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[ELF] --gc-sections: collect unused SHF_LINK_ORDER .gcc_except_table 

A SHF_LINK_ORDER .gcc_except_table is similar to a .gcc_except_table in
a section group. The associated text section is responsible for retaining it.

LLD still does not support GC of non-group non-SHF_LINK_ORDER .gcc_except_table -
but that is not necessary because we can teach the compiler to set SHF_LINK_ORDER.
This commit is contained in:
Fangrui Song 2021-02-05 21:35:27 -08:00
parent ea3175c15b
commit 6a1235211d
2 changed files with 26 additions and 12 deletions
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14
lld/ELF/MarkLive.cpp
View File

@ -107,13 +107,13 @@ void MarkLive<ELFT>::resolveReloc(InputSectionBase &sec, RelTy &rel,
// fromFDE being true means this is referenced by a FDE in a .eh_frame
// piece. The relocation points to the described function or to a LSDA. We
// only need to keep the LSDA live, so ignore anything that points to
// executable sections. If the LSDA is in a section group, we ignore the
// relocation as well because (a) if the associated text section is live,
// the LSDA will be retained due to section group rules (b) if the
// associated text section should be discarded, marking the LSDA will
// unnecessarily retain the text section.
if (!(fromFDE &&
((relSec->flags & SHF_EXECINSTR) || relSec->nextInSectionGroup)))
// executable sections. If the LSDA is in a section group or has the
// SHF_LINK_ORDER flag, we ignore the relocation as well because (a) if the
// associated text section is live, the LSDA will be retained due to section
// group/SHF_LINK_ORDER rules (b) if the associated text section should be
// discarded, marking the LSDA will unnecessarily retain the text section.
if (!(fromFDE && ((relSec->flags & (SHF_EXECINSTR | SHF_LINK_ORDER)) ||
relSec->nextInSectionGroup)))
enqueue(relSec, offset);
return;
}

24
lld/test/ELF/gc-sections-lsda.s
View File

@ -1,24 +1,26 @@
# REQUIRES: x86
# RUN: llvm-mc %s -o %t.o -filetype=obj -triple=x86_64-pc-linux
## Discard an unused .gcc_except_table in a COMDAT group if the associated text
## section is discarded.
## Discard an unused .gcc_except_table in a COMDAT group or having SHF_LINK_ORDER
## if the associated text section is discarded.
# RUN: ld.lld --gc-sections --print-gc-sections -u _Z3foov %t.o -o /dev/null | \
# RUN: FileCheck %s --implicit-check-not=.gcc_except_table
# CHECK: removing unused section {{.*}}.o:(.text._Z6comdatv)
# CHECK-NEXT: removing unused section {{.*}}.o:(.text._Z9linkorderv)
# CHECK-NEXT: removing unused section {{.*}}.o:(.gcc_except_table._Z6comdatv)
# CHECK-NEXT: removing unused section {{.*}}.o:(.gcc_except_table._Z9linkorderv)
## An unused non-group .gcc_except_table is not discarded.
## An unused non-group non-SHF_LINK_ORDER .gcc_except_table is not discarded.
# RUN: ld.lld --gc-sections --print-gc-sections -u _Z6comdatv %t.o -o /dev/null | \
# RUN: ld.lld --gc-sections --print-gc-sections -u _Z6comdatv -u _Z9linkorderv %t.o -o /dev/null | \
# RUN: FileCheck /dev/null --implicit-check-not=.gcc_except_table
## If the text sections are live, the .gcc_except_table sections are retained as
## well because they are referenced by .eh_frame pieces.
# RUN: ld.lld --gc-sections --print-gc-sections -u _Z3foov -u _Z6comdatv %t.o -o /dev/null | \
# RUN: ld.lld --gc-sections --print-gc-sections -u _Z3foov -u _Z6comdatv -u _Z9linkorderv %t.o -o /dev/null | \
# RUN: FileCheck %s --check-prefix=KEEP
# KEEP-NOT: .gcc_except_table
@ -39,6 +41,14 @@ _Z6comdatv:
.cfi_lsda 0x1b,.Lexception1
.cfi_endproc
.section .text._Z9linkorderv,"ax",@progbits
.globl _Z9linkorderv
_Z9linkorderv:
.cfi_startproc
ret
.cfi_lsda 0x1b,.Lexception2
.cfi_endproc
.section .gcc_except_table._Z3foov,"a",@progbits
.Lexception0:
.byte 255
@ -46,3 +56,7 @@ _Z6comdatv:
.section .gcc_except_table._Z6comdatv,"aG",@progbits,_Z6comdatv,comdat
.Lexception1:
.byte 255
.section .gcc_except_table._Z9linkorderv,"ao",@progbits,_Z9linkorderv
.Lexception2:
.byte 255