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llvm-project/lld/test/ELF/gc-sections-group.s

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[ELF] Improve --gc-sections compatibility with GNU ld regarding section groups Based on D70020 by serge-sans-paille. The ELF spec says: > Furthermore, there may be internal references among these sections that would not make sense if one of the sections were removed or replaced by a duplicate from another object. Therefore, such groups must be included or omitted from the linked object as a unit. A section cannot be a member of more than one group. GNU ld has 2 behaviors that we don't have: - Group members (nextInSectionGroup != nullptr) are subject to garbage collection. This includes non-SHF_ALLOC SHT_NOTE sections. In particular, discarding non-SHF_ALLOC SHT_NOTE sections is an expected behavior by the Annobin project. See https://developers.redhat.com/blog/2018/02/20/annobin-storing-information-binaries/ for more information. - Groups members are retained or discarded as a unit. Members may have internal references that are not expressed as SHF_LINK_ORDER, relocations, etc. It seems that we should be more conservative here: if a section is marked live, mark all the other member within the group. Both behaviors are reasonable. This patch implements them. A new field InputSectionBase::nextInSectionGroup tracks the next member within a group. on ELF64, this increases sizeof(InputSectionBase) froms 144 to 152. InputSectionBase::dependentSections tracks section dependencies, which is used by both --gc-sections and /DISCARD/. We can't overload it for the "next member" semantic, because we should allow /DISCARD/ to discard sections independent of --gc-sections (GNU ld behavior). This behavior may be reasonably used by `/DISCARD/ : { *(.ARM.exidx*) }` or `/DISCARD/ : { *(.note*) }` (new test `linkerscript/discard-group.s`). Reviewed By: ruiu Differential Revision: https://reviews.llvm.org/D70146
2019-11-19 13:56:58 +08:00
# REQUIRES: x86
## Check that group members are retained or discarded as a unit, and
[ELF] Refine section group --gc-sections rules to not discard .debug_types clang/gcc -fdebug-type-sections places .debug_types and .rela.debug_types in a section group, with a signature symbol which represents the type signature. The section group is for deduplication purposes. After D70146, we will discard such section groups. Refine the rule so that we will retain the group if no member has the SHF_ALLOC flag. GNU ld has a similar rule to retain the group if all members have the SEC_DEBUGGING flag. We try to be more general for future-proof purposes: if other non-SHF_ALLOC sections have deduplication needs, they may be placed in a section group. Don't discard them. Reviewed By: grimar Differential Revision: https://reviews.llvm.org/D71157
2019-12-07 08:26:55 +08:00
## non-SHF_ALLOC sections in a group are subject to garbage collection,
## if at least one member has the SHF_ALLOC flag.
[ELF] Improve --gc-sections compatibility with GNU ld regarding section groups Based on D70020 by serge-sans-paille. The ELF spec says: > Furthermore, there may be internal references among these sections that would not make sense if one of the sections were removed or replaced by a duplicate from another object. Therefore, such groups must be included or omitted from the linked object as a unit. A section cannot be a member of more than one group. GNU ld has 2 behaviors that we don't have: - Group members (nextInSectionGroup != nullptr) are subject to garbage collection. This includes non-SHF_ALLOC SHT_NOTE sections. In particular, discarding non-SHF_ALLOC SHT_NOTE sections is an expected behavior by the Annobin project. See https://developers.redhat.com/blog/2018/02/20/annobin-storing-information-binaries/ for more information. - Groups members are retained or discarded as a unit. Members may have internal references that are not expressed as SHF_LINK_ORDER, relocations, etc. It seems that we should be more conservative here: if a section is marked live, mark all the other member within the group. Both behaviors are reasonable. This patch implements them. A new field InputSectionBase::nextInSectionGroup tracks the next member within a group. on ELF64, this increases sizeof(InputSectionBase) froms 144 to 152. InputSectionBase::dependentSections tracks section dependencies, which is used by both --gc-sections and /DISCARD/. We can't overload it for the "next member" semantic, because we should allow /DISCARD/ to discard sections independent of --gc-sections (GNU ld behavior). This behavior may be reasonably used by `/DISCARD/ : { *(.ARM.exidx*) }` or `/DISCARD/ : { *(.note*) }` (new test `linkerscript/discard-group.s`). Reviewed By: ruiu Differential Revision: https://reviews.llvm.org/D70146
2019-11-19 13:56:58 +08:00
# RUN: llvm-mc -filetype=obj -triple=x86_64 %s -o %t.o
# RUN: ld.lld --gc-sections %t.o -o %t.dead
# RUN: llvm-readobj -S %t.dead | FileCheck %s --check-prefix=CHECK-DEAD
## .mynote.bar is retained because it is not in a group.
# CHECK-DEAD-NOT: Name: .myanote.foo
# CHECK-DEAD-NOT: Name: .mytext.foo
# CHECK-DEAD-NOT: Name: .mybss.foo
# CHECK-DEAD-NOT: Name: .mynote.foo
# CHECK-DEAD: Name: .mynote.bar
# RUN: ld.lld --gc-sections %t.o -o %t -e anote_foo
# RUN: llvm-readobj -S %t | FileCheck %s --check-prefix=CHECK-LIVE
# RUN: ld.lld --gc-sections %t.o -o %t -e foo
# RUN: llvm-readobj -S %t | FileCheck %s --check-prefix=CHECK-LIVE
# RUN: ld.lld --gc-sections %t.o -o %t -e bss_foo
# RUN: llvm-readobj -S %t | FileCheck %s --check-prefix=CHECK-LIVE
## note_foo as the entry point does not make much sense because it is defined
## in a non-SHF_ALLOC section. This is just to demonstrate the behavior.
# RUN: ld.lld --gc-sections %t.o -o %t -e note_foo
# RUN: llvm-readobj -S %t | FileCheck %s --check-prefix=CHECK-LIVE
# CHECK-LIVE: Name: .myanote.foo
# CHECK-LIVE: Name: .mytext.foo
# CHECK-LIVE: Name: .mybss.foo
# CHECK-LIVE: Name: .mynote.foo
# CHECK-LIVE: Name: .mynote.bar
.globl anote_foo, foo, bss_foo, note_foo
.section .myanote.foo,"aG",@note,foo,comdat
anote_foo:
.byte 0
.section .mytext.foo,"axG",@progbits,foo,comdat
foo:
.byte 0
.section .mybss.foo,"awG",@nobits,foo,comdat
bss_foo:
.byte 0
.section .mynote.foo,"G",@note,foo,comdat
note_foo:
.byte 0
.section .mynote.bar,"",@note
.byte 0