https://sourceware.org/bugzilla/show_bug.cgi?id=27109
Bug ID: 27109
Summary: gas: Omit unused STT_SECTION symbols
Product: binutils
Version: unspecified
Status: UNCONFIRMED
Severity: normal
Priority: P2
Component: gas
Assignee: unassigned at sourceware dot org
Reporter: i at maskray dot me
Target Milestone: ---
% cat a.s
call .text.0
.section .text.0,"ax",@progbits
.section .text.1,"ax",@progbits
% as a.s -o a.o
% llvm-mc -filetype=obj a.s -o b.o
% readelf -Ws a.o
Symbol table '.symtab' contains 6 entries:
Num: Value Size Type Bind Vis Ndx Name
0: 0000000000000000 0 NOTYPE LOCAL DEFAULT UND
1: 0000000000000000 0 SECTION LOCAL DEFAULT 1
2: 0000000000000000 0 SECTION LOCAL DEFAULT 3
3: 0000000000000000 0 SECTION LOCAL DEFAULT 4
4: 0000000000000000 0 SECTION LOCAL DEFAULT 5
5: 0000000000000000 0 SECTION LOCAL DEFAULT 6
% readelf -Ws b.o
Symbol table '.symtab' contains 2 entries:
Num: Value Size Type Bind Vis Ndx Name
0: 0000000000000000 0 NOTYPE LOCAL DEFAULT UND
1: 0000000000000000 0 SECTION LOCAL DEFAULT 4
(The only STT_SECTION symbol is for .text.0, which is referenced by a
relocation.)
LLVM integrated assembler omits unused STT_SECTION symbols.
This behavior was introduced in 2015 (https://reviews.llvm.org/rL239045)
and is a very efficient size optimization, especially for -ffunction-sections
-fdata-sections.
This is quite significant on ELF64 because on ELF64 an unused STT_SECTION costs
sizeof(Elf64_Shdr)=64 bytes.
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