Bug 104943 - Analyzer fails to purge state for local structs
Summary: Analyzer fails to purge state for local structs
Alias: None
Product: gcc
Classification: Unclassified
Component: analyzer (show other bugs)
Version: 12.0
: P3 normal
Target Milestone: ---
Assignee: David Malcolm
Depends on:
Blocks: 104954
  Show dependency treegraph
Reported: 2022-03-15 22:18 UTC by David Malcolm
Modified: 2022-03-18 23:26 UTC (History)
1 user (show)

See Also:
Known to work:
Known to fail:
Last reconfirmed:


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Description David Malcolm 2022-03-15 22:18:32 UTC
State purging only happens for SSA names, and locals of struct type aren't SSA names.

Given e.g.:

struct boxed {
  int value;

extern struct boxed boxed_add (struct boxed a, struct boxed b);
extern struct boxed boxed_mul (struct boxed a, struct boxed b);

struct boxed
test (struct boxed a, struct boxed b)
  struct boxed result = boxed_add (boxed_mul (a, a),
				   boxed_mul (b, b));
  return result;

without optimization we have this gimple:

struct boxed test (struct boxed a, struct boxed b)
  struct boxed result;
  struct boxed D.1994;
  struct boxed D.1993;
  struct boxed D.1992;

  <bb 2> :
  D.1992 = boxed_mul (b, b);
  D.1993 = boxed_mul (a, a);
  result = boxed_add (D.1993, D.1992);
  D.1994 = result;
  result ={v} {CLOBBER(eol)};

  <bb 3> :
  return D.1994;


and this final exploded node:

EN 11:
preds: EN: 10
callstring: []
after SN: 3
stack depth: 1
  frame (index 0): frame: ‘test’@1
clusters within frame: ‘test’@1
  cluster for: <anonymous>: CONJURED(D.1992 = boxed_mul (b, b);, <anonymous>)
  cluster for: <anonymous>: CONJURED(D.1993 = boxed_mul (a, a);, <anonymous>)
  cluster for: <anonymous>: CONJURED(result = boxed_add (D.1993, D.1992);, result)
  cluster for: <anonymous>: CONJURED(result = boxed_add (D.1993, D.1992);, result)
m_called_unknown_fn: TRUE
  equiv classes:

where the various <anonymous> are the values of the local temporaries of struct type, which ought to be purged; they don't matter anymore.

Seen on linux kernel: drivers/gpu/drm/amd/display/dc/calcs/dce_calcs.c which amongst other things has:

struct bw_fixed {
  int64_t value;

with numerous calls to manipulate values; the states get bloated with bindings for temporaries that persist far longer than they are needed.
Comment 1 CVS Commits 2022-03-18 23:20:44 UTC
The master branch has been updated by David Malcolm <dmalcolm@gcc.gnu.org>:


commit r12-7717-g1c1daca1cdf7bc0156d57bb2b9083ee70c66b000
Author: David Malcolm <dmalcolm@redhat.com>
Date:   Thu Mar 17 18:12:46 2022 -0400

    analyzer: add tests of boxed values [PR104943]
    This patch adds various regression tests as preparatory work for
    purging irrelevant local decls from state (PR analyzer/104943)
            PR analyzer/104943
            * gcc.dg/analyzer/boxed-malloc-1-29.c: New test.
            * gcc.dg/analyzer/boxed-malloc-1.c: New test.
            * gcc.dg/analyzer/taint-alloc-5.c: New test.
            * gcc.dg/analyzer/torture/boxed-int-1.c: New test.
            * gcc.dg/analyzer/torture/boxed-ptr-1.c: New test.
    Signed-off-by: David Malcolm <dmalcolm@redhat.com>
Comment 2 CVS Commits 2022-03-18 23:21:59 UTC
The master branch has been updated by David Malcolm <dmalcolm@gcc.gnu.org>:


commit r12-7718-gfaacafd2306ad7ece721a79dedbb6e44e0d65bdb
Author: David Malcolm <dmalcolm@redhat.com>
Date:   Tue Dec 7 19:22:47 2021 -0500

