While attempting to port a profiler designed to work with the Microsoft
compiler's /Gh and /GH (generate _penter and _pexit) options, I have
rediscovered a problem that was previously filed as bug #23296: "When compiling
code with gcc 4.0.0 or 4.0.1 and when specifying both the
-finstrument-functions and the -O3 options, then __cyg_profile_func_enter() is
called two or more times successively with exactly the same arguments (called
function pointer and call site pointer). This should never happen." This was
marked INVALID with the comment: "This is not a bug, this is how it works now
in 4.0.0 and above with respect with the inliner."
However, this makes it more difficult to implement an efficient and accurate
profiler. The inlined functions look just like regular ones, and can't be
billed to the called function, because the called function doesn't appear in
the binary, and its address wouldn't be given to __cyg_profile_func_enter even
if it were. So the only semantics a profiler could get out of this are "a
function would have been called here if it hadn't been inlined, but we don't
know what it was in any case." And when lots of functions are inlined, we'll
take a significant time hit (and possibly disturb pipelining and optimization)
to get this effectively useless knowledge. The best we can do is say
"-fno-inline" to turn off inlining altogether, and accept a slower and less
accurate profiler.
I would like to have another option (or to change the semantics of the existing
option) that would cause inlined function calls to behave as if they had
__attribute((no_instrument_function)). If this was too hard, it would also be
acceptable to make it be a little more heuristic and cause possibly-inlined
function calls to behave as if they had __attribute((no_instrument_function).
Although I'm not familiar with the gcc codebase, I can imagine this working by
making a change in expand_function_start in gcc/function.c:
current_function_profile
= (profile_flag
&& ! DECL_NO_INSTRUMENT_FUNCTION_ENTRY_EXIT (subr));
->
current_function_profile
= (profile_flag
&& ! DECL_NO_INSTRUMENT_FUNCTION_ENTRY_EXIT (subr)
&& ! tree_inlinable_function_p (subr));
--
Summary: Request an option to make -finstrument-functions not
apply to inlined function calls
Product: gcc
Version: 4.1.0
Status: UNCONFIRMED
Severity: enhancement
Priority: P3
Component: c++
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: joev dot dubach at nuance dot com
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=28205
