Working with existing passes¶
GCC organizes the optimization work it does as “passes”, and these form trees: passes can have both successors and child passes.
There are actually five “roots” to this tree:
gcc.Passholding all “lowering” passes, invoked per function within the callgraph, to turn high-level GIMPLE into lower-level forms (this wraps all_lowering_passes within gcc/passes.c).
gcc.Passholding all “small IPA” passes, working on the whole callgraph (IPA is “Interprocedural Analysis”; all_small_ipa_passes within gcc/passes.c)
gcc.Passholding all regular IPA passes (all_regular_ipa_passes within gcc/passes.c)
gcc.Passholding those passes relating to link-time-optimization (all_lto_gen_passes within gcc/passes.c)
- The “all other passes” gcc.Pass catchall, holding the majority of the passes. These are called on each function within the call graph (all_passes within gcc/passes.c)
Returns a 5-tuple of
gcc.Passinstances, giving the 5 top-level passes within GCC’s tree of passes, in the order described above.
gcc.Passinstance for the pass with the given name, raising ValueError if it isn’t found
This wraps one of GCC’s struct opt_pass * instances.
Beware: “pass” is a reserved word in Python, so use e.g. ps as a variable name for an instance of
The name of the pass, as a string
The first child pass of this pass (if any)
The next sibling pass of this pass (if any)
Currently these are int bitfields, expressing the flow of data betweeen the various passes.
They can be accessed using bitwise arithmetic:
if ps.properties_provided & gcc.PROP_cfg: print(fn.cfg)
Here are the bitfield flags:
Mask Meaning Which pass sets this up? Which pass clears this? gcc.PROP_gimple_any Is the full GIMPLE grammar allowed? (the frontend) “expand” gcc.PROP_gimple_lcf Has control flow been lowered? “lower” “expand” gcc.PROP_gimple_leh Has exception-handling been lowered? “eh” “expand” gcc.PROP_cfg Does the gcc.Function have a non-None “cfg”? “cfg” “*free_cfg” gcc.PROP_referenced_vars Do we have data on which functions reference which variables? (Dataflow analysis, aka DFA). This flag was removed in GCC 4.8 “*referenced_vars” (none) gcc.PROP_ssa Is the GIMPLE in SSA form? “ssa” “expand” gcc.PROP_no_crit_edges Have all critical edges within the CFG been split? “crited” (none) gcc.PROP_rtl Is the function now in RTL form? (rather than GIMPLE-SSA) “expand” “*clean_state” gcc.PROP_gimple_lomp Have OpenMP directives been lowered into explicit calls to the runtime library (libgomp) “omplower” “expand” gcc.PROP_cfglayout Are we reorganizing the CFG into a more efficient order? “into_cfglayout” “outof_cfglayout” gcc.PROP_gimple_lcx Have operations on complex numbers been lowered to scalar operations? “cplxlower” “cplxlower0”
(int) The number of this pass, used as a fragment of the dump file name. This is assigned automatically for custom passes.
(boolean) Is dumping enabled for this pass? Set this attribute to True to enable dumping. Not available from GCC 4.8 onwards
There are four subclasses of
gcc.Pass, signifying a pass called per-function on the GIMPLE representation of that function.
gcc.Pass, signifying a pass called per-function on the RTL representation of that function.
Creating new optimization passes¶
You can create new optimization passes. This involves three steps:
Here’s an example:
# Here's the (trivial) implementation of our new pass: class MyPass(gcc.GimplePass): # This is optional. # If present, it should return a bool, specifying whether or not # to execute this pass (and any child passes) def gate(self, fun): print('gate() called for %r' % fun) return True def execute(self, fun): print('execute() called for %r' % fun) # We now create an instance of the class: my_pass = MyPass(name='my-pass') # ...and wire it up, after the "cfg" pass: my_pass.register_after('cfg')
where fun is a
Unfortunately it doesn’t appear to be possible to implement gate() for gcc.IpaPass yet; for now, the gate() method on such passes will not be called. See http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54959
If an unhandled exception is raised within gate or execute, it will lead to a GCC error:
/home/david/test.c:36:1: error: Unhandled Python exception raised calling 'execute' method Traceback (most recent call last): File "script.py", line 79, in execute dot = gccutils.tree_to_dot(fun) NameError: global name 'gccutils' is not defined
Given the name of another pass, register this
gcc.Passto occur immediately after that other pass.
If the other pass occurs multiple times, the pass will be inserted at the specified instance number, or at every instance, if supplied 0.
If they don’t match, GCC won’t be able to find the other pass, giving an error like this:
cc1: fatal error: pass 'ssa' not found but is referenced by new pass 'my-ipa-pass'
As above, but this pass is registered immediately before the referenced pass.
As above, but replace the given pass. This method is included for completeness; the result is unlikely to work well.
Dumping per-pass information¶
GCC has a logging framework which supports per-pass logging (“dump files”).
By default, no logging is done; dumping must be explicitly enabled.
Dumping of passes can be enabled from the command-line in groups:
For more information, see http://gcc.gnu.org/onlinedocs/gcc/Debugging-Options.html
It’s not possible to directly enable dumping for a custom pass from the command-line (it would require adding new GCC command-line options). However, your script can directly enable dumping for a custom pass by writing to the dump_enabled attribute (perhaps in response to the arguments passed to plugin, or a driver script).
If enabled for a pass, then a file is written to the same directory as the output file, with a name based on the input file and the pass number.
For example, given a custom
gcc.Pass with name ‘test-pass’, then
when input.c is compiled to build/output.o:
$ gcc -fdump-tree-all -o build/output.o src/input.c
then a dump file input.c.225t.test-pass will be written to the directory build. In this case, 225 is the static_pass_number field, “t” signifies a tree pass, with the pass name appearing as the suffix.
Write str() of the argument to the current dump file. No newlines or other whitespace are added.
Note that dumping is disabled by default; in this case, the call will do nothing.
Get the name of the current dump file.
If called from within a pass for which dumping is enabled, it will return the filename in string form.
If dumping is disabled for this pass, it will return None.
The typical output of a dump file will contain:
;; Function bar (bar) (dumped information when handling function bar goes here) ;; Function foo (foo) (dumped information when handling function foo goes here)
class TestPass(gcc.GimplePass): def execute(self, fun): # Dumping of strings: gcc.dump('hello world') # Dumping of other objects: gcc.dump(42) ps = TestPass(name='test-pass') ps.register_after('cfg') ps.dump_enabled = True
would have a dump file like this:
;; Function bar (bar) hello world42 ;; Function foo (foo) hello world42
Alternatively, it can be simpler to create your own logging system, given that one can simply open a file and write to it.
Get the base file path and name prefix for GCC’s dump files.
You can use this when creating non-standard logfiles and other output.
For example, the libcpychecker code can write HTML reports on reference-counting errors within a function, writing the output to a file named:
filename = '%s.%s-refcount-errors.html' % (gcc.get_dump_base_name(), fun.decl.name)
given fun, a
By default, this is the name of the input file, but within the output file’s directory. (It can be overridden using the -dumpbase command-line option).