[PATCH] D38790: [ELF] Do not keep symbols if they referenced only from discarded sections.
Rafael Avila de Espindola via llvm-commits
llvm-commits at lists.llvm.org
Mon Nov 20 10:34:00 PST 2017
Taking a look
Igor Kudrin via Phabricator <reviews at reviews.llvm.org> writes:
> ikudrin updated this revision to Diff 123596.
> ikudrin edited the summary of this revision.
> ikudrin added a comment.
>
> - Extract corrections in the existing tests into https://reviews.llvm.org/D40253.
> - Use handling of DT_NEEDED from https://reviews.llvm.org/D40240.
> - Don't try to maintain actual `Live` flag for all symbols. Just mark symbols used in relocations in live sections and make the final decision later in `includeInSymtab`.
>
>
> https://reviews.llvm.org/D38790
>
> Files:
> ELF/MarkLive.cpp
> ELF/Symbols.h
> ELF/Writer.cpp
> test/ELF/gc-collect-undefined.s
> test/ELF/gc-sections-shared.s
>
> Index: test/ELF/gc-sections-shared.s
> ===================================================================
> --- test/ELF/gc-sections-shared.s
> +++ test/ELF/gc-sections-shared.s
> @@ -1,12 +1,15 @@
> # REQUIRES: x86
> # RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %p/Inputs/shared.s -o %t2.o
> # RUN: ld.lld -shared %t2.o -o %t2.so
> +# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %p/Inputs/shared3.s -o %t3.o
> +# RUN: ld.lld -shared %t3.o -o %t3.so
> # RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t.o
> -# RUN: ld.lld --gc-sections --export-dynamic-symbol foo -o %t %t.o --as-needed %t2.so
> +# RUN: ld.lld --gc-sections --export-dynamic-symbol foo -o %t %t.o --as-needed %t2.so %t3.so
> # RUN: llvm-readobj --dynamic-table --dyn-symbols %t | FileCheck %s
>
> # This test the property that we have a needed line for every undefined.
> -# It would also be OK to drop bar2 and the need for the .so
> +# '%t2.so' is dropped because 'bar2' is eliminated, whereas
> +# '%t3.so' is preserved because 'baz' is used.
>
> # CHECK: DynamicSymbols [
> # CHECK-NEXT: Symbol {
> @@ -19,40 +22,42 @@
> # CHECK-NEXT: Section: Undefined (0x0)
> # CHECK-NEXT: }
> # CHECK-NEXT: Symbol {
> -# CHECK-NEXT: Name: bar2
> +# CHECK-NEXT: Name: bar
> # CHECK-NEXT: Value:
> # CHECK-NEXT: Size:
> # CHECK-NEXT: Binding: Global
> # CHECK-NEXT: Type:
> # CHECK-NEXT: Other:
> -# CHECK-NEXT: Section: Undefined
> +# CHECK-NEXT: Section: .text
> # CHECK-NEXT: }
> # CHECK-NEXT: Symbol {
> -# CHECK-NEXT: Name: bar
> +# CHECK-NEXT: Name: foo
> # CHECK-NEXT: Value:
> # CHECK-NEXT: Size:
> # CHECK-NEXT: Binding: Global
> # CHECK-NEXT: Type:
> # CHECK-NEXT: Other:
> # CHECK-NEXT: Section: .text
> # CHECK-NEXT: }
> # CHECK-NEXT: Symbol {
> -# CHECK-NEXT: Name: foo
> +# CHECK-NEXT: Name: baz
> # CHECK-NEXT: Value:
> # CHECK-NEXT: Size:
> # CHECK-NEXT: Binding: Global
> # CHECK-NEXT: Type:
> # CHECK-NEXT: Other:
> -# CHECK-NEXT: Section: .text
> +# CHECK-NEXT: Section: Undefined
> # CHECK-NEXT: }
> # CHECK-NEXT: ]
>
> -# CHECK: NEEDED Shared library: [{{.*}}.so]
> +# CHECK-NOT: NEEDED Shared library: [{{.*}}2.so]
> +# CHECK: NEEDED Shared library: [{{.*}}3.so]
>
> .section .text.foo, "ax"
