insertion of profiling code

[Kenneth Zadeck <zadeck at naturalbridge dot com>]

When I was working on the detecting const and pure functions, I ran 
across the following comments in passes.c:


 /* It may make more sense to mark constant functions after dead code is
    eliminated by life_analysis, but we need to do it early, as 
-fprofile-arcs
    may insert code making function non-constant, but we still must 
consider
    it as constant, otherwise -fbranch-probabilities will not read data 
back.

    life_analysis rarely eliminates modification of external memory.

    FIXME: now with tree based profiling we are in the trap described above
    again.  It seems to be easiest to disable the optimization for time
    being before the problem is either solved by moving the transformation
    to the IPA level (we need the CFG for this) or the very early 
optimization
    passes are made to ignore the const/pure flags so code does not 
change.  */

Now that I am doing the detection of pure and const functions in the ipa 
pass there does appear to be the possibility of fixing this problem.  
(my code is still a secret, but I am planing to post a patch this 
weekend.) Unfortunately, my code still seems to do this detection too 
late since my analysis occurs over the cfg and the code appears to be 
being added when the cfg is being built.

The two possible solutions to this problem appear to be:
1) The addition of the profiling code can be decoupled from the creation 
of the cfg and could be added at any later pass where the cfg is still 
around.

2) Some annotation could be attached to the added code that I could 
teach my pass to ignore.

I am an advocate of the first plan.

Comments?

Kenny