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
