[llvm] [memprof] Add MemProf version (PR #86414)

[Teresa Johnson via llvm-commits]

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@@ -1230,10 +1231,38 @@ Error IndexedInstrProfReader::readHeader() {
             Header->MemProfOffset);
 
     const unsigned char *Ptr = Start + MemProfOffset;
-    // The value returned from RecordTableGenerator.Emit.
-    const uint64_t RecordTableOffset =
+
+    // Read the first 64-bit word, which may be RecordTableOffset in
+    // memprof::MemProfVersion0 or the MemProf version number in
+    // memprof::MemProfVersion1.
+    const uint64_t FirstWord =
         support::endian::readNext<uint64_t, llvm::endianness::little,
                                   unaligned>(Ptr);
+
+    memprof::MemProfVersion Version = memprof::MemProfVersion0;
+    if (static_cast<memprof::MemProfVersion>(FirstWord) ==
+        memprof::MemProfVersion1) {
+      // Everything is good.  We can proceed to deserialize the rest.
+      Version = memprof::MemProfVersion1;
+    } else if (FirstWord >= 24) {
+      // This is a heuristic/hack to detect memprof::MemProfVersion0,
+      // which does not have a version field in the header.
+      // In memprof::MemProfVersion0, FirstWord should be RecordTableOffset,
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teresajohnson wrote:

"will be RecordTableOffset"

https://github.com/llvm/llvm-project/pull/86414