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From: "wcohen at redhat dot com" <sourceware-bugzilla@sourceware.org>
To: dwz@sourceware.org
Subject: [Bug default/28568] New: Eliminate redundant DW_AT_entry_pc tags for
 inlined functions from debuginfo
Date: Tue, 09 Nov 2021 13:47:03 +0000
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https://sourceware.org/bugzilla/show_bug.cgi?id=3D28568

            Bug ID: 28568
           Summary: Eliminate redundant DW_AT_entry_pc tags for inlined
                    functions from debuginfo
           Product: dwz
           Version: unspecified
            Status: NEW
          Severity: normal
          Priority: P2
         Component: default
          Assignee: nobody at sourceware dot org
          Reporter: wcohen at redhat dot com
                CC: dwz at sourceware dot org
  Target Milestone: ---

According to the DWARF5.pdf standard DW_AT_entry_pc can be optional "2.18 E=
ntry
Address":

If no DW_AT_entry_pc attribute is present, then the entry address is assume=
d to
be the same as the base address of the containing scope.

Eliminating those unneeded DW_AT_entry_pc attributes would save some space =
in
the generated debuginfo.  To determine the possible saving the following dw=
grep
(https://pmachata.github.io/dwgrep/) script was used to find how often this
occurs in the Fedora 34 kernel and showed over 200,000 instances where the
DW_AT_low_pc and DW_AT_entry_pc were the same:


$  dwgrep /usr/lib/debug/lib/modules/5.14.15-200.fc34.x86_64/vmlinux -e '
let A :=3D entry (?TAG_inlined_subroutine) ?DW_AT_low_pc ?DW_AT_entry_pc;
let LOW :=3D ((A ?DW_AT_low_pc @DW_AT_low_pc) || (0xdeadbeef));
let ENTRY :=3D ((A ?DW_AT_entry_pc @DW_AT_entry_pc) || (0xdeadbeef));
(LOW =3D=3D ENTRY) ([A name, ENTRY, LOW ])' | grep \\[  | wc
 214604  643812 12804153

Initially filed a gcc bug to
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D103134 for the redundant
DW_AT_entry_pc. It was suggested that dwz would be likely be able to elimin=
ate
these unneeded DW_AT_entry_pc tags in the debuginfo.

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