Bring r329960 back

[• espindola]

Summary

Now that I have debugged this further, I don't think this is the best solution.

But there is an easy fix to the old patch (setting Used) and it is probably better to first commit this and then review the alternative.

[• espindola]

Actually, I misread the location of the call to demoteSharedSymbols. I now think this is the proper fix as it just simplifies the symbols before we start processing the relocations.

[ruiu]

LGTM

ELF/Driver.cpp

1127

Please add {} here and one line below.

[• espindola]

r330788.

[• espindola]

330788

ELF/Driver.cpp

template <class ELFT> static void handleUndefined(StringRef Name) {
   // Since symbol S may not be used inside the program, LTO may
   // eliminate it. Mark the symbol as "used" to prevent it.
   Sym->IsUsedInRegularObj = true;
 
   if (Sym->isLazy())
     Symtab->fetchLazy<ELFT>(Sym);
 }
 
+// If all references to a DSO happen to be weak, the DSO is not added
+// to DT_NEEDED. If that happens, we need to eliminate shared symbols
+// created from the DSO. Otherwise, they become dangling references
+// that point to a non-existent DSO.
+template <class ELFT> static void demoteSharedSymbols() {
+  for (Symbol *Sym : Symtab->getSymbols())
+    if (auto *S = dyn_cast<SharedSymbol>(Sym))
+      if (!S->getFile<ELFT>().IsNeeded) {
+        bool Used = S->Used;
+        replaceSymbol<Undefined>(S, nullptr, S->getName(), STB_WEAK, S->StOther,
+                                 S->Type);
+        S->Used = Used;
+      }
+}
+
 // Do actual linking. Note that when this function is called,
 // all linker scripts have already been parsed.
 template <class ELFT> void LinkerDriver::link(opt::InputArgList &Args) {
   Target = getTarget();
 
   Config->MaxPageSize = getMaxPageSize(Args);
   Config->ImageBase = getImageBase(Args);
 
template <class ELFT> void LinkerDriver::link(opt::InputArgList &Args) {
   // before decompressAndMergeSections because the .comment section is a
   // mergeable section.
   if (!Config->Relocatable)
     InputSections.push_back(createCommentSection());
 
   // Do size optimizations: garbage collection, merging of SHF_MERGE sections
   // and identical code folding.
   markLive<ELFT>();
+  demoteSharedSymbols<ELFT>();
   decompressSections();
   mergeSections();
   if (Config->ICF)
     doIcf<ELFT>();
 
   // Read the callgraph now that we know what was gced or icfed
   if (auto *Arg = Args.getLastArg(OPT_call_graph_ordering_file))
     if (Optional<MemoryBufferRef> Buffer = readFile(Arg->getValue()))
       readCallGraph(*Buffer);
 
   // Write the result to the file.
   writeResult<ELFT>();
 }

test/ELF/as-needed-weak.s

+# REQUIRES: x86
+
+# RUN: echo '.globl foo; .type foo, @function; foo:' | \
+# RUN:   llvm-mc -filetype=obj -triple=x86_64-unknown-linux - -o %t1.o
+# RUN: ld.lld -shared -o %t1.so -soname libfoo %t1.o
+
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t2.o
+# RUN: ld.lld -o %t.exe %t2.o --as-needed %t1.so
+# RUN: llvm-readelf -dynamic-table -dyn-symbols %t.exe | FileCheck %s
+
+# CHECK-NOT: libfoo
+
+# CHECK:      Symbol table of .hash for image:
+# CHECK-NEXT: Num Buc:    Value          Size   Type   Bind Vis      Ndx Name
+# CHECK-NEXT:   1   1: 0000000000000000     0 FUNC    WEAK   DEFAULT UND foo@
+
+.globl _start
+.weak foo
+
+_start:
+  mov $foo, %eax
+  callq foo

