[lld] Optimize undefined weak symbols in LTO link.

[eugenis]

Summary

This is the LLD side of D25854

[eugenis]

ELF/LTO.cpp

115

I'm not sure this condition is correct. isUndefined() is true for non-prevailing definitions, which I try to avoid.

ELF/LTO.cpp

for (const lto::InputFile::Symbol &ObjSym : Obj.symbols()) {
     // Once IRObjectFile is fixed to report only one symbol this hack can
     // be removed.
     R.Prevailing =
         !(ObjSym.getFlags() & object::BasicSymbolRef::SF_Undefined) &&
         B->File == &F;
 
     R.VisibleToRegularObj =
         Sym->IsUsedInRegularObj || (R.Prevailing && Sym->includeInDynsym());
+
+    R.Undefined = B->isUndefined() &&
+                  (ObjSym.getFlags() & object::BasicSymbolRef::SF_Undefined);
+
     if (R.Prevailing)
       undefine(Sym);
   }
   checkError(LtoObj->add(std::move(F.Obj), Resols));
 }
 
 // Merge all the bitcode files we have seen, codegen the result
 // and return the resulting ObjectFile(s).

test/ELF/lto/Inputs/extern_weak.ll

+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+define void @bar() {
+  ret void
+}

test/ELF/lto/extern_weak.ll

+; REQUIRES: x86
+; RUN: llvm-as %s -o %t1.o
+; RUN: llvm-as %S/Inputs/extern_weak.ll -o %t2.o
+; RUN: ld.lld -m elf_x86_64 %t1.o %t2.o -o %t -shared -save-temps
+; RUN: llvm-dis %t.0.4.opt.bc -o - | FileCheck %s --check-prefix=DEF
+; RUN: ld.lld -m elf_x86_64 %t1.o -o %t -shared -save-temps
+; RUN: llvm-dis %t.0.4.opt.bc -o - | FileCheck %s --check-prefix=UNDEF
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+declare extern_weak void @bar()
+
+define i32 @foo() {
+  ret i32 select (i1 icmp ne (void ()* @bar, void ()* null), i32 1, i32 2)
+}
+
+; DEF: ret i32 1
+; UNDEF: ret i32 2