[ELF] - Do not error out if "entry symbol not found" warning and --fatal-warnings are used together.

[grimar]

Summary

We produce warning "entry symbol not found". If it is used together with --fatal-warnings,
lld errors out. That is different from whar ld do, ld ignores that flag with this warning.
btw gold disagree with ld here, but I assume we want to be compatible with ld, so
I introduced new method info() that solves that problem.

Initially I fixed it as PR30521, but as Ed noticed, PR30521 links shared which does not have entry,
so this fix is not about that case, but still about difference of ld/gold.
Do we want it ?

[emaste]

The case that prompted PR30521 was a shared library. Lack of an entry symbol there should not be a warning I think - I'm not sure we ought to mention it at all.

[grimar]

In D24908#552119, @emaste wrote:

The case that prompted PR30521 was a shared library. Lack of an entry symbol there should not be a warning I think - I'm not sure we ought to mention it at all.

Right, shared case also requires patch, I`ll do it.
What do you think of this one then ?

[ruiu]

I wonder if this is the right thing to do. That message is a warning message, so it seems to me that it naturally should turn into an error when --fatal-warning is given, even if that's different from what GNU ld does.

[emaste]

We need to emit no warning when the output is a shared library, and then I think --fatal-warnings should indeed make the warning (in an exe) an error.

[ruiu]

Agreed.

[grimar]

This was ineffectual one.

ELF/Driver.cpp

if (!Config->Entry.empty()) {
     // -e was not specified. Use the default start symbol name
     // if it is resolvable.
     Config->Entry = (Config->EMachine == EM_MIPS) ? "__start" : "_start";
   }
   if (!HasEntryAddr) {
     if (Symtab.find(Config->Entry))
       Config->EntrySym = Symtab.addUndefined(Config->Entry);
     else
-      warning("entry symbol " + Config->Entry + " not found, assuming 0");
+      info("entry symbol " + Config->Entry + " not found, assuming 0");
   }
 
   if (HasError)
     return; // There were duplicate symbols or incompatible files
 
   Symtab.scanUndefinedFlags();
   Symtab.scanShlibUndefined();
   Symtab.scanDynamicList();

ELF/Error.h

 
 namespace lld {
 namespace elf {
 
 extern bool HasError;
 extern llvm::raw_ostream *ErrorOS;
 
 void log(const Twine &Msg);
+void info(const Twine &Msg);
 void warning(const Twine &Msg);
 
 void error(const Twine &Msg);
 void error(std::error_code EC, const Twine &Prefix);
 
 template <typename T> void error(const ErrorOr<T> &V, const Twine &Prefix) {
   error(V.getError(), Prefix);
 }

ELF/Error.cpp

 bool elf::HasError;
 raw_ostream *elf::ErrorOS;
 
 void elf::log(const Twine &Msg) {
   if (Config->Verbose)
     outs() << Msg << "\n";
 }
 
+void elf::info(const Twine &Msg) { *ErrorOS << Msg << "\n"; }
+
 void elf::warning(const Twine &Msg) {
   if (Config->FatalWarnings)
     error(Msg);
   else
     *ErrorOS << Msg << "\n";
 }
 
 void elf::error(const Twine &Msg) {

test/ELF/entry.s

 # RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t1
-# RUN: ld.lld -e foobar %t1 -o %t2 2>&1 | FileCheck -check-prefix=WARN %s
+# RUN: ld.lld -e foobar %t1 -o %t2 2>&1 | FileCheck -check-prefix=INFO %s
+# RUN: ld.lld --fatal-warnings -e foobar %t1 -o %t2 2>&1 | FileCheck -check-prefix=INFO %s
 # RUN: llvm-readobj -file-headers %t2 | FileCheck -check-prefix=NOENTRY %s
 # RUN: ld.lld %t1 -o %t2 -e entry
 # RUN: llvm-readobj -file-headers %t2 | FileCheck -check-prefix=SYM %s
 # RUN: ld.lld %t1 -shared -o %t2 -e entry
 # RUN: llvm-readobj -file-headers %t2 | FileCheck -check-prefix=DSO %s
 # RUN: ld.lld %t1 -o %t2 --entry=4096
 # RUN: llvm-readobj -file-headers %t2 | FileCheck -check-prefix=DEC %s
 # RUN: ld.lld %t1 -o %t2 --entry 0xcafe
 # RUN: llvm-readobj -file-headers %t2 | FileCheck -check-prefix=HEX %s
 # RUN: ld.lld %t1 -o %t2 -e 0777
 # RUN: llvm-readobj -file-headers %t2 | FileCheck -check-prefix=OCT %s
 
-# WARN: entry symbol foobar not found, assuming 0
+# INFO: entry symbol foobar not found, assuming 0
 
 # NOENTRY: Entry: 0x0
 # SYM: Entry: 0x11000
 # DSO: Entry: 0x1000
 # DEC: Entry: 0x1000
 # HEX: Entry: 0xCAFE
 # OCT: Entry: 0x1FF
 
 .globl entry
 entry: