[llvm] [SampleProfile] Fix UB in Demangler invocation. (PR #137659)

[Snehasish Kumar via llvm-commits]

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@@ -737,14 +737,13 @@ bool SampleProfileMatcher::functionMatchesProfileHelper(
     auto FunctionName = FName.str();
     if (Demangler.partialDemangle(FunctionName.c_str()))
       return std::string();
-    constexpr size_t MaxBaseNameSize = 65536;
-    std::vector<char> BaseNameBuf(MaxBaseNameSize, 0);
-    size_t BaseNameSize = MaxBaseNameSize;
-    char *BaseNamePtr =
-        Demangler.getFunctionBaseName(BaseNameBuf.data(), &BaseNameSize);
-    return (BaseNamePtr && BaseNameSize)
-               ? std::string(BaseNamePtr, BaseNameSize)
-               : std::string();
+    size_t BaseNameSize = 0;
+    char *BaseNamePtr = Demangler.getFunctionBaseName(nullptr, &BaseNameSize);
+    std::string Result = (BaseNamePtr && BaseNameSize)
+                             ? std::string(BaseNamePtr, BaseNameSize)
+                             : std::string();
+    free(BaseNamePtr);
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snehasish wrote:

Perhaps add a comment above this to explain the behaviour (nullptr and need to free) and point to https://gcc.gnu.org/onlinedocs/libstdc++/libstdc++-html-USERS-4.3/a01696.html ?

https://github.com/llvm/llvm-project/pull/137659