Index: llvm/test/tools/llvm-profdata/Inputs/same-name-1.proftext
===================================================================
--- /dev/null
+++ llvm/test/tools/llvm-profdata/Inputs/same-name-1.proftext
@@ -0,0 +1,10 @@
+# IR level Instrumentation Flag
+:ir
+main
+# Func Hash:
+12884901887
+# Num Counters:
+1
+# Counter Values:
+1
+
Index: llvm/test/tools/llvm-profdata/Inputs/same-name-2.proftext
===================================================================
--- /dev/null
+++ llvm/test/tools/llvm-profdata/Inputs/same-name-2.proftext
@@ -0,0 +1,10 @@
+# IR level Instrumentation Flag
+:ir
+main
+# Func Hash:
+12884901887
+# Num Counters:
+1
+# Counter Values:
+2
+
Index: llvm/test/tools/llvm-profdata/Inputs/same-name-3.proftext
===================================================================
--- /dev/null
+++ llvm/test/tools/llvm-profdata/Inputs/same-name-3.proftext
@@ -0,0 +1,16 @@
+_Z3bari:20301:1437
+ 1: 1437
+_Z3fooi:7711:610
+ 1: 610
+main:184019:0
+ 4: 534
+ 4.2: 534
+ 5: 1075
+ 5.1: 1075
+ 6: 2080
+ 7: 534
+ 9: 2064 _Z3bari:1471 _Z3fooi:631
+ 10: inline1:1000
+  1: 1000
+ 10: inline2:2000
+  1: 2000
Index: llvm/test/tools/llvm-profdata/Inputs/same-name-4.proftext
===================================================================
--- /dev/null
+++ llvm/test/tools/llvm-profdata/Inputs/same-name-4.proftext
@@ -0,0 +1,16 @@
+_Z3bari:40602:2874
+ 1: 2874
+_Z3fooi:15422:1220
+ 1: 1220
+main:368038:0
+ 4: 1068
+ 4.2: 1068
+ 5: 2150
+ 5.1: 2150
+ 6: 4160
+ 7: 1068
+ 9: 4128 _Z3bari:2942 _Z3fooi:1262
+ 10: inline1:2000
+  1: 2000
+ 10: inline2:4000
+  1: 4000
Index: llvm/test/tools/llvm-profdata/same-filename.test
===================================================================
--- /dev/null
+++ llvm/test/tools/llvm-profdata/same-filename.test
@@ -0,0 +1,27 @@
+Test the input file names overlap with the output file names.
+
+This is OK for merging instrument profiles.
+RUN: cp %S/Inputs/same-name-1.proftext %t.0.proftext
+RUN: llvm-profdata merge -o %t.0.proftext -text %t.0.proftext %t.0.proftext
+RUN: llvm-profdata show -counts -all-functions %t.0.proftext -o %t_show_0
+RUN: llvm-profdata show -counts -all-functions %S/Inputs/same-name-2.proftext -o %t_show_1
+RUN: diff %t_show_0 %t_show_1
+RUN: llvm-profdata merge -o %t.1.profdata %S/Inputs/same-name-1.proftext
+RUN: llvm-profdata merge -o %t.1.profdata %t.1.profdata %t.1.profdata
+RUN: llvm-profdata show -counts -all-functions %t.1.profdata -o %t_show_2
+RUN: diff %t_show_2 %t_show_1
+
+We report error for the show command.
+RUN: not llvm-profdata show -o %t.1.profdata %t.1.profdata 2>&1 | FileCheck %s
+CHECK: llvm-profdata show: Input file name cannot be the same as the output file name!
+
+This is OK for merging sample fdo profiles.
+RUN: cp %S/Inputs/same-name-3.proftext %t.3.proftext
+RUN: llvm-profdata merge --sample -o %t.3.proftext -text %t.3.proftext %t.3.proftext
+RUN: llvm-profdata show --sample -counts -all-functions %t.3.proftext -o %t_show_3
+RUN: llvm-profdata show --sample -counts -all-functions %S/Inputs/same-name-4.proftext -o %t_show_4
+RUN: diff %t_show_3 %t_show_4
+RUN: llvm-profdata merge --sample -o %t.5.profdata %S/Inputs/same-name-3.proftext
+RUN: llvm-profdata merge --sample -o %t.5.profdata %t.5.profdata %t.5.profdata
+RUN: llvm-profdata show --sample -counts -all-functions %t.5.profdata -o %t_show_6
+RUN: diff %t_show_6 %t_show_4
Index: llvm/tools/llvm-profdata/llvm-profdata.cpp
===================================================================
--- llvm/tools/llvm-profdata/llvm-profdata.cpp
+++ llvm/tools/llvm-profdata/llvm-profdata.cpp
@@ -317,11 +317,6 @@
       OutputFormat != PF_Text)
     exitWithError("Unknown format is specified.");
 
-  std::error_code EC;
-  raw_fd_ostream Output(OutputFilename.data(), EC, sys::fs::F_None);
-  if (EC)
-    exitWithErrorCode(EC, OutputFilename);
-
   std::mutex ErrorLock;
   SmallSet<instrprof_error, 4> WriterErrorCodes;
 
@@ -384,6 +379,11 @@
            WC->ErrWhence);
   }
 
+  std::error_code EC;
+  raw_fd_ostream Output(OutputFilename.data(), EC, sys::fs::F_None);
+  if (EC)
+    exitWithErrorCode(EC, OutputFilename);
+
   InstrProfWriter &Writer = Contexts[0]->Writer;
   if (OutputFormat == PF_Text) {
     if (Error E = Writer.writeText(Output))
@@ -433,12 +433,6 @@
                                StringRef OutputFilename,
                                ProfileFormat OutputFormat) {
   using namespace sampleprof;
-  auto WriterOrErr =
-      SampleProfileWriter::create(OutputFilename, FormatMap[OutputFormat]);
-  if (std::error_code EC = WriterOrErr.getError())
-    exitWithErrorCode(EC, OutputFilename);
-
-  auto Writer = std::move(WriterOrErr.get());
   StringMap<FunctionSamples> ProfileMap;
   SmallVector<std::unique_ptr<sampleprof::SampleProfileReader>, 5> Readers;
   LLVMContext Context;
@@ -473,6 +467,12 @@
       }
     }
   }
+  auto WriterOrErr =
+      SampleProfileWriter::create(OutputFilename, FormatMap[OutputFormat]);
+  if (std::error_code EC = WriterOrErr.getError())
+    exitWithErrorCode(EC, OutputFilename);
+
+  auto Writer = std::move(WriterOrErr.get());
   Writer->write(ProfileMap);
 }
 
@@ -1020,6 +1020,12 @@
   if (OutputFilename.empty())
     OutputFilename = "-";
 
+  if (!Filename.compare(OutputFilename)) {
+    errs() << sys::path::filename(argv[0])
+           << ": Input file name cannot be the same as the output file name!\n";
+    return 1;
+  }
+
   std::error_code EC;
   raw_fd_ostream OS(OutputFilename.data(), EC, sys::fs::F_Text);
   if (EC)