[v2] limit repeated DIE comparisons
Commit Message
Exponential explosion of DIE comparison has been possible since the
limit of at most 5 pending struct/union DIE comparison pairs was
lifted.
This commit adds two things to control this (with a negligible chance of
falsely finding that two DIEs are equivalent when they are not).
- DIE self-comparisons immediately return true
- once a DIE pair has been compared 10000 times, always return true
* src/abg-dwarf-reader.cc (read_context): Add mutable
die_comparison_visits_ member.
(compare_dies): Return true if this is a self-comparison.
Return true if we have visited this comparison 10000 times.
Signed-off-by: Giuliano Procida <gprocida@google.com>
---
src/abg-dwarf-reader.cc | 12 ++++++++++++
1 file changed, 12 insertions(+)
Comments
Hello,
Giuliano Procida <gprocida@google.com> a écrit:
I have applied the patch, thanks a lot for it!
I just have one useless comment below, for my own understanding.
> Exponential explosion of DIE comparison has been possible since the
> limit of at most 5 pending struct/union DIE comparison pairs was
> lifted.
>
> This commit adds two things to control this (with a negligible chance of
> falsely finding that two DIEs are equivalent when they are not).
>
> - DIE self-comparisons immediately return true
> - once a DIE pair has been compared 10000 times, always return true
>
> * src/abg-dwarf-reader.cc (read_context): Add mutable
> die_comparison_visits_ member.
> (compare_dies): Return true if this is a self-comparison.
> Return true if we have visited this comparison 10000 times.
>
> Signed-off-by: Giuliano Procida <gprocida@google.com>
> ---
> src/abg-dwarf-reader.cc | 12 ++++++++++++
> 1 file changed, 12 insertions(+)
>
> diff --git a/src/abg-dwarf-reader.cc b/src/abg-dwarf-reader.cc
> index 7bf2375d..27dd4faf 100644
> --- a/src/abg-dwarf-reader.cc
> +++ b/src/abg-dwarf-reader.cc
> @@ -2094,6 +2094,9 @@ public:
> /// A map that associates a function type representations to
> /// function types, inside a translation unit.
> mutable istring_fn_type_map_type per_tu_repr_to_fn_type_maps_;
> + mutable std::unordered_map<std::pair<Dwarf_Off, Dwarf_Off>,
> + size_t,
> + dwarf_offset_pair_hash> die_comparison_visits_;
>
> die_class_or_union_map_type die_wip_classes_map_;
> die_class_or_union_map_type alternate_die_wip_classes_map_;
> @@ -10216,6 +10219,15 @@ compare_dies(const read_context& ctxt,
>
> Dwarf_Off l_offset = dwarf_dieoffset(const_cast<Dwarf_Die*>(l)),
> r_offset = dwarf_dieoffset(const_cast<Dwarf_Die*>(r));
> +
> + if (l_offset == r_offset)
> + return true;
> + auto& visit = ctxt.die_comparison_visits_[std::make_pair(l_offset, r_offset)];
> + if (visit == 10000)
> + return true;
Interesting. I'll play with this and see what happens when we hit
this. I hope I'll find a binary in the testsuite that hits this line,
so that I can understand why the type is compared so many times.
Thanks a lot!
[...]
Cheers,
@@ -2094,6 +2094,9 @@ public:
/// A map that associates a function type representations to
/// function types, inside a translation unit.
mutable istring_fn_type_map_type per_tu_repr_to_fn_type_maps_;
+ mutable std::unordered_map<std::pair<Dwarf_Off, Dwarf_Off>,
+ size_t,
+ dwarf_offset_pair_hash> die_comparison_visits_;
die_class_or_union_map_type die_wip_classes_map_;
die_class_or_union_map_type alternate_die_wip_classes_map_;
@@ -10216,6 +10219,15 @@ compare_dies(const read_context& ctxt,
Dwarf_Off l_offset = dwarf_dieoffset(const_cast<Dwarf_Die*>(l)),
r_offset = dwarf_dieoffset(const_cast<Dwarf_Die*>(r));
+
+ if (l_offset == r_offset)
+ return true;
+ auto& visit = ctxt.die_comparison_visits_[std::make_pair(l_offset, r_offset)];
+ if (visit == 10000)
+ return true;
+ else
+ ++visit;
+
Dwarf_Off l_canonical_die_offset = 0, r_canonical_die_offset = 0;
const die_source l_die_source = ctxt.get_die_source(l);
const die_source r_die_source = ctxt.get_die_source(r);