[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
Fri Nov 24 10:03:39 PST 2017
LGTM
Good catch on the undefined weak in the gc case!
Thanks,
Rafael
Igor Kudrin via Phabricator <reviews at reviews.llvm.org> writes:
> ikudrin updated this revision to Diff 124139.
> ikudrin added a comment.
>
> - Rebased on the tip.
> - This is mostly Rafael's version of the patch.
> - Added a check for a weak symbol when setting `IsNeeded`.
> - Added the test to check that a weak symbol doesn't cause adding DT_NEEDED; this test might be extracted to a separate patch.
>
>
> https://reviews.llvm.org/D38790
>
> Files:
> ELF/MarkLive.cpp
> ELF/SymbolTable.cpp
> ELF/Symbols.h
> ELF/Writer.cpp
> test/ELF/Inputs/gc-sections-shared2.s
> 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
> @@ -1,14 +1,18 @@
> # REQUIRES: x86
> # RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %p/Inputs/shared.s -o %t2.o
> # RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %p/Inputs/gc-sections-shared.s -o %t3.o
> +# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %p/Inputs/gc-sections-shared2.s -o %t4.o
> # RUN: ld.lld -shared %t2.o -o %t2.so
> # RUN: ld.lld -shared %t3.o -o %t3.so
> +# RUN: ld.lld -shared %t4.o -o %t4.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 %t3.so
> +# RUN: ld.lld --gc-sections --export-dynamic-symbol foo -o %t %t.o --as-needed %t2.so %t3.so %t4.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
> +# It would also be OK to keep bar2 and the need for %t2.so
> +# At the same time, weak symbols should not cause adding DT_NEEDED;
> +# this case is checked with symbol qux and %t4.so.
>
> # CHECK: DynamicSymbols [
> # CHECK-NEXT: Symbol {
> @@ -21,31 +25,31 @@
> # 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: qux
> # CHECK-NEXT: Value:
> # CHECK-NEXT: Size:
> -# CHECK-NEXT: Binding: Global
> +# CHECK-NEXT: Binding: Weak
> # CHECK-NEXT: Type:
> # CHECK-NEXT: Other:
> -# CHECK-NEXT: Section: .text
> +# CHECK-NEXT: Section: Undefined
> # CHECK-NEXT: }
> # CHECK-NEXT: Symbol {
> # CHECK-NEXT: Name: baz
> @@ -59,7 +63,6 @@
> # CHECK-NEXT: ]
>
> # CHECK-NOT: NEEDED
> -# CHECK: NEEDED Shared library: [{{.*}}2.so]
> # CHECK: NEEDED Shared library: [{{.*}}3.so]
> # CHECK-NOT: NEEDED
>
> @@ -75,8 +78,10 @@
>
> .section .text._start, "ax"
> .globl _start
> +.weak qux
> _start:
> call baz
> +call qux
> ret
>
> .section .text.unused, "ax"
> Index: test/ELF/gc-collect-undefined.s
> ===================================================================
> --- /dev/null
> +++ test/ELF/gc-collect-undefined.s
> @@ -0,0 +1,19 @@
> +# 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-NOT: qux
> +# 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: test/ELF/Inputs/gc-sections-shared2.s
> ===================================================================
> --- /dev/null
> +++ test/ELF/Inputs/gc-sections-shared2.s
> @@ -0,0 +1,3 @@
> +.global qux
> +.type qux, @function
> +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.Used;
> }
>
> // 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), Used(!Config->GcSections),
> + Type(Type), StOther(StOther), Name(Name) {}
>
> const unsigned SymbolKind : 8;
>
> @@ -157,6 +157,9 @@
>
> unsigned IsPreemptible : 1;
>
> + // True if an undefined or shared symbol is used from a live section.
> + unsigned Used : 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
> Index: ELF/SymbolTable.cpp
> ===================================================================
> --- ELF/SymbolTable.cpp
> +++ ELF/SymbolTable.cpp
> @@ -304,7 +304,8 @@
> if (!S->isDefined())
> S->Binding = Binding;
> if (auto *SS = dyn_cast<SharedSymbol>(S))
> - SS->getFile<ELFT>()->IsNeeded = true;
> + if (!Config->GcSections)
> + SS->getFile<ELFT>()->IsNeeded = true;
> }
> if (auto *L = dyn_cast<Lazy>(S)) {
> // An undefined weak will not fetch archive members. See comment on Lazy in
> @@ -500,7 +501,7 @@
> Sym.st_value, Sym.st_size, Alignment, Verdef);
> if (!WasInserted) {
> S->Binding = Binding;
> - if (!S->isWeak())
> + if (!S->isWeak() && !Config->GcSections)
> File->IsNeeded = true;
> }
> }
> Index: ELF/MarkLive.cpp
> ===================================================================
> --- ELF/MarkLive.cpp
> +++ ELF/MarkLive.cpp
> @@ -64,6 +64,12 @@
> 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 used.
> + B.Used = true;
> + if (auto *SS = dyn_cast<SharedSymbol>(&B))
> + if (!SS->isWeak())
> + SS->getFile<ELFT>()->IsNeeded = true;
> +
> if (auto *D = dyn_cast<Defined>(&B)) {
> if (!D->Section)
> return;
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