[llvm] [ProfData] Improve efficiency of reader (PR #169730)

[Snehasish Kumar via llvm-commits]

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@@ -401,12 +401,27 @@ std::error_code ProfileSymbolList::read(const uint8_t *Data,
   const char *ListStart = reinterpret_cast<const char *>(Data);
   uint64_t Size = 0;
   uint64_t StrNum = 0;
+  uint64_t ExpectedCount = 0;
+
+  // Scan forward to see how many elements we expect.
+  while (Size < ListSize) {
+    if (ListStart[Size] == '\0') ExpectedCount++;
+    Size++;
+  }
+
+  reserve(ExpectedCount);
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snehasish wrote:

Should this be `reserve(std::min(ExpectedCount, ProfileSymbolListCutOff))`?

https://github.com/llvm/llvm-project/pull/169730