+bb8:                                              ; preds = %bb8.preheader, %bb8
+  %tmp2.03 = phi i32 [ %tmp12, %bb8 ], [ 0, %bb8.preheader ]
+  %tmp1.02 = phi i32 [ %tmp10, %bb8 ], [ %tmp4, %bb8.preheader ]
+  call void @llvm.dbg.value(metadata i32 %tmp2.03, metadata !44, metadata !DIExpression()), !dbg !45
+  %tmp10 = lshr i32 %tmp1.02, 1, !dbg !49
+  %tmp12 = add nuw nsw i32 %tmp2.03, 1, !dbg !51
+  call void @llvm.dbg.value(metadata i32 %tmp12, metadata !44, metadata !DIExpression()), !dbg !45
+  %tmp7 = icmp eq i32 %tmp10, 0, !dbg !45
+  br i1 %tmp7, label %bb13.loopexit, label %bb8, !dbg !48, !llvm.loop !52
+
+bb13.loopexit:                                    ; preds = %bb8
+  %tmp12.lcssa = phi i32 [ %tmp12, %bb8 ], !dbg !51
+  br label %bb13, !dbg !54
+
+bb13:                                             ; preds = %bb13.loopexit, %bb
+  %tmp2.0.lcssa = phi i32 [ 0, %bb ], [ %tmp12.lcssa, %bb13.loopexit ], !dbg !55
+  call void @llvm.dbg.value(metadata i32 %tmp2.0.lcssa, metadata !44, metadata !DIExpression()), !dbg !45
+  ret i32 %tmp2.0.lcssa, !dbg !54
+}
+
+; Function Attrs: nounwind readnone speculatable
+declare void @llvm.dbg.value(metadata, metadata, metadata) #2
+
+attributes #0 = { nounwind readnone speculatable "target-cpu"="z13" }
+attributes #1 = { norecurse nounwind readnone "target-cpu"="z13" "use-soft-float"="false" }
+attributes #2 = { nounwind readnone speculatable }
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!36, !37}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "clang version 9.0.0 (ijonpan@m35lp38.lnxne.boe:llvm/llvm-dev-2/tools/clang a87ff88c6466fbedd6281513b9480a2cad6c08c8) (llvm/llvm-dev-2 922a3b1b3254bf3310c467e880a5419c1e13c87f)", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !2, retainedTypes: !2, globals: !2, nameTableKind: None)
+!1 = !DIFile(filename: "configfile.c", directory: "/home/ijonpan/minispec-2006/spec-llvm/464.h264ref/build")
+!2 = !{}
+!4 = !DIFile(filename: "./global.h", directory: "/home/ijonpan/minispec-2006/spec-llvm/464.h264ref/build")
+!5 = !DIBasicType(name: "unsigned int", size: 32, encoding: DW_ATE_unsigned)
+!36 = !{i32 2, !"Debug Info Version", i32 3}
+!37 = !{i32 1, !"wchar_size", i32 4}
+!38 = distinct !DISubprogram(name: "CeilLog2", scope: !1, file: !1, line: 599, type: !39, scopeLine: 600, flags: DIFlagPrototyped, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !41)
+!39 = !DISubroutineType(types: !40)
+!40 = !{!5, !5}
+!41 = !{!42, !43, !44}
+!42 = !DILocalVariable(name: "uiVal", arg: 1, scope: !38, file: !1, line: 599, type: !5)
+!43 = !DILocalVariable(name: "uiTmp", scope: !38, file: !1, line: 601, type: !5)
+!44 = !DILocalVariable(name: "uiRet", scope: !38, file: !1, line: 602, type: !5)
+!45 = !DILocation(line: 601, column: 25, scope: !38)
+!48 = !DILocation(line: 604, column: 3, scope: !38)
+!49 = !DILocation(line: 606, column: 11, scope: !50)
+!50 = distinct !DILexicalBlock(scope: !38, file: !1, line: 605, column: 3)
+!51 = !DILocation(line: 607, column: 10, scope: !50)
+!52 = distinct !{!52, !48, !53}
+!53 = !DILocation(line: 608, column: 3, scope: !38)
+!54 = !DILocation(line: 609, column: 3, scope: !38)
+!55 = !DILocation(line: 0, scope: !38)