diff --git a/llvm/lib/CodeGen/TargetInstrInfo.cpp b/llvm/lib/CodeGen/TargetInstrInfo.cpp
--- a/llvm/lib/CodeGen/TargetInstrInfo.cpp
+++ b/llvm/lib/CodeGen/TargetInstrInfo.cpp
@@ -1017,6 +1017,17 @@
   InsInstrs.push_back(MIB2);
   DelInstrs.push_back(&Prev);
   DelInstrs.push_back(&Root);
+
+  // We transformed:
+  // B = A op X (Prev)
+  // C = B op Y (Root)
+  // Into:
+  // B = X op Y (MIB1)
+  // C = A op B (MIB2)
+  // C has the same value as before, B doesn't; as such, keep the debug number
+  // of C but not of B.
+  if (unsigned OldRootNum = Root.peekDebugInstrNum())
+    MIB2.getInstr()->setDebugInstrNum(OldRootNum);
 }
 
 void TargetInstrInfo::genAlternativeCodeSequence(
diff --git a/llvm/test/CodeGen/X86/machine-combiner-dbg.mir b/llvm/test/CodeGen/X86/machine-combiner-dbg.mir
new file mode 100644
--- /dev/null
+++ b/llvm/test/CodeGen/X86/machine-combiner-dbg.mir
@@ -0,0 +1,58 @@
+# RUN: llc -mtriple=x86_64-gnu-linux -run-pass=machine-combiner %s -o - | FileCheck %s
+
+--- |
+  define float @reassoc_me(float %f1, float %f2, float %f3, float %f4) !dbg !4 {
+    %add = fadd reassoc nsz float %f1, %f2, !dbg !10
+    %add1 = fadd reassoc nsz float %add, %f3, !dbg !10
+    %add2 = fadd reassoc nsz float %add1, %f4, !dbg !10
+    call void @llvm.dbg.value(metadata float %add2, metadata !9, metadata !DIExpression()), !dbg !10
+    ret float %add2, !dbg !10
+  }
+
+  declare void @llvm.dbg.value(metadata, metadata, metadata)
+
+  !llvm.dbg.cu = !{!0}
+  !llvm.module.flags = !{!2, !3}
+
+  !0 = distinct !DICompileUnit(language: DW_LANG_C11, file: !1, producer: "hello", isOptimized: true, emissionKind: FullDebug)
+  !1 = !DIFile(filename: "reassoc.c", directory: "blah")
+  !2 = !{i32 7, !"Dwarf Version", i32 4}
+  !3 = !{i32 2, !"Debug Info Version", i32 3}
+  !4 = distinct !DISubprogram(name: "reassoc_me", scope: !1, file: !1, line: 1, type: !5, scopeLine: 1, unit: !0, retainedNodes: !8)
+  !5 = !DISubroutineType(types: !6)
+  !6 = !{!7, !7, !7, !7, !7}
+  !7 = !DIBasicType(name: "float", size: 32, encoding: DW_ATE_float)
+  !8 = !{!9}
+  !9 = !DILocalVariable(name: "temp3", scope: !4, file: !1, line: 4, type: !7)
+  !10 = !DILocation(line: 0, scope: !4)
+
+...
+---
+name:            reassoc_me
+alignment:       16
+debugInstrRef:   true
+body:             |
+  bb.0 (%ir-block.0):
+    liveins: $xmm0, $xmm1, $xmm2, $xmm3
+
+    %3:fr32 = COPY $xmm3
+    %2:fr32 = COPY $xmm2
+    %1:fr32 = COPY $xmm1
+    %0:fr32 = COPY $xmm0
+    %4:fr32 = nsz reassoc nofpexcept ADDSSrr %0, %1, implicit $mxcsr, debug-location !10
+    %5:fr32 = nsz reassoc nofpexcept ADDSSrr %4, %2, implicit $mxcsr, debug-location !10
+    %6:fr32 = nsz reassoc nofpexcept ADDSSrr %5, %3, implicit $mxcsr, debug-instr-number 1, debug-location !10
+    DBG_INSTR_REF !9, !DIExpression(DW_OP_LLVM_arg, 0), dbg-instr-ref(1, 0), debug-location !10
+    $xmm0 = COPY %6, debug-location !10
+    RET 0, $xmm0, debug-location !10
+
+  ; CHECK: ![[VAR:.*]] = !DILocalVariable(name: "temp3"
+  ; CHECK:      %[[arg3:.*]]:fr32 = COPY $xmm3
+  ; CHECK-NEXT: %[[arg2:.*]]:fr32 = COPY $xmm2
+  ; CHECK-NEXT: %[[arg1:.*]]:fr32 = COPY $xmm1
+  ; CHECK-NEXT: %[[arg0:.*]]:fr32 = COPY $xmm0
+  ; CHECK-NEXT: %[[add1:.*]]:fr32 = {{.*}} ADDSSrr %[[arg0]], %[[arg1]]
+  ; CHECK-NEXT: %[[add2:.*]]:fr32 = {{.*}} ADDSSrr %[[arg2]], %[[arg3]]
+  ; CHECK-NEXT:                            ADDSSrr %[[add1]], killed %[[add2]], {{.*}} debug-instr-number 1
+  ; CHECK-NEXT: DBG_INSTR_REF ![[VAR]], !DIExpression(DW_OP_LLVM_arg, 0), dbg-instr-ref(1, 0)
+...