https://sourceware.org/bugzilla/show_bug.cgi?id=26622
Bug ID: 26622
Summary: Support --gc-sections for SHF_MERGE sections
Product: binutils
Version: unspecified
Status: UNCONFIRMED
Severity: normal
Priority: P2
Component: ld
Assignee: unassigned at sourceware dot org
Reporter: i at maskray dot me
Target Milestone: ---
% cat a.c
__attribute__((noinline)) void ppp(const char *a) {}
void used1(){ ppp("foo"); }
void used2(){ ppp("bar"); }
void used3(){ ppp("bar foo"); }
void unused(){ ppp("unused"); }
% clang -c a.c -ffunction-sections -fdata-sections -fPIC && re -S a.o G rodata
[12] .rodata.str1.1 PROGBITS 0000000000000000 0000c2 000017 01 AMS
0 0 1
% ld.bfd --gc-sections -e 0 a.o -o a -u used1 -u used2 -u used3 && strings a |
grep unused
2010 unused
`unused` is in the linked image because GNU ld (and gold) do not implement
--gc-sections for SHF_MERGE sections.
Since May 2015, GCC -ffunction-sections -fdata-sections -fmerge-constants
places string literals into separate sections
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=192#c16
% gcc -c a.c -ffunction-sections -fdata-sections -fPIC -fmerge-constants && re
-S a.o G rodata
[ 5] .rodata.used1.str1.1 PROGBITS 0000000000000000 00004b 000004 01
AMS 0 0 1
[ 8] .rodata.used2.str1.1 PROGBITS 0000000000000000 000062 000004 01
AMS 0 0 1
[11] .rodata.used3.str1.1 PROGBITS 0000000000000000 000079 000008 01
AMS 0 0 1
[14] .rodata.unused.str1.1 PROGBITS 0000000000000000 000094 000007 01
AMS 0 0 1
This is to make --gc-sections discard unused section pieces. However, this does
not look like a good idea
(https://gcc.gnu.org/bugzilla/show_bug.cgi?id=192#c14) because the cost of a
section header (sizeof(Elf64_Shdr)=64) + a section name is quite large. The
object file sizes are bloated even if the final linked image is optimal.
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