Getting started writing a GIMPLE analysis plugin

[Florian Weimer]

Is there some simple example plugin which contains a simple GIMPLE 
analysis pass which doesn't change anything?  I have written something 
very much like the warn_unused_result pass, iterating over 
gimple_body(current_function_decl), and the compiler crashes much later 
in the compilation process.

The gcc-python plugin (which is much more complex) seems to work in my 
GCC installation, but it's not clear to me what I'm doing differently, 
so that my own plugin registration doesn't work.

Here's the backtrace I eventually get when I call gimple_call_fndecl on 
a GIMPLE_CALL statement from my pass:

==1089== Invalid read of size 4
==1089==    at 0x657F60: dom_info_available_p(cdi_direction) 
(dominance.c:1479)
==1089==    by 0x985F98: cleanup_tree_cfg() (tree-cfgcleanup.c:674)
==1089==    by 0x86543A: execute_function_todo(void*) (passes.c:1756)
==1089==    by 0x8659E6: do_per_function(void (*)(void*), void*) 
(passes.c:1646)
==1089==    by 0x865B92: execute_todo(unsigned int) (passes.c:1834)
==1089==    by 0x868C41: execute_one_pass(opt_pass*) (passes.c:2180)
==1089==    by 0x868EEE: execute_pass_list(opt_pass*) (passes.c:2212)
==1089==    by 0x61AA58: cgraph_analyze_function(cgraph_node*) 
(cgraphunit.c:663)
==1089==    by 0x61D3F5: cgraph_analyze_functions() (cgraphunit.c:937)
==1089==    by 0x61E6DD: finalize_compilation_unit() (cgraphunit.c:2080)
==1089==    by 0x4F31C6: c_write_global_declarations() (c-decl.c:10116)
==1089==    by 0x9603E2: compile_file() (toplev.c:560)

If I call gimple_call_fndecl on the same statements, I get this instead:

==1699== Invalid read of size 8
==1699==    at 0x979391: verify_gimple_in_cfg(function*) (tree-cfg.c:4544)
==1699==    by 0x865562: execute_function_todo(void*) (passes.c:1801)
==1699==    by 0x8659E6: do_per_function(void (*)(void*), void*) 
(passes.c:1646)
==1699==    by 0x865B92: execute_todo(unsigned int) (passes.c:1834)
==1699==    by 0x868C41: execute_one_pass(opt_pass*) (passes.c:2180)
==1699==    by 0x868EEE: execute_pass_list(opt_pass*) (passes.c:2212)
==1699==    by 0x61AA58: cgraph_analyze_function(cgraph_node*) 
(cgraphunit.c:663)
==1699==    by 0x61D3F5: cgraph_analyze_functions() (cgraphunit.c:937)
==1699==    by 0x61E6DD: finalize_compilation_unit() (cgraphunit.c:2080)
==1699==    by 0x4F31C6: c_write_global_declarations() (c-decl.c:10116)
==1699==    by 0x9603E2: compile_file() (toplev.c:560)
==1699==    by 0x961F67: toplev_main(int, char**) (toplev.c:1863)
==1699==  Address 0x0 is not stack'd, malloc'd or (recently) free'd

This happens with GCC 4.7.0 from Fedora 17, and with GCC 4.8 trunk.  I 
tried to compile the latter with --enable-checking=all, but I don't get 
any illuminating results, either.  For GCC 4.7, it doesn't matter if I 
compile the plugin in C or C++ mode.

Any idea what might be the cause?

I've attached the plugin source code for reference.

-- 
Florian Weimer / Red Hat Product Security Team
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