curious gprof behaviour on trivial testcase

[James E Wilson <wilson at specifix dot com>]

This came up on the mips linux kernel mailing list.
http://www.linux-mips.org/archives/linux-mips/2006-04/msg00231.html

Suppose I am building a toolchain for the first time, and want to verify
that gprof works.  So you compile a trivial testcase with -pg, run it,
run gprof on it, and you get an error.  This is very confusing for a
naive user.

The problem here is that a trivial testcase has only one function, and
hence no call graph info.  However, gprof by default assumes you want to
emit call graph info, and if there is none, it gives an error.

I suspect this used to work OK, because in the past we used static
libraries by default, and usually had static profiled libraries
available to link with, so there was generally always a call graph. 
Even if it only included start and main.  However, nowadays, we use
shared libraries by default, and we don't link with profiled libraries
unless you add extra options because this only works right for static
libraries, so it is much more likely that there may be no call graph.

I think it would be better if we emit a flat profile if no call graph is
present.  Of course, the flaw here is that people may now ask why they
got a result but no call graph.  However, I think this is easier to deal
with.  If you see a flat profile with only one function, then it is
clear what is wrong.  If you see an error with gprof, it isn't clear
what is wrong, since the problem could be in gcc, glibc, or gprof or
even somewhere else entirely.

I tested this patch on x86_64-linux.  There were no regressions.  Not
very meaningful since there is no gprof testsuite, but I tested it by
hand on a trivial and non-trivial testcase.  Without the patch I get an
error on a trivial testcase.  With the patch, I get a flat profile for a
trivial testcase.

Not checked in yet in case someone wanted to comment.
-- 
Jim Wilson, GNU Tools Support, http://www.specifix.com

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