    analyzer: extend state-purging to locals [PR104943]
    The existing analyzer code attempts to purge the state of SSA names
    where it can in order to minimize the size of program_state instances,
    and to increase the chances of being able to reuse exploded_node
    instances whilst exploring the user's code.
    PR analyzer/104943 identifies that we fail to purge state of local
    variables, based on behavior seen in PR analyzer/104954 when attempting
    to profile slow performance of -fanalyzer on a particular file in the
    Linux kernel, where that testcase has many temporary "boxed" values of
    structs containing ints, which are never cleaned up, leading to bloat
    of the program_state instances (specifically, of the store objects).
    This patch generalizes the state purging from just being on SSA names
    to also work on local variables.  Doing so requires that we detect where
    addresses to a local variable (or within them) are taken; we assume that
    once a pointer has been taken, it's not longer safe to purge the value
    of that decl at any successor point within the function.
    Doing so speeds up the PR analyzer/104954 Linux kernel analyzer testcase
    from taking 254 seconds to "just" 186 seconds (and I have a followup
    patch in development that seems to further reduce this to 37 seconds).
    The patch may also help with scaling up taint-detection so that it can
    eventually be turned on by default, but we're not quite there (this
    is PR analyzer/103533).
            PR analyzer/104943
            PR analyzer/104954
            PR analyzer/103533
            * analyzer.h (class state_purge_per_decl): New forward decl.
            * engine.cc (impl_run_checkers): Pass region_model_manager to
            state_purge_map ctor.
            * program-point.cc (function_point::final_stmt_p): New.
            (function_point::get_next): New.
            * program-point.h (function_point::final_stmt_p): New decl.
            (function_point::get_next): New decl.
            * program-state.cc (program_state::prune_for_point): Generalize to
            purge local decls as well as SSA names.
            (program_state::can_purge_base_region_p): New.
            * program-state.h (program_state::can_purge_base_region_p): New
            * region-model.cc (struct append_ssa_names_cb_data): Rename to...
            (struct append_regions_cb_data): ...this.
            (region_model::get_ssa_name_regions_for_current_frame): Rename
            (region_model::get_regions_for_current_frame): ...this, updating
            for other renamings.
            (region_model::append_ssa_names_cb): Rename to...
            (region_model::append_regions_cb): ...this, and drop the requirement
            that the subregion be a SSA name.
            * region-model.h (struct append_ssa_names_cb_data): Rename decl
            (struct append_regions_cb_data): ...this.
            (region_model::get_ssa_name_regions_for_current_frame): Rename
            decl to...
            (region_model::get_regions_for_current_frame): ...this.
            (region_model::append_ssa_names_cb): Rename decl to...
            (region_model::append_regions_cb): ...this.
            * state-purge.cc: Include "tristate.h", "selftest.h",
            "analyzer/store.h", "analyzer/region-model.h", and
            (get_candidate_for_purging): New.
            (class gimple_op_visitor): New.
            (my_load_cb): New.
            (my_store_cb): New.
            (my_addr_cb): New.
            (state_purge_map::state_purge_map): Add "mgr" param.  Update for
            renamings.  Find uses of local variables.
            (state_purge_map::~state_purge_map): Update for renaming of m_map
            to m_ssa_map.  Clean up m_decl_map.
            (state_purge_map::get_or_create_data_for_decl): New.
            (state_purge_per_ssa_name::state_purge_per_ssa_name): Update for
            inheriting from state_purge_per_tree.
            (state_purge_per_ssa_name::add_to_worklist): Likewise.
            (state_purge_per_decl::state_purge_per_decl): New.
            (state_purge_per_decl::add_needed_at): New.
            (state_purge_per_decl::add_pointed_to_at): New.
            (state_purge_per_decl::process_worklists): New.
            (state_purge_per_decl::add_to_worklist): New.
            (same_binding_p): New.
            (fully_overwrites_p): New.
            (state_purge_per_decl::process_point_backwards): New.
            (state_purge_per_decl::process_point_forwards): New.
            (state_purge_per_decl::needed_at_point_p): New.
            (state_purge_annotator::print_needed): Generalize to print local
            decls as well as SSA names.
            * state-purge.h (class state_purge_map): Update leading comment.
            (state_purge_map::map_t): Rename to...
            (state_purge_map::ssa_map_t): ...this.
            (state_purge_map::iterator): Rename to...
            (state_purge_map::ssa_iterator): ...this.
            (state_purge_map::decl_map_t): New typedef.
            (state_purge_map::decl_iterator): New typedef.
            (state_purge_map::state_purge_map): Add "mgr" param.
            (state_purge_map::get_data_for_ssa_name): Update for renaming.
            (state_purge_map::get_any_data_for_decl): New.
            (state_purge_map::get_or_create_data_for_decl): New decl.
            (state_purge_map::begin): Rename to...
            (state_purge_map::begin_ssas): ...this.
            (state_purge_map::end): Rename to...
            (state_purge_map::end_ssa): ...this.
            (state_purge_map::begin_decls): New.
            (state_purge_map::end_decls): New.
            (state_purge_map::m_map): Rename to...
            (state_purge_map::m_ssa_map): ...this.
            (state_purge_map::m_decl_map): New field.
            (class state_purge_per_tree): New class.
            (class state_purge_per_ssa_name): Inherit from state_purge_per_tree.
            (state_purge_per_ssa_name::get_function): Move to base class.
            (state_purge_per_ssa_name::point_set_t): Likewise.
            (state_purge_per_ssa_name::m_fun): Likewise.
            (class state_purge_per_decl): New.
            PR analyzer/104943
            PR analyzer/104954
            PR analyzer/103533
            * gcc.dg/analyzer/torture/boxed-ptr-1.c: Update expected number
            of exploded nodes to reflect improvements in state purging.
    Signed-off-by: David Malcolm <dmalcolm@redhat.com>
Comment 3 David Malcolm 2022-03-18 23:26:43 UTC
Should be fixed by the above commit.