What is the performance implication of converting to bool in C++?

[This question is related to but not the same as this one.]

My compiler warns about implicitly converting or casting certain types to bool whereas explicit conversions do not produce a warning:

long t = 0;
bool b = false;
b = t;                    // performance warning: forcing long to bool
b = (bool)t;              // performance warning
b = bool(t);              // performance warning
b = static_cast<bool>(t); // performance warning
b = t ? true : false;     // ok, no warning
b = t != 0;               // ok
b = !!t;                  // ok

This is with Visual C++ 2008 but I suspect other compilers may have similar warnings.

So my question is: what is the performance implication of casting/converting to bool? Does explicit conversion have better performance in some circumstance (e.g., for certain target architectures or processors)? Does implicit conversion somehow confuse the optimizer?

Microsoft's explanation of their warning is not particularly helpful. They imply that there is a good reason but they don't explain it.

I was puzzled by this behaviour, until I found this link:

http://connect.microsoft.com/VisualStudio/feedback/ViewFeedback.aspx?FeedbackID=99633

Apparently, coming from the Microsoft Developer who "owns" this warning:

This warning is surprisingly helpful, and found a bug in my code just yesterday. I think Martin is taking "performance warning" out of context.

It's not about the generated code, it's about whether or not the programmer has signalled an intent to change a value from int to bool. There is a penalty for that, and the user has the choice to use "int" instead of "bool" consistently (or more likely vice versa) to avoid the "boolifying" codegen. [...]

It is an old warning, and may have outlived its purpose, but it's behaving as designed here.

So it seems to me the warning is more about style and avoiding some mistakes than anything else.

Hope this will answer your question...

:-p

The performance is identical across the board. It involves a couple of instructions on x86, maybe 3 on some other architectures.

On x86 / VC++, they all do

cmp DWORD PTR [whatever], 0
setne al

GCC generates the same thing, but without the warnings (at any warning-level).

The performance warning does actually make a little bit of sense. I've had it as well and my curiousity led me to investigate with the disassembler. It is trying to tell you that the compiler has to generate some code to coerce the value to either 0 or 1. Because you are insisting on a bool, the old school C idea of 0 or anything else doesn't apply.

You can avoid that tiny performance hit if you really want to. The best way is to avoid the cast altogether and use a bool from the start. If you must have an int, you could just use if( int ) instead of if( bool ). The code generated will simply check whether the int is 0 or not. No extra code to make sure the value is 1 if it's not 0 will be generated.

Sounds like premature optimization to me. Are you expecting that the performance of the cast to seriously effect the performance of your app? Maybe if you are writing kernel code or device drivers but in most cases, they all should be ok.

As far as I know, there is no warning on any other compiler for this. The only way I can think that this would cause a performance loss is that the compiler has to compare the entire integer to 0 and then assign the bool appropriately (unlike a conversion such as a char to bool, where the result can be copied over because a bool is one byte and so they are effectively the same), or an integral conversion which involves copying some or all of the source to the destination, possibly after a zero of the destination if it's bigger than the source (in terms of memory).

It's yet another one of Microsoft's useless and unhelpful ideas as to what constitutes good code, and leads us to have to put up with stupid definitions like this:

template <typename T>
inline bool to_bool (const T& t)
  { return t ? true : false; }