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[ELF] Support -r --gc-sections
-r --gc-sections is usually not useful because it just makes intermediate output smaller. https://bugs.llvm.org/show_bug.cgi?id=46700#c7 mentions a use case: validating the absence of undefined symbols ealier than in the final link. After D84129 (SHT_GROUP support in -r links), we can support -r --gc-sections without extra code. So let's allow it. Reviewed By: grimar, jhenderson Differential Revision: https://reviews.llvm.org/D84131
This commit is contained in:
Fangrui Song
Fangrui Song
parent
23d1800433
commit
4e80c768c2
@ -330,8 +330,6 @@ static void checkOptions() {
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if (config->relocatable) {
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if (config->shared)
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error("-r and -shared may not be used together");
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if (config->gcSections)
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error("-r and --gc-sections may not be used together");
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if (config->gdbIndex)
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error("-r and --gdb-index may not be used together");
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if (config->icf != ICFLevel::None)
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@ -926,20 +926,6 @@ InputSectionBase *ObjFile<ELFT>::createInputSection(const Elf_Shdr &sec) {
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this->sections[sec.sh_info] = target;
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}
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// This section contains relocation information.
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// If -r is given, we do not interpret or apply relocation
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// but just copy relocation sections to output.
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if (config->relocatable) {
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InputSection *relocSec = make<InputSection>(*this, sec, name);
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// We want to add a dependency to target, similar like we do for
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// -emit-relocs below. This is useful for the case when linker script
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// contains the "/DISCARD/". It is perhaps uncommon to use a script with
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// -r, but we faced it in the Linux kernel and have to handle such case
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// and not to crash.
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target->dependentSections.push_back(relocSec);
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return relocSec;
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}
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if (target->firstRelocation)
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fatal(toString(this) +
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": multiple relocation sections to one section are not supported");
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@ -957,17 +943,17 @@ InputSectionBase *ObjFile<ELFT>::createInputSection(const Elf_Shdr &sec) {
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}
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assert(isUInt<31>(target->numRelocations));
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// Relocation sections processed by the linker are usually removed
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// from the output, so returning `nullptr` for the normal case.
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// However, if -emit-relocs is given, we need to leave them in the output.
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// (Some post link analysis tools need this information.)
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if (config->emitRelocs) {
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InputSection *relocSec = make<InputSection>(*this, sec, name);
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// We will not emit relocation section if target was discarded.
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target->dependentSections.push_back(relocSec);
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return relocSec;
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}
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return nullptr;
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// Relocation sections are usually removed from the output, so return
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// `nullptr` for the normal case. However, if -r or --emit-relocs is
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// specified, we need to copy them to the output. (Some post link analysis
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// tools specify --emit-relocs to obtain the information.)
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if (!config->relocatable && !config->emitRelocs)
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return nullptr;
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InputSection *relocSec = make<InputSection>(*this, sec, name);
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// If the relocated section is discarded (due to /DISCARD/ or
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// --gc-sections), the relocation section should be discarded as well.
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target->dependentSections.push_back(relocSec);
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return relocSec;
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}
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}
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@ -34,10 +34,6 @@
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# RUN: not ld.lld -r -shared %t -o /dev/null 2>&1 | FileCheck -check-prefix=ERR2 %s
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# ERR2: -r and -shared may not be used together
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## Attempt to use -r and --gc-sections together
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# RUN: not ld.lld -r --gc-sections %t -o /dev/null 2>&1 | FileCheck -check-prefix=ERR3 %s
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# ERR3: -r and --gc-sections may not be used together
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## Attempt to use -r and --gdb-index together
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# RUN: not ld.lld -r --gdb-index %t -o /dev/null 2>&1 | FileCheck -check-prefix=ERR4 %s
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# ERR4: -r and --gdb-index may not be used together
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80
lld/test/ELF/relocatable-gc.s
Normal file
80
lld/test/ELF/relocatable-gc.s
Normal file
@ -0,0 +1,80 @@
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# REQUIRES: x86
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## Test -r --gc-sections.
