Allow undefined symbols in shared library by default

[emaste]

Summary

Change from D8886 with updated testcase.

(LLVM phabricator is not set to allow reopening a review, it seems.)

[• rafael]

Fine by me, but someone from the lld side should LGTM it

From the discussion on the list, there seem to be two consistent ways to handle this

• Say that a .so's dependency is the job of the dynamic linker (this patch new default and what gold does) (this is by far my preference).
• Fully implement what a dynamic linker does. In particular, resolve a symbol from a executable to an indirect dependency. This is *not* done by gold or bfd ld.

lib/Core/LinkingContext.cpp

 LinkingContext::LinkingContext()
     : _deadStrip(false), _allowDuplicates(false),
       _globalsAreDeadStripRoots(false),
       _searchArchivesToOverrideTentativeDefinitions(false),
       _searchSharedLibrariesToOverrideTentativeDefinitions(false),
       _warnIfCoalesableAtomsHaveDifferentCanBeNull(false),
       _warnIfCoalesableAtomsHaveDifferentLoadName(false),
       _printRemainingUndefines(true), _allowRemainingUndefines(false),
-      _logInputFiles(false), _allowShlibUndefines(false),
+      _logInputFiles(false), _allowShlibUndefines(true),
       _outputFileType(OutputFileType::Default), _nextOrdinal(0) {}
 
 LinkingContext::~LinkingContext() {}
 
 bool LinkingContext::validate(raw_ostream &diagnostics) {
   return validateImpl(diagnostics);
 }
 

test/elf/dynamic-undef.test

 #
 # This test creates a executable and tests the options that are used to
 # to create an executable and a shared library
 #
 # This test will fail because there are unresolved symbols from the shared
-# library and we are not passing --allow-shlib-undefined
+# library and we are passing --no-allow-shlib-undefined
 RUN: not lld -flavor gnu -target x86_64-linux %p/Inputs/tls.x86-64 \
-RUN:   %p/Inputs/shared.so-x86-64 -o %t -e main 2> %t1
+RUN:   %p/Inputs/shared.so-x86-64 -o %t -e main --no-allow-shlib-undefined 2> %t1
 RUN: FileCheck -check-prefix=EXEC %s < %t1
 # This test will pass because of --allow-shlib-undefined
 RUN: lld -flavor gnu -target x86_64-linux %p/Inputs/tls.x86-64 \
 RUN:   %p/Inputs/shared.so-x86-64 -o %t -e main --allow-shlib-undefined \
 RUN:   --defsym=__tls_get_addr=0
+# This test will pass becase --allow-shlib-undefined is the default.
+RUN: lld -flavor gnu -target x86_64-linux %p/Inputs/tls.x86-64 \
+RUN:   %p/Inputs/shared.so-x86-64 -o %t -e main \
+RUN:   --defsym=__tls_get_addr=0
 # Building shared libraries should not fail when there is a undefined symbol.
 # Test creation of shared library, this should pass because we are using
 # shared option and by default, dynamic library wouldn't create undefined atoms
 # from the input shared library
 RUN: lld -flavor gnu -target x86_64-linux %p/Inputs/tls.x86-64 \
 RUN:   %p/Inputs/shared.so-x86-64 -o %t.usenoundefines -e main -shared
 RUN: llvm-readobj -symbols %t.usenoundefines | FileCheck %s -check-prefix=SHLIB-NOUNDEF
 # Test creation of shared library, this should fail because we are using