Index: lib/Transforms/Scalar/MergeICmps.cpp
===================================================================
--- lib/Transforms/Scalar/MergeICmps.cpp
+++ lib/Transforms/Scalar/MergeICmps.cpp
@@ -183,6 +183,13 @@
 // BCE atoms, returns the comparison.
 BCECmpBlock visitICmp(const ICmpInst *const CmpI,
                       const ICmpInst::Predicate ExpectedPredicate) {
+  // The comparison can only be used once:
+  //  - For intermediate blocks, as a branch condition.
+  //  - For the final block, as an incoming value for the Phi.
+  if (!CmpI->hasOneUse()) {
+    DEBUG(dbgs() << "cmp has several uses\n");
+    return {};
+  }
   if (CmpI->getPredicate() == ExpectedPredicate) {
     DEBUG(dbgs() << "cmp "
                  << (ExpectedPredicate == ICmpInst::ICMP_EQ ? "eq" : "ne")
Index: test/Transforms/MergeICmps/X86/pr36557.ll
===================================================================
--- /dev/null
+++ test/Transforms/MergeICmps/X86/pr36557.ll
@@ -0,0 +1,119 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt < %s -mergeicmps -mtriple=x86_64-unknown-unknown -S | FileCheck %s --check-prefix=X86
+
+source_filename = "qabstractitemmodeltester.cpp"
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64--linux-gnu"
+
+%class.B = type { i32, i32 }
+%class.D = type { i32 }
+%class.C = type { i8 }
+%class.QMessageLogger = type { i8 }
+
+$_ZN1D7compareI1BS1_EEbRKT_RKT0_PKcS9_S9_i = comdat any
+
+@.str = private unnamed_addr constant [1 x i8] zeroinitializer, align 1
+
+; Function Attrs: uwtable
+define linkonce_odr dso_local zeroext i1 @_ZN1D7compareI1BS1_EEbRKT_RKT0_PKcS9_S9_i(%class.D* %this, %class.B* dereferenceable(8) %p1, %class.B* dereferenceable(8) %p2, i8*, i8*, i8*, i32) local_unnamed_addr #0 comdat align 2 {
+; X86-LABEL: @_ZN1D7compareI1BS1_EEbRKT_RKT0_PKcS9_S9_i(
+; X86-NEXT:  entry:
+; X86-NEXT:    [[REF_TMP:%.*]] = alloca [[CLASS_C:%.*]], align 1
+; X86-NEXT:    [[C_I:%.*]] = getelementptr inbounds [[CLASS_B:%.*]], %class.B* [[P2:%.*]], i64 0, i32 0
+; X86-NEXT:    [[TMP4:%.*]] = load i32, i32* [[C_I]], align 4, !tbaa !2
+; X86-NEXT:    [[C2_I:%.*]] = getelementptr inbounds [[CLASS_B]], %class.B* [[P1:%.*]], i64 0, i32 0
+; X86-NEXT:    [[TMP5:%.*]] = load i32, i32* [[C2_I]], align 4, !tbaa !2
+; X86-NEXT:    [[CMP_ENTRY:%.*]] = icmp eq i32 [[TMP4]], [[TMP5]]
+; X86-NEXT:    br i1 [[CMP_ENTRY]], label [[BB1:%.*]], label [[BB2:%.*]]
+; X86:       bb1:
+; X86-NEXT:    [[M_I:%.*]] = getelementptr inbounds [[CLASS_B]], %class.B* [[P2]], i64 0, i32 1
+; X86-NEXT:    [[TMP6:%.*]] = load i32, i32* [[M_I]], align 4, !tbaa !7
+; X86-NEXT:    [[M3_I:%.*]] = getelementptr inbounds [[CLASS_B]], %class.B* [[P1]], i64 0, i32 1
+; X86-NEXT:    [[TMP7:%.*]] = load i32, i32* [[M3_I]], align 4, !tbaa !7
+; X86-NEXT:    [[CMP1:%.*]] = icmp eq i32 [[TMP6]], [[TMP7]]
+; X86-NEXT:    br label [[BB2]]
+; X86:       bb2:
+; X86-NEXT:    [[TMP8:%.*]] = phi i1 [ false, [[ENTRY:%.*]] ], [ [[CMP1]], [[BB1]] ]
+; X86-NEXT:    [[FAILUREREPORTINGMODE:%.*]] = getelementptr inbounds [[CLASS_D:%.*]], %class.D* [[THIS:%.*]], i64 0, i32 0
+; X86-NEXT:    [[TMP9:%.*]] = load i32, i32* [[FAILUREREPORTINGMODE]], align 4, !tbaa !8
+; X86-NEXT:    [[COND:%.*]] = icmp eq i32 [[TMP9]], 0
+; X86-NEXT:    br i1 [[COND]], label [[BB3:%.*]], label [[SW_EPILOG:%.*]]
+; X86:       bb3:
+; X86-NEXT:    br i1 [[CMP_ENTRY]], label [[BB4:%.*]], label [[BB5:%.*]]
+; X86:       bb4:
+; X86-NEXT:    [[M_I_I:%.*]] = getelementptr inbounds [[CLASS_B]], %class.B* [[P2]], i64 0, i32 1
+; X86-NEXT:    [[TMP10:%.*]] = load i32, i32* [[M_I_I]], align 4, !tbaa !7
+; X86-NEXT:    [[M3_I_I:%.*]] = getelementptr inbounds [[CLASS_B]], %class.B* [[P1]], i64 0, i32 1
