[Bug default/25141] New: Doing dwz single-file optimization before dwz multifile optimization reduces effectiveness

[Bug default/25141] New: Doing dwz single-file optimization before dwz multifile optimization reduces effectiveness

[vries at gcc dot gnu.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=25141

            Bug ID: 25141
           Summary: Doing dwz single-file optimization before dwz
                    multifile optimization reduces effectiveness
           Product: dwz
           Version: unspecified
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: default
          Assignee: nobody at sourceware dot org
          Reporter: vries at gcc dot gnu.org
                CC: dwz at sourceware dot org
  Target Milestone: ---

When doing the self-multi-file optimization on a hello world executable, we
manage a total reduction of 78%:
...
$ cp hello 1; cp 1 2; ./dwz -m 3 1 2
$ ../measure/diff.sh hello 1
.debug_info      red: 66.19%    1467  496
.debug_abbrev    red: 50.65%    624  308
.debug_str       red: 100.00%   1595  0
total            red: 78.19%    3686 804
...

However, if we first do the single-file mode optimization step using a separate
invocation, we achieve a total reduction of only 54%:
...
$ cp hello 4; ./dwz 4; cp 4 5; ./dwz -m 6 4 5
./dwz: 4: DWARF compression not beneficial - old size 3400 new size 3400
./dwz: 5: DWARF compression not beneficial - old size 3400 new size 3400
$ ../measure/diff.sh hello 4
.debug_info      red: 24.20%    1467  1112
.debug_abbrev    red: 8.82%     624  569
.debug_str       red: 100.00%   1595  0
total            red: 54.40%    3686 1681
...

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