[ELF] Move IsNeeded logic from SymbolTable::addShared to MarkLive, and check IsUsedInRegularObj

Summary:
In glibc, libc.so is a linker script with an as-needed dependency on ld-linux-x86-64.so.2

    GROUP ( /lib/x86_64-linux-gnu/libc.so.6 /usr/lib/x86_64-linux-gnu/libc_nonshared.a  AS_NEEDED ( /lib/x86_64-linux-gnu/ld-linux-x86-64.so.2 ) )

ld-linux-x86-64.so.2 (as-needed) defines some symbols which resolve undefined references in libc.so.6, it will therefore be added as a DT_NEEDED entry, which isn't necessary.

The test case as-needed-not-in-regular.s emulates the libc.so scenario, where ld.bfd and gold don't add DT_NEEDED for a.so

The relevant code in gold/resolve.cc:

  // If we have a non-WEAK reference from a regular object to a
  // dynamic object, mark the dynamic object as needed.
  if (to->is_from_dynobj() && to->in_reg() && !to->is_undef_binding_weak())
    to->object()->set_is_needed();

in_reg() appears to do something similar to IsUsedInRegularObj.

This patch makes lld do the similar thing, but moves the check from
addShared to a later stage MarkLive where all symbols are scanned.

Reviewers: ruiu, pcc, espindola

Reviewed By: ruiu

Subscribers: emaste, arichardson, llvm-commits

Differential Revision: https://reviews.llvm.org/D55902

llvm-svn: 349849

lld/ELF/MarkLive.cpp

@@ -267,10 +267,16 @@ template <class ELFT> static void doGcSections() {
 // input sections. This function make some or all of them on
 // so that they are emitted to the output file.
 template <class ELFT> void elf::markLive() {
-  // If -gc-sections is missing, no sections are removed.
   if (!Config->GcSections) {
+    // If -gc-sections is missing, no sections are removed.
     for (InputSectionBase *Sec : InputSections)
       Sec->Live = true;
+
+    // If a DSO defines a symbol referenced in a regular object, it is needed.
+    for (Symbol *Sym : Symtab->getSymbols())
+      if (auto *S = dyn_cast<SharedSymbol>(Sym))
+        if (S->IsUsedInRegularObj && !S->isWeak())
+          S->getFile<ELFT>().IsNeeded = true;
     return;
   }
 

lld/ELF/SymbolTable.cpp

@@ -503,14 +503,11 @@ void SymbolTable::addShared(StringRef Name, SharedFile<ELFT> &File,
 
   if ((S->isUndefined() || S->isLazy()) && S->Visibility == STV_DEFAULT) {
     uint8_t Binding = S->Binding;
-    bool WasUndefined = S->isUndefined();
     replaceSymbol<SharedSymbol>(S, File, Name, Sym.getBinding(), Sym.st_other,
                                 Sym.getType(), Sym.st_value, Sym.st_size,
                                 Alignment, VerdefIndex);
 
     S->Binding = Binding;
-    if (!S->isWeak() && !Config->GcSections && WasUndefined)
-      File.IsNeeded = true;
   }
 }
 

lld/test/ELF/as-needed-in-regular.s

@@ -0,0 +1,24 @@
+# REQUIRES: x86
+
+# RUN: echo '.globl a; .type a, @function; .type a, @function; a: ret' | \
+# RUN:   llvm-mc -filetype=obj -triple=x86_64-unknown-linux - -o %ta.o
+# RUN: ld.lld %ta.o --shared --soname=a.so -o %ta.so
+
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t.o
+# RUN: ld.lld %t.o --as-needed %ta.so -o %t
+# RUN: llvm-readelf -d %t | FileCheck %s
+# RUN: ld.lld %t.o --as-needed %ta.so --gc-sections -o %t
+# RUN: llvm-readelf -d %t | FileCheck %s
+
+# The order of %ta.so and %t.o does not matter.
+
+# RUN: ld.lld --as-needed %ta.so %t.o -o %t
+# RUN: llvm-readelf -d %t | FileCheck %s
+# RUN: ld.lld --as-needed %ta.so %t.o --gc-sections -o %t
+# RUN: llvm-readelf -d %t | FileCheck %s
+
+# CHECK: a.so
+
+.global _start
+_start:
+  jmp a@PLT

lld/test/ELF/as-needed-not-in-regular.s

@@ -0,0 +1,26 @@
+# REQUIRES: x86
+
+# RUN: echo '.globl a1, a2; .type a1, @function; .type a2, @function; a1: a2: ret' | \
+# RUN:   llvm-mc -filetype=obj -triple=x86_64-unknown-linux - -o %ta.o
+# RUN: ld.lld %ta.o --shared --soname=a.so -o %ta.so
+
+# RUN: echo '.globl b; .type b, @function; b: jmp a1@PLT' | \
+# RUN:   llvm-mc -filetype=obj -triple=x86_64-unknown-linux - -o %tb.o
+# RUN: ld.lld %tb.o %ta.so --shared --soname=b.so -o %tb.so
+
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t.o
+# RUN: ld.lld %t.o %tb.so --as-needed %ta.so -o %t
+# RUN: llvm-readelf -d %t | FileCheck %s
+
+# RUN: ld.lld %t.o %tb.so --as-needed %ta.so --gc-sections -o %t
+# RUN: llvm-readelf -d %t | FileCheck %s
+
+# The symbol a1 (defined in a.so) is not referenced by a regular object,
+# the reference to a2 is weak, don't add a DT_NEEDED entry for a.so.
+# CHECK-NOT: a.so
+
+.global _start
+.weak a2
+_start:
+  jmp b@PLT
+  jmp a2