> .globl foo
> foo:
> call bar
> +call baz
>
> .section .text.bar, "ax"
> .globl bar
> Index: test/ELF/gc-collect-undefined.s
> ===================================================================
> --- /dev/null
> +++ test/ELF/gc-collect-undefined.s
> @@ -0,0 +1,18 @@
> +# REQUIRES: x86
> +# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t
> +# RUN: ld.lld %t -o %tout --gc-sections -shared
> +# RUN: llvm-nm -D %tout | FileCheck %s
> +
> +# CHECK: bar
> +# CHECK-NOT: qux
> +
> + .global foo,bar,qux
> + .local baz
> +
> + .section .data.foo,"aw",%progbits
> +foo:
> + .dc.a bar
> +
> + .section .bata.baz,"aw",%progbits
> +baz:
> + .dc.a qux
> Index: ELF/Writer.cpp
> ===================================================================
> --- ELF/Writer.cpp
> +++ ELF/Writer.cpp
> @@ -441,8 +441,9 @@
> if (auto *S = dyn_cast<MergeInputSection>(Sec))
> if (!S->getSectionPiece(D->Value)->Live)
> return false;
> + return true;
> }
> - return true;
> + return B.ForceLive || !Config->GcSections;
> }
>
> // Local symbols are not in the linker's symbol table. This function scans
> Index: ELF/Symbols.h
> ===================================================================
> --- ELF/Symbols.h
> +++ ELF/Symbols.h
> @@ -134,8 +134,8 @@
> uint8_t Type)
> : Binding(Binding), SymbolKind(K), NeedsPltAddr(false),
> IsInGlobalMipsGot(false), Is32BitMipsGot(false), IsInIplt(false),
> - IsInIgot(false), IsPreemptible(false), Type(Type), StOther(StOther),
> - Name(Name) {}
> + IsInIgot(false), IsPreemptible(false), ForceLive(false), Type(Type),
> + StOther(StOther), Name(Name) {}
>
> const unsigned SymbolKind : 8;
>
> @@ -157,6 +157,9 @@
>
> unsigned IsPreemptible : 1;
>
> + // True if GC marked this symbol alive because it is used in a live section.
> + unsigned ForceLive : 1;
> +
> // The following fields have the same meaning as the ELF symbol attributes.
> uint8_t Type; // symbol type
> uint8_t StOther; // st_other field value
> @@ -363,6 +366,7 @@
> S->CanInline = Sym.CanInline;
> S->Traced = Sym.Traced;
> S->InVersionScript = Sym.InVersionScript;
> + S->ForceLive = Sym.ForceLive;
>
> // Print out a log message if --trace-symbol was specified.
> // This is for debugging.
> Index: ELF/MarkLive.cpp
> ===================================================================
> --- ELF/MarkLive.cpp
> +++ ELF/MarkLive.cpp
> @@ -64,6 +64,9 @@
> std::function<void(InputSectionBase *, uint64_t)> Fn) {
> Symbol &B = Sec.getFile<ELFT>()->getRelocTargetSym(Rel);
>
> + // If a symbol is referenced in a live section, it is alive.
> + B.ForceLive = true;
> +
> if (auto *Sym = dyn_cast<SharedSymbol>(&B))
> if (!Sym->isWeak())
> cast<SharedFile<ELFT>>(Sym->File)->IsNeeded = true;
> @@ -216,10 +219,6 @@
> };
>
> auto MarkSymbol = [&](Symbol *Sym) {
> - if (auto *S = dyn_cast_or_null<SharedSymbol>(Sym))
> - if (!S->isWeak())
> - cast<SharedFile<ELFT>>(S->File)->IsNeeded = true;
> -
> if (auto *D = dyn_cast_or_null<Defined>(Sym))
> if (auto *IS = cast_or_null<InputSectionBase>(D->Section))
> Enqueue(IS, D->Value);
More information about the llvm-commits
mailing list