+; X86-NEXT:    [[TMP11:%.*]] = load i32, i32* [[M3_I_I]], align 4, !tbaa !7
+; X86-NEXT:    [[CMP4:%.*]] = icmp eq i32 [[TMP10]], [[TMP11]]
+; X86-NEXT:    br label [[BB5]]
+; X86:       bb5:
+; X86-NEXT:    [[TMP12:%.*]] = phi i1 [ false, [[BB3]] ], [ [[CMP4]], [[BB4]] ]
+; X86-NEXT:    [[TMP13:%.*]] = getelementptr inbounds [[CLASS_C]], %class.C* [[REF_TMP]], i64 0, i32 0
+; X86-NEXT:    br label [[SW_EPILOG]]
+; X86:       sw.epilog:
+; X86-NEXT:    ret i1 [[TMP8]]
+;
+entry:
+  %ref.tmp = alloca %class.C, align 1
+  %c.i = getelementptr inbounds %class.B, %class.B* %p2, i64 0, i32 0
+  %4 = load i32, i32* %c.i, align 4, !tbaa !2
+  %c2.i = getelementptr inbounds %class.B, %class.B* %p1, i64 0, i32 0
+  %5 = load i32, i32* %c2.i, align 4, !tbaa !2
+  %cmp_entry = icmp eq i32 %4, %5
+  br i1 %cmp_entry, label %bb1, label %bb2
+
+bb1:                                       ; preds = %entry
+  %m.i = getelementptr inbounds %class.B, %class.B* %p2, i64 0, i32 1
+  %6 = load i32, i32* %m.i, align 4, !tbaa !7
+  %m3.i = getelementptr inbounds %class.B, %class.B* %p1, i64 0, i32 1
+  %7 = load i32, i32* %m3.i, align 4, !tbaa !7
+  %cmp1 = icmp eq i32 %6, %7
+  br label %bb2
+
+bb2:                               ; preds = %entry, %bb1
+  %8 = phi i1 [ false, %entry ], [ %cmp1, %bb1 ]
+  %failureReportingMode = getelementptr inbounds %class.D, %class.D* %this, i64 0, i32 0
+  %9 = load i32, i32* %failureReportingMode, align 4, !tbaa !8
+  %cond = icmp eq i32 %9, 0
+  br i1 %cond, label %bb3, label %sw.epilog
+
+bb3:                                            ; preds = %bb2
+  br i1 %cmp_entry, label %bb4, label %bb5
+
+bb4:                                     ; preds = %bb3
+  %m.i.i = getelementptr inbounds %class.B, %class.B* %p2, i64 0, i32 1
+  %10 = load i32, i32* %m.i.i, align 4, !tbaa !7
+  %m3.i.i = getelementptr inbounds %class.B, %class.B* %p1, i64 0, i32 1
+  %11 = load i32, i32* %m3.i.i, align 4, !tbaa !7
+  %cmp4 = icmp eq i32 %10, %11
+  br label %bb5
+
+bb5:                          ; preds = %bb3, %bb4
+  %12 = phi i1 [ false, %bb3 ], [ %cmp4, %bb4 ]
+  %13 = getelementptr inbounds %class.C, %class.C* %ref.tmp, i64 0, i32 0
+  br label %sw.epilog
+
+sw.epilog:                                        ; preds = %bb2
+  ret i1 %8
+}
+
+attributes #0 = { uwtable "correctly-rounded-divide-sqrt-fp-math"="false" "disable-tail-calls"="false" "less-precise-fpmad"="false" "no-frame-pointer-elim"="false" "no-infs-fp-math"="false" "no-jump-tables"="false" "no-nans-fp-math"="false" "no-signed-zeros-fp-math"="false" "no-trapping-math"="false" "stack-protector-buffer-size"="8" "target-cpu"="x86-64" "target-features"="+fxsr,+mmx,+sse,+sse2,+x87" "unsafe-fp-math"="false" "use-soft-float"="false" }
+attributes #1 = { "correctly-rounded-divide-sqrt-fp-math"="false" "disable-tail-calls"="false" "less-precise-fpmad"="false" "no-frame-pointer-elim"="false" "no-infs-fp-math"="false" "no-nans-fp-math"="false" "no-signed-zeros-fp-math"="false" "no-trapping-math"="false" "stack-protector-buffer-size"="8" "target-cpu"="x86-64" "target-features"="+fxsr,+mmx,+sse,+sse2,+x87" "unsafe-fp-math"="false" "use-soft-float"="false" }
+attributes #2 = { argmemonly nounwind }
+attributes #3 = { nounwind }
+
+!llvm.module.flags = !{!0}
+!llvm.ident = !{!1}
+
+!0 = !{i32 1, !"wchar_size", i32 4}
+!1 = !{!"clang version 7.0.0 (trunk 326236) (llvm/trunk 326244)"}
+!2 = !{!3, !4, i64 0}
+!3 = !{!"_ZTS1B", !4, i64 0, !4, i64 4}
+!4 = !{!"int", !5, i64 0}
+!5 = !{!"omnipotent char", !6, i64 0}
+!6 = !{!"Simple C++ TBAA"}
+!7 = !{!3, !4, i64 4}
+!8 = !{!9, !10, i64 0}
+!9 = !{!"_ZTS1D", !10, i64 0}
+!10 = !{!"_ZTSN1A20FailureReportingModeE", !5, i64 0}