Re: RFA: Inserting NOPs for profiling.

[Nick Clifton <nickc at redhat dot com>]

Hi Philip,

> >  If profiling is enabled (with -pg) then GCC will emit a NOP at the
> >  start of the file, between the .gcc2_compiled symbol and the start
> >  of the first function.  This ensures that the address of the first
> >  function is not the same as the address of the .gcc2_compiled
> >  symbol, and so ensures that the profiler's output does not refer to
> >  the first function as if it were the .gcc2_compiled symbol.
> >
> >  Unfortunately the code that is currently in GCC to generate this NOP
> >  does not work if the NOP pattern is a define_expand or if it is a
> >  define_insn which has a C body to compute the emitted assembler
> >  string.  In both cases the code will abort inside
> >  get_insn_template.  One particular case where this happens is the
> >  ARM port where the NOP pattern is complicated by the fact that there
> >  are two NOP instructions, one for ARM mode and one for Thumb mode.
> 
> Hi Nick,
> 
> Was there ever a resolution to this issue?  Profiling still seems to
> be busted  at least on the 3.0 branch. 

The issue was never resolved.  Jim Wilson pointed out that using the
special symbol .gcc2_compiled was actually a bad idea:

  http://gcc.gnu.org/ml/gcc-patches/2000-10/msg00477.html

It was suggested that putting symbols into a special section might be
a better solution:

  http://gcc.gnu.org/ml/gcc-patches/2000-10/msg00537.html

But I do not think anyone has actually gotten around to doing this.

The patch was never officially accepted, however, which is why it is
not in the sources.

Cheers
        Nick