Index: llvm/lib/Transforms/Utils/SimplifyCFG.cpp
===================================================================
--- llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -2006,6 +2006,21 @@
   BasicBlock *BB = BI->getParent();
   BasicBlock *EndBB = ThenBB->getTerminator()->getSuccessor(0);
 
+  // Check how expensive it will be to insert the necessary selects.
+  unsigned CostOfSelects = 0;
+  for (PHINode &PN : EndBB->phis()) {
+    unsigned OrigI = PN.getBasicBlockIndex(BB);
+    unsigned ThenI = PN.getBasicBlockIndex(ThenBB);
+    Value *OrigV = PN.getIncomingValue(OrigI);
+    Value *ThenV = PN.getIncomingValue(ThenI);
+    if (OrigV != ThenV)
+      CostOfSelects +=
+          TTI.getCmpSelInstrCost(Instruction::Select, PN.getType(), nullptr,
+                                 TargetTransformInfo::TCK_SizeAndLatency);
+  }
+  if (CostOfSelects > PHINodeFoldingThreshold * TargetTransformInfo::TCC_Basic)
+    return false;
+
   // If ThenBB is actually on the false edge of the conditional branch, remember
   // to swap the select operands later.
   bool Invert = false;
Index: llvm/test/Transforms/SimplifyCFG/ARM/select-costs.ll
===================================================================
--- /dev/null
+++ llvm/test/Transforms/SimplifyCFG/ARM/select-costs.ll
@@ -0,0 +1,105 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -mtriple=thumbv8.1m.main -simplifycfg -S %s -o - | FileCheck %s --check-prefix=V8M
+; RUN: opt -mtriple=thumbv8m.main -simplifycfg -S %s -o - | FileCheck %s --check-prefix=V8M
+; RUN: opt -mtriple=thumbv8m.base -simplifycfg -S %s -o - | FileCheck %s --check-prefix=V8M
+; RUN: opt -mtriple=armv8a -simplifycfg -S %s -o - | FileCheck %s --check-prefix=V8A
+
+; Test that the phis from for.inc.preheader aren't hoisted into sw.bb92 because
+; the cost is too great - we can make a better decision later on.
+define i32 @multiple_spec_select_costs(i8* %a, i32* %idx, i8 %in) {
+; V8M-LABEL: @multiple_spec_select_costs(
+; V8M-NEXT:  entry:
+; V8M-NEXT:    [[GEP_A_2:%.*]] = getelementptr inbounds i8, i8* [[A:%.*]], i32 2
+; V8M-NEXT:    [[INCDEC_PTR109_C4:%.*]] = getelementptr inbounds i8, i8* [[A]], i32 3
+; V8M-NEXT:    [[ZERO:%.*]] = icmp ne i8 [[IN:%.*]], 0
+; V8M-NEXT:    br i1 [[ZERO]], label [[FOR_INC_PREHEADER:%.*]], label [[SW_BB92:%.*]]
+; V8M:       sw.bb92:
+; V8M-NEXT:    [[C_OFF_I150:%.*]] = add i8 [[IN]], -48
+; V8M-NEXT:    [[UGT_9:%.*]] = icmp ugt i8 [[C_OFF_I150]], 9
+; V8M-NEXT:    br i1 [[UGT_9]], label [[FOR_INC_PREHEADER]], label [[SELECT_UNFOLD198:%.*]]
+; V8M:       select.unfold198:
+; V8M-NEXT:    br label [[FOR_INC_PREHEADER]]
+; V8M:       for.inc.preheader:
+; V8M-NEXT:    [[STR_PH_0:%.*]] = phi i8* [ [[INCDEC_PTR109_C4]], [[SELECT_UNFOLD198]] ], [ [[INCDEC_PTR109_C4]], [[SW_BB92]] ], [ [[GEP_A_2]], [[ENTRY:%.*]] ]
+; V8M-NEXT:    [[CMP:%.*]] = phi i1 [ true, [[SELECT_UNFOLD198]] ], [ false, [[SW_BB92]] ], [ false, [[ENTRY]] ]
+; V8M-NEXT:    [[PHI_RES:%.*]] = phi i32 [ 7, [[SELECT_UNFOLD198]] ], [ 1, [[SW_BB92]] ], [ 1, [[ENTRY]] ]
+; V8M-NEXT:    br label [[FOR_INC:%.*]]
+; V8M:       for.inc:
+; V8M-NEXT:    [[STR_PH_1:%.*]] = phi i8* [ [[INCDEC_PTR109:%.*]], [[FOR_BODY:%.*]] ], [ [[STR_PH_0]], [[FOR_INC_PREHEADER]] ]
+; V8M-NEXT:    [[INCDEC_PTR109]] = getelementptr inbounds i8, i8* [[STR_PH_1]], i32 1
+; V8M-NEXT:    [[LOAD_1:%.*]] = load i8, i8* [[INCDEC_PTR109]], align 1
