[ELF] Eliminate symbols demoted due to /DISCARD/ discarded sections (#85167)
#69295 demoted Defined symbols relative to discarded sections.
If such a symbol is unreferenced, the desired behavior is to
eliminate it from .symtab just like --gc-sections discarded
definitions.
Linux kernel's CONFIG_DEBUG_FORCE_WEAK_PER_CPU=y configuration expects
that the unreferenced `unused` is not emitted to .symtab
(https://github.com/ClangBuiltLinux/linux/issues/2006).
For relocations referencing demoted symbols, the symbol index restores
to 0 like older lld (`R_X86_64_64 0` in `discard-section.s`).
Fix #85048
GitOrigin-RevId: 8fe3e70e810b409dce36f6d415e86f0f9b1cf22d
diff --git a/ELF/Writer.cpp b/ELF/Writer.cpp
index a9292b3..d8782af 100644
--- a/ELF/Writer.cpp
+++ b/ELF/Writer.cpp
@@ -261,6 +261,9 @@
Undefined(sym.file, sym.getName(), binding, sym.stOther, sym.type,
/*discardedSecIdx=*/map.lookup(sym.section))
.overwrite(sym);
+ // Eliminate from the symbol table, otherwise we would leave an undefined
+ // symbol if the symbol is unreferenced in the absence of GC.
+ sym.isUsedInRegularObj = false;
}
// If all references to a DSO happen to be weak, the DSO is not added to
diff --git a/test/ELF/linkerscript/discard-section.s b/test/ELF/linkerscript/discard-section.s
index 24f3b2b..0bbebac 100644
--- a/test/ELF/linkerscript/discard-section.s
+++ b/test/ELF/linkerscript/discard-section.s
@@ -9,6 +9,9 @@
# RUN: ld.lld -r -T a.lds a.o b.o -o a.ro 2>&1 | FileCheck %s --check-prefix=WARNING --implicit-check-not=warning:
# RUN: llvm-readelf -r -s a.ro | FileCheck %s --check-prefix=RELOC
+# RUN: ld.lld -r --gc-sections -T a.lds a.o b.o -o a.gc.ro --no-fatal-warnings
+# RUN: llvm-readelf -r -s a.gc.ro | FileCheck %s --check-prefix=RELOC-GC
+
# LOCAL: error: relocation refers to a discarded section: .aaa
# LOCAL-NEXT: >>> defined in a.o
# LOCAL-NEXT: >>> referenced by a.o:(.bbb+0x0)
@@ -32,16 +35,18 @@
# WARNING: warning: relocation refers to a discarded section: .aaa
# WARNING-NEXT: >>> referenced by a.o:(.rela.bbb+0x0)
+## GNU ld reports "defined in discarded secion" errors even in -r mode.
+## We set the symbol index to 0.
# RELOC: Relocation section '.rela.bbb' at offset {{.*}} contains 1 entries:
# RELOC-NEXT: Offset Info Type Symbol's Value Symbol's Name + Addend
# RELOC-NEXT: 0000000000000000 0000000000000000 R_X86_64_NONE 0
# RELOC-EMPTY:
# RELOC-NEXT: Relocation section '.rela.data' at offset {{.*}} contains 4 entries:
# RELOC-NEXT: Offset Info Type Symbol's Value Symbol's Name + Addend
-# RELOC-NEXT: 0000000000000000 0000000500000001 R_X86_64_64 0000000000000000 global + 0
-# RELOC-NEXT: 0000000000000008 0000000700000001 R_X86_64_64 0000000000000000 weak + 0
-# RELOC-NEXT: 0000000000000010 0000000600000001 R_X86_64_64 0000000000000000 weakref1 + 0
-# RELOC-NEXT: 0000000000000018 0000000800000001 R_X86_64_64 0000000000000000 weakref2 + 0
+# RELOC-NEXT: 0000000000000000 0000000000000001 R_X86_64_64 0
+# RELOC-NEXT: 0000000000000008 0000000000000001 R_X86_64_64 0
+# RELOC-NEXT: 0000000000000010 0000000000000001 R_X86_64_64 0
+# RELOC-NEXT: 0000000000000018 0000000000000001 R_X86_64_64 0
# RELOC: Num: Value Size Type Bind Vis Ndx Name
# RELOC-NEXT: 0: 0000000000000000 0 NOTYPE LOCAL DEFAULT UND
@@ -49,23 +54,25 @@
# RELOC-NEXT: 2: 0000000000000000 0 SECTION LOCAL DEFAULT 2 .bbb
# RELOC-NEXT: 3: 0000000000000000 0 SECTION LOCAL DEFAULT 4 .data
# RELOC-NEXT: 4: 0000000000000000 0 NOTYPE GLOBAL DEFAULT 1 _start
-# RELOC-NEXT: 5: 0000000000000000 0 NOTYPE GLOBAL DEFAULT UND global
-# RELOC-NEXT: 6: 0000000000000000 0 NOTYPE GLOBAL DEFAULT UND weakref1
-# RELOC-NEXT: 7: 0000000000000000 0 NOTYPE GLOBAL DEFAULT UND weak
-# RELOC-NEXT: 8: 0000000000000000 0 NOTYPE GLOBAL DEFAULT UND weakref2
# RELOC-EMPTY:
+# RELOC-GC: There are no relocations in this file.
+
#--- a.s
.globl _start
_start:
.section .aaa,"a"
-.globl global, weakref1
+.globl global, weakref1, unused
.weak weak, weakref2
global:
weak:
weakref1:
weakref2:
+## Eliminate `unused` just like GC discarded definitions.
+## Linux kernel's CONFIG_DEBUG_FORCE_WEAK_PER_CPU=y configuration expects
+## that the unreferenced `unused` is not emitted to .symtab.
+unused:
.quad 0
.section .bbb,"aw"
