[compiler-rt] [llvm] [InstrFDO]Allow indexed profile reader to parse compatible future versions and returns errors for incompatible ones. (PR #88212)

[Mingming Liu via llvm-commits]

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@@ -710,9 +745,28 @@ Error InstrProfWriter::writeImpl(ProfOStream &OS) {
   OS.write(0);
 
   uint64_t VTableNamesOffset = OS.tell();
-  if (!WritePrevVersion)
+  OS.write(0);
+
+  // Record the offset of 'Header::Size' field.
+  const uint64_t OnDiskHeaderSizeOffset = OS.tell();
+
+  if (!WritePrevVersion) {
+    // Reserve the space for `OnDiskByteSize` to allow back patching later.
+    OS.write(0);
+
+    // Write the minimum compatible version required.
+    OS.write(minProfileReaderVersion());
+  }
+
+  // This is a test-only path to append dummy header fields.
+  // NOTE: please write all other header fields before this one.
+  if (AppendAdditionalHeaderFields)
     OS.write(0);
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minglotus-6 wrote:

I wonder if I should move this to a member function `InstrProfWriter::TestOnlyAppendAdditionalHeaaderFields` so people don't need to carefully write real header fields before this line (or spend time debugging what goes wrong).

An alternative (that i'm leaning towards) is to introduce `InstrProfWriter::writeHeader` method to encapsulate the write of header, but with back patching (`InstrProfWriter::writeHeader` records the byte offset to patch after payloads are written), it's a bit of refactoring work so decided to ask for thoughts first. 

https://github.com/llvm/llvm-project/pull/88212