+; V8M-NEXT:    [[TOBOOL:%.*]] = icmp ne i8 [[LOAD_1]], 0
+; V8M-NEXT:    [[AND:%.*]] = and i1 [[CMP]], [[TOBOOL]]
+; V8M-NEXT:    br i1 [[AND]], label [[FOR_BODY]], label [[EXIT:%.*]]
+; V8M:       for.body:
+; V8M-NEXT:    [[CMP_1:%.*]] = icmp eq i8 [[LOAD_1]], 1
+; V8M-NEXT:    br i1 [[CMP_1]], label [[FOR_INC]], label [[SW_BB92]]
+; V8M:       exit:
+; V8M-NEXT:    ret i32 [[PHI_RES]]
+;
+; V8A-LABEL: @multiple_spec_select_costs(
+; V8A-NEXT:  entry:
+; V8A-NEXT:    [[GEP_A_2:%.*]] = getelementptr inbounds i8, i8* [[A:%.*]], i32 2
+; V8A-NEXT:    [[INCDEC_PTR109_C4:%.*]] = getelementptr inbounds i8, i8* [[A]], i32 3
+; V8A-NEXT:    [[ZERO:%.*]] = icmp ne i8 [[IN:%.*]], 0
+; V8A-NEXT:    br i1 [[ZERO]], label [[FOR_INC_PREHEADER:%.*]], label [[SW_BB92:%.*]]
+; V8A:       sw.bb92:
+; V8A-NEXT:    [[C_OFF_I150:%.*]] = add i8 [[IN]], -48
+; V8A-NEXT:    [[UGT_9:%.*]] = icmp ugt i8 [[C_OFF_I150]], 9
+; V8A-NEXT:    [[SPEC_SELECT:%.*]] = select i1 [[UGT_9]], i1 false, i1 true
+; V8A-NEXT:    [[SPEC_SELECT1:%.*]] = select i1 [[UGT_9]], i32 1, i32 7
+; V8A-NEXT:    br label [[FOR_INC_PREHEADER]]
+; V8A:       for.inc.preheader:
+; V8A-NEXT:    [[STR_PH_0:%.*]] = phi i8* [ [[GEP_A_2]], [[ENTRY:%.*]] ], [ [[INCDEC_PTR109_C4]], [[SW_BB92]] ]
+; V8A-NEXT:    [[CMP:%.*]] = phi i1 [ false, [[ENTRY]] ], [ [[SPEC_SELECT]], [[SW_BB92]] ]
+; V8A-NEXT:    [[PHI_RES:%.*]] = phi i32 [ 1, [[ENTRY]] ], [ [[SPEC_SELECT1]], [[SW_BB92]] ]
+; V8A-NEXT:    br label [[FOR_INC:%.*]]
+; V8A:       for.inc:
+; V8A-NEXT:    [[STR_PH_1:%.*]] = phi i8* [ [[INCDEC_PTR109:%.*]], [[FOR_BODY:%.*]] ], [ [[STR_PH_0]], [[FOR_INC_PREHEADER]] ]
+; V8A-NEXT:    [[INCDEC_PTR109]] = getelementptr inbounds i8, i8* [[STR_PH_1]], i32 1
+; V8A-NEXT:    [[LOAD_1:%.*]] = load i8, i8* [[INCDEC_PTR109]], align 1
+; V8A-NEXT:    [[TOBOOL:%.*]] = icmp ne i8 [[LOAD_1]], 0
+; V8A-NEXT:    [[AND:%.*]] = and i1 [[CMP]], [[TOBOOL]]
+; V8A-NEXT:    br i1 [[AND]], label [[FOR_BODY]], label [[EXIT:%.*]]
+; V8A:       for.body:
+; V8A-NEXT:    [[CMP_1:%.*]] = icmp eq i8 [[LOAD_1]], 1
+; V8A-NEXT:    br i1 [[CMP_1]], label [[FOR_INC]], label [[SW_BB92]]
+; V8A:       exit:
+; V8A-NEXT:    ret i32 [[PHI_RES]]
+;
+entry:
+  %gep.a.2 = getelementptr inbounds i8, i8* %a, i32 2
+  %incdec.ptr109.c4 = getelementptr inbounds i8, i8* %a, i32 3
+  %zero = icmp ne i8 %in, 0
+  br i1 %zero , label %for.inc.preheader, label %sw.bb92
+
+sw.bb92:
+  %c.off.i150 = add i8 %in, -48
+  %ugt.9 = icmp ugt i8 %c.off.i150, 9
+  br i1 %ugt.9, label %for.inc.preheader, label %select.unfold198
+
+select.unfold198:
+  br label %for.inc.preheader
+
+for.inc.preheader:
+  %str.ph.0 = phi i8* [ %incdec.ptr109.c4, %select.unfold198 ], [ %incdec.ptr109.c4, %sw.bb92 ], [ %gep.a.2, %entry ]
+  %cmp = phi i1 [ true, %select.unfold198 ], [ false, %sw.bb92 ], [ false, %entry ]
+  %phi.res = phi i32 [ 7, %select.unfold198 ], [ 1, %sw.bb92 ], [ 1, %entry ]
+  br label %for.inc
+
+for.inc:
+  %str.ph.1 = phi i8* [ %incdec.ptr109, %for.body ], [ %str.ph.0, %for.inc.preheader ]
+  %incdec.ptr109 = getelementptr inbounds i8, i8* %str.ph.1, i32 1
+  %load.1 = load i8, i8* %incdec.ptr109, align 1
+  %tobool = icmp ne i8 %load.1, 0
+  %and = and i1 %cmp, %tobool
+  br i1 %and, label %for.body, label %exit
+
+for.body:
+  %cmp.1 = icmp eq i8 %load.1, 1
+  br i1 %cmp.1, label %for.inc, label %sw.bb92
+
+exit:
+  ret i32 %phi.res
+}
+