Re: Questions about __attribute((cold))

[John Fine <johnsfine at verizon dot net>]

I want to clarify a few things that probably weren't clear in my initial 
question.

I'm asking primarily about a feature which I think is new in GCC 4.3.  I 
downloaded, built, and tested just version 4.4.0 in case there has been 
recent further work on this new feature, but I'm asking this question 
primarily about version 4.3.  If you can help me understand how 4.3 
behaves, please don't decide not to reply based on my use of 4.4.0 for 
my own testing.

In the example below, I would like to know if there is some way to 
define rarely_executed() so it's use will tell the compiler that xxx4 
will be executed far less often than xxx2 and xxx5.  I thought 
__attribute__((cold)) might do that and I tried that.  But if some other 
way of defining rarely_executed() will do that, I'd like to know.

I would also like to know whether __attribute__((cold)) does or doesn't 
change the compilation of the block containing the call.  The 
documentation seems to say it does.  I looked at the gcc source code and 
there seems to be code to achieve that.  But I haven't found the effect 
in the actual generated asm code.

John Fine wrote:

> I am using (GCC) 4.4.0 20080319 (experimental)


> inline void __attribute__((cold)) rarely_execute() {}
> while (xxx1) {
>   xxx2;
>   if (xxx3) {
>       rarely_execute();
>       xxx4; }
>   xxx5; }