Re: -ffunction-sections and profiling

[Jeff Law <law at redhat dot com>]

Tim Abbott wrote:
> I'd like to be able to compile the Linux kernel with both 
> -ffunction-sections and profiling (-pg).  Using gcc with both 
> -ffunction-sections and -pg causes the following warning to be printed:
>
> "warning: -ffunction-sections disabled; it makes profiling impossible"
>
> and indeed -ffunction-sections is disabled.  I've not been able to 
> find any documentation or mailing list threads that explain this 
> conflict, though the discussion starting at [1] speculates it is 
> related to the labels emitted for profiling support, and notes that 
> the code for disabling -ffunction-sections was written when 
> -ffunction-sections was introduced back in 1996.
>
> However, I've compiled gcc 4.3.2 on i386 after removing the code that 
> disables -ffunction-sections when profiling is enabled.  It does 
> generate working -ffunction-sections code that has mcount calls at the 
> start of each function.  I've also tried gprof with a simple program 
> compiled with -ffunction-sections -pg, and it gave similar (though 
> definitely not identical) results to those from using gprof with the 
> same program compiled with just -pg.
>
> So, why does gcc disable -ffunction-sections when compiling with -pg?
Because it didn't work way back when the code was originally written.  
If you wanted to do some testing, test function-sections with -pg on the 
PA  (HP-UX) or Sparc (Solaris)  targets.  That's where the problem is 
most likely to manifest itself.  If I had to hazard a guess I'd suspect 
something along the lines of not being able to ensure that one or more 
marker labels are in the same section as the function itself. 

Jeff