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# RUN: llvm-mc -filetype=obj -triple=x86_64 %s -o %t.o
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## By default all regular sections are discarded. We currently don't track
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## usage of group signature symbols and will retain them and their associated
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## STT_SECTION symbols.
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# RUN: ld.lld -r --gc-sections --print-gc-sections %t.o -o %t
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# RUN: llvm-readelf -S -s %t | FileCheck %s
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# CHECK: [ 1] .group
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# CHECK-NEXT: [ 2] .note.GNU-stack
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# CHECK: Symbol table '.symtab' contains 3 entries:
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# CHECK-NEXT: Num:
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# CHECK-NEXT: 0:
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# CHECK-NEXT: 1: {{.*}} NOTYPE LOCAL DEFAULT 1 group
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# CHECK-NEXT: 2: {{.*}} SECTION LOCAL DEFAULT 1
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## -u keeps .text.bar alive. Other group members are kept alive as well.
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# RUN: ld.lld -r --gc-sections -u bar %t.o -o - | llvm-readelf -Ss - | \
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# RUN: FileCheck %s --check-prefix=KEEP_GROUP
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## -e, --init and --fini are similar.
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# RUN: ld.lld -r --gc-sections -e bar %t.o -o - | llvm-readelf -Ss - | \
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# RUN: FileCheck %s --check-prefix=KEEP_GROUP
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# RUN: ld.lld -r --gc-sections --init=bar %t.o -o - | llvm-readelf -Ss - | \
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# RUN: FileCheck %s --check-prefix=KEEP_GROUP
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# RUN: ld.lld -r --gc-sections --fini=bar %t.o -o - | llvm-readelf -Ss - | \
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# RUN: FileCheck %s --check-prefix=KEEP_GROUP
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# KEEP_GROUP: [ 1] .group
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# KEEP_GROUP-NEXT: [ 2] .text.bar
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# KEEP_GROUP-NEXT: [ 3] .text.foo
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# KEEP_GROUP-NEXT: [ 4] .note.GNU-stack
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# KEEP_GROUP: Symbol table '.symtab' contains 7 entries:
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# KEEP_GROUP: 4: {{.*}} SECTION
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# KEEP_GROUP-NEXT: 5: {{.*}} 2 bar
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# KEEP_GROUP-NEXT: 6: {{.*}} 3 foo
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## If .text is retained, its referenced qux and .fred are retained as well.
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## fred_und is used (by .fred) and thus emitted.
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## Note, GNU ld does not retain qux.
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# RUN: ld.lld -r --gc-sections -e _start %t.o -o %tstart.ro
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# RUN: llvm-readelf -Ss %tstart.ro | FileCheck %s --check-prefix=KEEP_START
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# KEEP_START: [ 1] .text
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# KEEP_START-NEXT: [ 2] .rela.text
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# KEEP_START-NEXT: [ 3] qux
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# KEEP_START-NEXT: [ 4] .group
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# KEEP_START-NEXT: [ 5] .fred
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# KEEP_START-NEXT: [ 6] .rela.fred
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# KEEP_START-NEXT: [ 7] .note.GNU-stack
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# KEEP_START: Symbol table '.symtab' contains 10 entries:
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# KEEP_START: 5: {{.*}} SECTION
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# KEEP_START-NEXT: 6: {{.*}} UND __start_qux
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# KEEP_START-NEXT: 7: {{.*}} 1 _start
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# KEEP_START-NEXT: 8: {{.*}} 5 fred
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# KEEP_START-NEXT: 9: {{.*}} UND fred_und
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.section qux,"a",@progbits
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.byte 0
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.text
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.globl _start, bar, foo, fred
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_start:
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call fred
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.quad __start_qux
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.section .text.bar,"axG",@progbits,group,comdat
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bar:
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.byte 1
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.section .text.foo,"axG",@progbits,group,comdat
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foo:
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.byte 2
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.section .fred,"ax",@progbits
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fred:
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call fred_und
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