Another VC compiler optimization bug:(

Last week, my leader had another annoying problem, a snippet like below is not working at all. The GetBool function will assign the 2nd argument “value” to true internally, while when returning, it is still false.

   1: IMyNameValues src;

   2:  

   3: ...

   4:  

   5: if (destField == nullptr)

   6: {

   7:     bool value;

   8:     System::String^ key = strConverter.Utf8ToString(srcField.Name);

   9:     mGuts->CheckCode(src.GetBool(srcField.Name, value));

  10:     dest->AddField(key, value);

  11: }

  12: else if (destField->Type->DotNetType->Equals(System::Boolean::typeid))

  13: {

  14:     bool value;

  15:     const char* pName = srcField.Name;

  16:     mGuts->CheckCode((long)src.GetBool(srcField.Name, value));

  17:     dest->AddField(destField, value);

  18: }

  19:  

  20: ...

  21:  

  22: //the GetBool function's signature is like:

  23: long GetBool(const string& name, bool& value);

After debugging the codes, and found an interesting thing, the “&value” before entering the GetBool function is different from the one inside the function. Then, reading the disassembly code, and manually calculating the local variable address on the call stack (ebp + xxxx), it is really different from the actual address of the local variable.

Then, if I changed the name of the local variable “value”, making different for the if/else branches, and it works!

   1: if (destField == nullptr)

   2: {

   3:     bool value;

   4:     System::String^ key = strConverter.Utf8ToString(srcField.Name);

   5:     mGuts->CheckCode(src.GetBool(srcField.Name, value));

   6:     dest->AddField(key, value);

   7: }

   8: else if (destField->Type->DotNetType->Equals(System::Boolean::typeid))

   9: {

  10:     bool value1;

  11:     const char* pName = srcField.Name;

  12:     mGuts->CheckCode((long)src.GetBool(srcField.Name, value1));

  13:     dest->AddField(destField, value1);

  14: }

And if disabling compiler optimization, also fine. It seems to be a compiler optimization bug, And I tried to reproduce it with some sample small snippet, could not reproduce. Since the product codes are kind of over-designed, introducing too many unnecessary concepts, for instance, virtual inheritance, RTTI, CLI, and etc, also involving a bunch of mangled classes. I had no time to trim them to get a simple case.

Another simple solution is move the inner local variable forward, it also works, e.g.:

   1: bool value;

   2: if (destField == nullptr)

   3: {

   4:     System::String^ key = strConverter.Utf8ToString(srcField.Name);

   5:     mGuts->CheckCode(src.GetBool(srcField.Name, value));

   6:     dest->AddField(key, value);

   7: }

   8: else if (destField->Type->DotNetType->Equals(System::Boolean::typeid))

   9: {

  10:     const char* pName = srcField.Name;

  11:     mGuts->CheckCode((long)src.GetBool(srcField.Name, value));

  12:     dest->AddField(destField, value);

  13: }