Index: lib/Transforms/Utils/SimplifyCFG.cpp
===================================================================
--- lib/Transforms/Utils/SimplifyCFG.cpp
+++ lib/Transforms/Utils/SimplifyCFG.cpp
@@ -1372,6 +1372,13 @@
     }
   }
 
+  // As the parent basic block terminator is a branch instruction which is
+  // removed at the end of the current transformation, use its previous
+  // instruction, as the reference insertion point.
+  Instruction *InsertPt = BIParent->size() > 1
+                              ? BIParent->getTerminator()->getPrevNode()
+                              : BIParent->getTerminator();
+
   // Okay, it is safe to hoist the terminator.
   Instruction *NT = I1->clone();
   BIParent->getInstList().insert(BI->getIterator(), NT);
@@ -1381,6 +1388,14 @@
     NT->takeName(I1);
   }
 
+  // The instruction NT being hoisted, is the terminator for the true branch,
+  // with debug location (DILocation) within that branch. We can't retain
+  // its original debug location value, otherwise 'select' instructions that
+  // are created from any PHI nodes, will take its debug location, giving
+  // the impression that those 'select' instructions are in the true branch,
+  // causing incorrect stepping, affecting the debug experience.
+  NT->setDebugLoc(InsertPt->getDebugLoc());
+
   IRBuilder<NoFolder> Builder(NT);
   // Hoisting one of the terminators from our successor is a great thing.
   // Unfortunately, the successors of the if/else blocks may have PHI nodes in
Index: test/CodeGen/X86/pr39187.ll
===================================================================
--- test/CodeGen/X86/pr39187.ll
+++ test/CodeGen/X86/pr39187.ll
@@ -0,0 +1,79 @@
+; RUN: opt < %s -S -simplifycfg | FileCheck %s
+
+; SimplifyCFG can hoist any common code in the 'then' and 'else' blocks to
+; the 'if' basic block.
+;
+; For the special case, when hoisting the terminator instruction, its debug
+; location keep references to its basic block, causing the debug information
+; to become ambiguous. It causes the debugger to display unreached lines.
+
+; Change the debug location associated with the hoisted instruction, to
+; the debug location from the insertion point in the 'if' block.
+
+; IR generated with:
+; clang -S -g -gno-column-info -O2 -emit-llvm pr39187.cpp -o pr39187.ll -mllvm -opt-bisect-limit=10
+
+; // pr39187.cpp
+; int main() {
+;   volatile int foo = 0;
+;
+;   int beards = 0;
+;   if (foo == 4)
+;     beards = 8;
+;   else
+;     beards = 4;
+;
+;   return beards;
+; }
+
+; CHECK-LABEL: entry
+; CHECK:  %foo = alloca i32, align 4
+; CHECK:  %foo.0..sroa_cast = bitcast i32* %foo to i8*
+; CHECK:  store volatile i32 0, i32* %foo, align 4
+; CHECK:  %foo.0. = load volatile i32, i32* %foo, align 4
+; CHECK:  %cmp = icmp eq i32 %foo.0., 4, !dbg !5
+; CHECK:  %. = select i1 %cmp, i32 8, i32 4, !dbg !5
+
+; ModuleID = 'pr39187.cpp'
+source_filename = "pr39187.cpp"
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-pc-linux-gnu"
+
+; Function Attrs: norecurse nounwind uwtable
+define dso_local i32 @main() local_unnamed_addr #0 {
+entry:
+  %foo = alloca i32, align 4
+  %foo.0..sroa_cast = bitcast i32* %foo to i8*
+  store volatile i32 0, i32* %foo, align 4
+  %foo.0. = load volatile i32, i32* %foo, align 4
+  %cmp = icmp eq i32 %foo.0., 4, !dbg !21
+  br i1 %cmp, label %if.then, label %if.else
+
+if.then:                                          ; preds = %entry
+  br label %if.end
+
+if.else:                                          ; preds = %entry
+  br label %if.end
+
+if.end:                                           ; preds = %if.else, %if.then!19
+  %beards.0 = phi i32 [ 8, %if.then ], [ 4, %if.else ]
+  ret i32 %beards.0
+}
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!3, !4}
+!llvm.ident = !{!6}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus, file: !1, producer: "clang version 8.0.0 (trunk 345575)", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, nameTableKind: None)
+!1 = !DIFile(filename: "pr39187.cpp", directory: ".")
+!3 = !{i32 2, !"Dwarf Version", i32 4}
+!4 = !{i32 2, !"Debug Info Version", i32 3}
+!6 = !{!"clang version 8.0.0 (trunk 345575)"}
+!7 = distinct !DISubprogram(name: "main", scope: !1, file: !1, line: 1, type: !8, isLocal: false, isDefinition: true, scopeLine: 1, flags: DIFlagPrototyped, isOptimized: true, unit: !0, retainedNodes: !11)
+!8 = !DISubroutineType(types: !9)
+!9 = !{!10}
+!10 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!11 = !{!14}
+!14 = !DILocalVariable(name: "beards", scope: !7, file: !1, line: 4, type: !10)
+!21 = !DILocation(line: 5, scope: !22)
+!22 = distinct !DILexicalBlock(scope: !7, file: !1, line: 5)