[SampleFDO] Allow forward compatibility when adding a new section for extbinary format.

[wmi]

Summary

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.

[hoy]

lgtm

[wenlei]

That's a helpful change, thanks!

llvm/include/llvm/ProfileData/SampleProf.h

static inline std::string getSecName(SecType Type) {
   case SecFuncOffsetTable:
     return "FuncOffsetTableSection";
   case SecFuncMetadata:
     return "FunctionMetadata";
   case SecCSNameTable:
     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
 // and SampleProfileExtBinaryBaseWriter.
 struct SecHdrTableEntry {
   SecType Type;
   uint64_t Flags;
   uint64_t Offset;

llvm/include/llvm/ProfileData/SampleProfReader.h

public:
   virtual void setSkipFlatProf(bool Skip) override { SkipFlatProf = Skip; }
 };
 
 class SampleProfileReaderExtBinary : public SampleProfileReaderExtBinaryBase {
 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;
   };
 
 public:
   SampleProfileReaderExtBinary(std::unique_ptr<MemoryBuffer> B, LLVMContext &C,
                                SampleProfileFormat Format = SPF_Ext_Binary)
       : SampleProfileReaderExtBinaryBase(std::move(B), C, Format) {}
 

llvm/test/tools/llvm-profdata/forward-compatible.test

+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