test/ELF/gc-sections-shared.s

 # CHECK-NEXT:     Value:
 # CHECK-NEXT:     Size:
 # CHECK-NEXT:     Binding: Local
 # CHECK-NEXT:     Type:
 # CHECK-NEXT:     Other:
 # CHECK-NEXT:     Section: Undefined (0x0)
 # CHECK-NEXT:   }
 # CHECK-NEXT:   Symbol {
+# CHECK-NEXT:     Name: qux
+# CHECK-NEXT:     Value:
+# CHECK-NEXT:     Size:
+# CHECK-NEXT:     Binding: Weak
+# CHECK-NEXT:     Type:
+# CHECK-NEXT:     Other:
+# CHECK-NEXT:     Section: Undefined
+# CHECK-NEXT:   }
+# CHECK-NEXT:   Symbol {
 # CHECK-NEXT:     Name: bar
 # CHECK-NEXT:     Value:
 # CHECK-NEXT:     Size:
 # CHECK-NEXT:     Binding: Global
 # CHECK-NEXT:     Type:
 # CHECK-NEXT:     Other:
 # CHECK-NEXT:     Section: .text
 # CHECK-NEXT:   }

 # 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: qux
-# CHECK-NEXT:     Value:
-# CHECK-NEXT:     Size:
-# CHECK-NEXT:     Binding: Weak
-# CHECK-NEXT:     Type:
-# CHECK-NEXT:     Other:
-# CHECK-NEXT:     Section: Undefined
-# CHECK-NEXT:   }
 # CHECK-NEXT: ]
 
 # CHECK-NOT: NEEDED
 # CHECK:     NEEDED Shared library: [{{.*}}3.so]
 # CHECK-NOT: NEEDED
 
 # Test with %t.o at the end too.
 # RUN: ld.lld --gc-sections --export-dynamic-symbol foo -o %t --as-needed %t2.so %t3.so %t4.so %t.o
-# RUN: llvm-readobj --dynamic-table --dyn-symbols %t | FileCheck --check-prefix=CHECK2 %s
-
-# CHECK2:      DynamicSymbols [
-# CHECK2-NEXT:   Symbol {
-# CHECK2-NEXT:     Name:
-# CHECK2-NEXT:     Value:
-# CHECK2-NEXT:     Size:
-# CHECK2-NEXT:     Binding: Local
-# CHECK2-NEXT:     Type:
-# CHECK2-NEXT:     Other:
-# CHECK2-NEXT:     Section: Undefined (0x0)
-# CHECK2-NEXT:   }
-# CHECK2-NEXT:   Symbol {
-# CHECK2-NEXT:     Name: bar
-# CHECK2-NEXT:     Value:
-# CHECK2-NEXT:     Size:
-# CHECK2-NEXT:     Binding: Global
-# CHECK2-NEXT:     Type:
-# CHECK2-NEXT:     Other:
-# CHECK2-NEXT:     Section: .text
-# CHECK2-NEXT:   }
-# CHECK2-NEXT:   Symbol {
-# CHECK2-NEXT:     Name: baz
-# CHECK2-NEXT:     Value:
-# CHECK2-NEXT:     Size:
-# CHECK2-NEXT:     Binding: Global
-# CHECK2-NEXT:     Type:
-# CHECK2-NEXT:     Other:
-# CHECK2-NEXT:     Section: Undefined
-# CHECK2-NEXT:   }
-# CHECK2-NEXT:   Symbol {
-# CHECK2-NEXT:     Name: qux
-# CHECK2-NEXT:     Value:
-# CHECK2-NEXT:     Size:
-# CHECK2-NEXT:     Binding: Weak
-# CHECK2-NEXT:     Type:
-# CHECK2-NEXT:     Other:
-# CHECK2-NEXT:     Section: Undefined
-# CHECK2-NEXT:   }
-# CHECK2-NEXT:   Symbol {
-# CHECK2-NEXT:     Name: foo
-# CHECK2-NEXT:     Value:
-# CHECK2-NEXT:     Size:
-# CHECK2-NEXT:     Binding: Global
-# CHECK2-NEXT:     Type:
-# CHECK2-NEXT:     Other:
-# CHECK2-NEXT:     Section: .text
-# CHECK2-NEXT:   }
-# CHECK2-NEXT: ]
-
-# CHECK2-NOT: NEEDED
-# CHECK2:     NEEDED Shared library: [{{.*}}3.so]
-# CHECK2-NOT: NEEDED
+# RUN: llvm-readobj --dynamic-table --dyn-symbols %t | FileCheck --check-prefix=CHECK %s
 
 .section .text.foo, "ax"
 .globl foo
 foo:
 .long bar - .
 
 .section .text.bar, "ax"
 .globl bar