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Re: Help with getting started with adding IPA-related attribute
- From: Martin Jambor <mjambor at suse dot cz>
- To: Daniel Santos <daniel dot santos at pobox dot com>
- Cc: gcc <gcc at gcc dot gnu dot org>, Jan Hubicka <jh at suse dot cz>
- Date: Wed, 3 May 2017 10:42:35 +0200
- Subject: Re: Help with getting started with adding IPA-related attribute
- Authentication-results: sourceware.org; auth=none
- References: <email@example.com>
On Sat, Apr 29, 2017 at 06:28:31AM -0500, Daniel Santos wrote:
> Brievity is not my forte, so let me start with the questions. Can somebody
> please point me to the pass and/or function where gcc
> 1.) decides rather or not to inline a function,
There are two inlining passes. Early-inliner that works in
topological order and inlines (almost) only when the result is
expected to be smaller than uninlined code. Then there is a full IPA
inlining pass, which can take advantage of LTO and heuristics that
allows code to grow.
To understand both, you really need to read most of ipa-inline*.[ch]
files. The exact places where the decisions are made are:
- early_inline_small_functions() and want_early_inline_function_p()
in ipa-inline.c for early inlining, and
- the IPA inliner has a few phases and sub-phases itself, see the
helper functions in ipa_inline() and especially in
inline_small_functions(). Alternatively, you want to check out
all functions which call inline_call().
> 2.) decides rather or not to make a .constprop version of a
Check out the places that call create_specialized_node() in ipa-cp.c,
namely templated decide_about_value() and the end of
decide_whether_version_node(), which however just checks the flag
do_clone_for_all_contexts set earlier in estimate_local_effects().
> 3.) a good pass (when all constant propagation is done) to search for fn
> parameters and variables (marked with an attribute) that were not
> constproped away, and finally
The attributes must be accepted by the front-end. For example in C
and C++ this is done in gcc/c-family/c-attribs.c. Afterwards any pass
or any code in the compiler can use lookup_attribute on the
PARM_DECL tree representing the parameter.
> 4.) what mechanism should I use for that search? (iterate through the tree
> to find them and then see if they have rtl? I haven't worked in this area
No, I do not think you want to operate on the RTL level (though I have
only very briefly skimmed through the rest of your email). Use the
gimple/tree representation as I wrote above. Look at for example
ipa_populate_param_decls() in ipa-prop.c to see how to iterate over
all PARM_DECLs of a function.
Hope this helps,