https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103134
Bug ID: 103134
Summary: Redundant DW_AT_entry_pc tags for inlined functions
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: debug
Assignee: unassigned at gcc dot gnu.org
Reporter: wcohen at redhat dot com
Target Milestone: ---
According to the DWARF5.pdf standard DW_AT_entry_pc can be optional "2.18 Entry
Address":
If no DW_AT_entry_pc attribute is present, then the entry address is assumed 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 dwgrep
(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 := entry (?TAG_inlined_subroutine) ?DW_AT_low_pc ?DW_AT_entry_pc;
let LOW := ((A ?DW_AT_low_pc @DW_AT_low_pc) || (0xdeadbeef));
let ENTRY := ((A ?DW_AT_entry_pc @DW_AT_entry_pc) || (0xdeadbeef));
(LOW == ENTRY) ([A name, ENTRY, LOW ])' | grep \\[ | wc
214604 643812 12804153
There were less than 3500 cases where the DW_AT_entry_pc was omitted:
$ dwgrep /usr/lib/debug/lib/modules/5.14.15-200.fc34.x86_64/vmlinux -e '
let A := entry (?TAG_inlined_subroutine);
let RANGE := ((A ?DW_AT_ranges @DW_AT_ranges) || (A ?DW_AT_low_pc
@DW_AT_low_pc) || (0xdeadbeef));
let ENTRY := ((A ?DW_AT_entry_pc @DW_AT_entry_pc) || (0xdeadbeef));
(ENTRY == 0xdeadbeef) ([A name, ENTRY, RANGE ])' | grep \\[ | wc
3448 12235 278068
There were even fewer cases of DW_AT_entry_pc != DW_AT_low_pc (some might be
due to Bug 99654):
]$ dwgrep /usr/lib/debug/lib/modules/5.14.15-200.fc34.x86_64/vmlinux -e '
let A := entry (?TAG_inlined_subroutine) ?DW_AT_low_pc ?DW_AT_entry_pc;
let LOW := ((A ?DW_AT_low_pc @DW_AT_low_pc) || (0xdeadbeef));
let ENTRY := ((A ?DW_AT_entry_pc @DW_AT_entry_pc) || (0xdeadbeef));
(LOW != ENTRY) ([A name, ENTRY, LOW ])' | grep \\[ | sort > entry_pc_diff.log
$ wc entry_pc_diff.log
445 1335 27515 entry_pc_diff.log
According to koji build of the package
(https://koji.fedoraproject.org/koji/buildinfo?buildID=1849253) the kernel was
built with gcc-11.2.1-1.fc34
