[ELF] Do not keep symbols if they referenced only from discarded sections.

This patch also ensures that in case of "--as-needed" is used,
DT_NEEDED entries are not created if they are required only by
these eliminated symbols.

Differential Revision: https://reviews.llvm.org/D38790

llvm-svn: 319008

diff --git a/lld/ELF/MarkLive.cpp b/lld/ELF/MarkLive.cpp
index 3520abe..e619a95 100644 (file)
--- a/lld/ELF/MarkLive.cpp
+++ b/lld/ELF/MarkLive.cpp
@@ -64,6 +64,12 @@ static void resolveReloc(InputSectionBase &Sec, RelT &Rel,
                          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;

diff --git a/lld/ELF/SymbolTable.cpp b/lld/ELF/SymbolTable.cpp
index e45ce22..4943a2e 100644 (file)
--- a/lld/ELF/SymbolTable.cpp
+++ b/lld/ELF/SymbolTable.cpp
@@ -304,7 +304,8 @@ Symbol *SymbolTable::addUndefined(StringRef Name, uint8_t Binding,
     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 @@ void SymbolTable::addShared(StringRef Name, SharedFile<ELFT> *File,
                                 Sym.st_value, Sym.st_size, Alignment, Verdef);
     if (!WasInserted) {
       S->Binding = Binding;
-      if (!S->isWeak())
+      if (!S->isWeak() && !Config->GcSections)
         File->IsNeeded = true;
     }
   }

diff --git a/lld/ELF/Symbols.h b/lld/ELF/Symbols.h
index b7b26e9..533d43b 100644 (file)
--- a/lld/ELF/Symbols.h
+++ b/lld/ELF/Symbols.h
@@ -134,8 +134,8 @@ protected:
          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 @@ public:
 
   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

diff --git a/lld/ELF/Writer.cpp b/lld/ELF/Writer.cpp
index e74289c..0ab33f5 100644 (file)
--- a/lld/ELF/Writer.cpp
+++ b/lld/ELF/Writer.cpp
@@ -448,8 +448,9 @@ static bool includeInSymtab(const Symbol &B) {
     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

diff --git a/lld/test/ELF/Inputs/gc-sections-shared2.s b/lld/test/ELF/Inputs/gc-sections-shared2.s
new file mode 100644 (file)
index 0000000..fb69c0e
--- /dev/null
+++ b/lld/test/ELF/Inputs/gc-sections-shared2.s
@@ -0,0 +1,3 @@
+.global qux
+.type qux, @function
+qux:

diff --git a/lld/test/ELF/gc-collect-undefined.s b/lld/test/ELF/gc-collect-undefined.s
new file mode 100644 (file)
index 0000000..7ade554
--- /dev/null
+++ b/lld/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

diff --git a/lld/test/ELF/gc-sections-shared.s b/lld/test/ELF/gc-sections-shared.s
index 80b3d61..b9ce5de 100644 (file)
--- a/lld/test/ELF/gc-sections-shared.s
+++ b/lld/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 {
@@ -21,16 +25,16 @@
 # 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
@@ -39,13 +43,13 @@
 # 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 @@ ret
 
 .section .text._start, "ax"
 .globl _start
+.weak qux
 _start:
 call baz
+call qux
 ret
 
 .section .text.unused, "ax"