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Re: Fix removal of ctors/dtors
- From: "H.J. Lu" <hjl dot tools at gmail dot com>
- To: Jan Hubicka <hubicka at ucw dot cz>
- Cc: gcc-patches at gcc dot gnu dot org
- Date: Mon, 8 Nov 2010 11:03:31 -0800
- Subject: Re: Fix removal of ctors/dtors
- References: <20101026141604.GD20390@kam.mff.cuni.cz>
On Tue, Oct 26, 2010 at 7:16 AM, Jan Hubicka <firstname.lastname@example.org> wrote:
> while implementing code removing pure & const constructors I considered correct option of
> dropping CONSTRUCTOR/DESTRUCTOR flag for functions found to be const or pure or slightly hacky
> of simply teaching dead function removal to consider them dead. ?The second is suboptimal
> for static constructors & destructors that are also called dirrectly. ?I concluded that no one
> does that, but Zdenek did and it triggers one overparanoid sanity check.
> So this patch takes the cleaner route of just dropping the flags and letting
> unreachable code removal to remove the functions later. ?This happens in
> visibility pass where we sanitize flags for pure/const constructors comming
> from forntends or via cgraph_set_pure_flag/cgraph_set_const_flags when they are
> discovered pure/const later.
> While working on it I also noticed that we don't handle correctly extern
> declarations (we consider them extern inlines and keep them in the cgraph.
> This is wasteful especially for LTO) and extern inlines (we keep them in the
> program even if they are obviously dead - they are never called nor have address
> taken up to the IPA inliner). Cleaning this issue shown that both inliner and
> ipa-cp compute code size costs incorrectly by assuming that optimizing out
> external copy of extern function it will save code size.
> Finally both inliner and ipa-cp confuse handling of functions with address
> taken. ?ipa-cp is considering functions with address taken to be cloning candidates.
> This is wrong. ?We should clone only when we think original function is not called at all
> but we can't say for sure since it is externally visible. ?Clonning is of course possible,
> but current impementation leads to wrong code when updating call sites of calls promoted to
> be direct.
> Inliner on the other hand produce invalid callgraph with inline clones having
> address taken.
> Bootstrapped/regtested x86_64-linux, lto-bootstrapped x86_64-linux and tested
> with Firefox builds. ? I also tested that tramp3d does not care about the
> inliner mertric fixes.
> ? ? ? ?* cgraph.c (cgraph_set_readonly_flag): Rename to...
> ? ? ? ?(cgraph_set_const_flags) ... this one; get also looping argument;
> ? ? ? ?clear constructor/destructor flags.
> ? ? ? ?(cgraph_set_pure_flag): Likewise.
> ? ? ? ?(cgraph_set_looping_const_or_pure_flag): Remove.
> ? ? ? ?(cgraph_can_remove_if_no_direct_calls_and_refs): Do not try
> ? ? ? ?to optimize away static ctors/dtors; it does not work on inline clones;
> ? ? ? ?external functions can always be rmeoved.
> ? ? ? ?(cgraph_will_be_removed_from_program_if_no_direct_calls): Assert on inline
> ? ? ? ?clones; in LTO external functions always can go.
> ? ? ? ?(cgraph_used_from_object_file_p): Handle EXTERNAL functions correctly.
> ? ? ? ?(cgraph_mark_address_taken_node): Assert that we are not taking address of
> ? ? ? ?inline clone.
> ? ? ? ?(cgraph_can_remove_if_no_direct_calls_p): We always eventually remove
> ? ? ? ?external functions.
> ? ? ? ?* ipa-cp.c (ipcp_cloning_candidate_p): Do not clone functions with address taken.
> ? ? ? ?(ipcp_initialize_node_lattices): Only local functions can be handled without cloning.
> ? ? ? ?* cgraph.h (cgraph_set_readonly_flag,
> ? ? ? ?cgraph_set_looping_const_or_pure_flag): Remove.
> ? ? ? ?(cgraph_set_const_flag): Declare.
> ? ? ? ?(cgraph_set_pure_flag): Update.
> ? ? ? ?* ipa-pure-const (propagate_pure_const, local_pure_const): Update
> ? ? ? ?flags setting code.
> ? ? ? ?* ipa.c (cgraph_remove_unreachable_nodes): Fix formating; do not look at inline
> ? ? ? ?clones; fix handling of external definitions.
> ? ? ? ?(cgraph_postorder): Do not look at inline clones in the first pass.
> ? ? ? ?(function_and_variable_visibility): Drop constructors/destructor
> ? ? ? ?flags at pure and const functions.
> ? ? ? ?* tree-profile.c (tree_profiling): Update.
> ? ? ? ?* ipa-inline.c (cgraph_clone_inlined_nodes): Always clone functions with
> ? ? ? ?address taken; external functions do not account to whole program size.
> ? ? ? ?(cgraph_decide_inlining): Likewise; do not try to inline functions already
> ? ? ? ?inlined.
> ? ? ? ?* testsuite/gcc.dg/lto/pr45736_0.c: New function.
This patch caused: