commit 8eb617d719bdc6a4ed7773925d2421b9bbdd4b7a
Author: Wei Mi <wmi@google.com>
Date:   Tue Sep 7 19:33:36 2021 -0700

    [SampleFDO] Allow forward compatibility when adding a new section for extbinary
    format.
    
    Currently when we add a new section in the profile format and generate a profile
    containing the new section, older compiler which reads the new profile will
    issue an error. The forward incompatibility can cause unnecessary churn when
    extending the profile. This patch removes the incompatibility when adding a new
    section for extbinary format.
    
    Differential Revision: https://reviews.llvm.org/D109398

diff --git a/llvm/include/llvm/ProfileData/SampleProf.h b/llvm/include/llvm/ProfileData/SampleProf.h
index 043e40260307..124e07a93071 100644
--- a/llvm/include/llvm/ProfileData/SampleProf.h
+++ b/llvm/include/llvm/ProfileData/SampleProf.h
@@ -149,8 +149,9 @@ static inline std::string getSecName(SecType Type) {
     return "CSNameTableSection";
   case SecLBRProfile:
     return "LBRProfileSection";
+  default:
+    return "UnknownSection";
   }
-  llvm_unreachable("A SecType has no name for output");
 }
 
 // Entry type of section header table used by SampleProfileExtBinaryBaseReader
diff --git a/llvm/include/llvm/ProfileData/SampleProfReader.h b/llvm/include/llvm/ProfileData/SampleProfReader.h
index f9a7830335bb..169ba28965fb 100644
--- a/llvm/include/llvm/ProfileData/SampleProfReader.h
+++ b/llvm/include/llvm/ProfileData/SampleProfReader.h
@@ -786,6 +786,8 @@ private:
   virtual std::error_code verifySPMagic(uint64_t Magic) override;
   virtual std::error_code
   readCustomSection(const SecHdrTableEntry &Entry) override {
+    // Update the data reader pointer to the end of the section.
+    Data += Entry.Size;
     return sampleprof_error::success;
   };
 
diff --git a/llvm/test/tools/llvm-profdata/Inputs/unknown.section.extbin.profdata b/llvm/test/tools/llvm-profdata/Inputs/unknown.section.extbin.profdata
new file mode 100644
index 000000000000..b7d0ddbf3657
Binary files /dev/null and b/llvm/test/tools/llvm-profdata/Inputs/unknown.section.extbin.profdata differ
diff --git a/llvm/test/tools/llvm-profdata/forward-compatible.test b/llvm/test/tools/llvm-profdata/forward-compatible.test
new file mode 100644
index 000000000000..50c79a0d3e51
--- /dev/null
+++ b/llvm/test/tools/llvm-profdata/forward-compatible.test
@@ -0,0 +1,26 @@
+RUN: llvm-profdata show -sample %S/Inputs/unknown.section.extbin.profdata | FileCheck %s
+RUN: llvm-profdata show -sample -show-sec-info-only %S/Inputs/unknown.section.extbin.profdata | FileCheck %s -check-prefix=HDR
+
+# The input unknown.section.extbin.profdata contains an unknown section type
+# which uses an enum value which won't be used in the near future. Check
+# llvm-profdata can read it without problem. This is to ensure adding new
+# section in extbinary format for SampleFDO won't cause forward compatibility
+# issue.
+
+CHECK: Function: main: 366846, 0, 3 sampled lines
+CHECK-NEXT: Samples collected in the function's body {
+CHECK-NEXT:   2.1: 60401
+CHECK-NEXT:   3: 0
+CHECK-NEXT:   4: 0
+CHECK-NEXT: }
+
+HDR: ProfileSummarySection - Offset: 242, Size: 121, Flags: {}
+HDR-NEXT: NameTableSection - Offset: 363, Size: 6, Flags: {}
+HDR-NEXT: UnknownSection - Offset: 369, Size: 1, Flags: {}
+HDR-NEXT: FuncOffsetTableSection - Offset: 391, Size: 3, Flags: {}
+HDR-NEXT: LBRProfileSection - Offset: 370, Size: 21, Flags: {}
+HDR-NEXT: ProfileSymbolListSection - Offset: 391, Size: 0, Flags: {}
+HDR-NEXT: FunctionMetadata - Offset: 394, Size: 0, Flags: {}
+HDR-NEXT: Header Size: 242
+HDR-NEXT: Total Sections Size: 152
+HDR-NEXT: File Size: 394