[ELF] Make --trace-symbol track preempted shared definitions

[MaskRay]

Summary

Note, we still name a preempted SharedSymbol "shared definition",
instead of "reference" as printed by GNU ld. This difference should not matter.

// GNU ld
ld.bfd: t: definition of f@v1
ld.bfd: t.so: reference to f@v1

[MaskRay]

Delete an empty line

[MaskRay]

Simplify

[psmith]

LGTM.

lld/ELF/Symbols.cpp

716–717

I know some prefer an unrelated refactoring change to be split out. Maybe worth committing this separately as an NFC white space change.

[MaskRay]

Drop unneeded changes

lld/ELF/Symbols.cpp

if (isWeak()) {
     type = ty;
     binding = STB_WEAK;
     return;
   }
 
   other.fetch();
 }
 
 void Symbol::resolveShared(const SharedSymbol &other) {
   if (isCommon()) {
     // See the comment in resolveCommon() above.
     if (other.size > cast<CommonSymbol>(this)->size)
       cast<CommonSymbol>(this)->size = other.size;
     return;
   }
   if (visibility == STV_DEFAULT && (isUndefined() || isLazy())) {
     // An undefined symbol with non default visibility must be satisfied
     // in the same DSO.
     uint8_t bind = binding;
     replace(other);
     binding = bind;
-  }
+  } else if (traced)
+    printTraceSymbol(&other);
 }

lld/test/ELF/trace-symbols.s

 # REQUIRES: x86
 # Test -y symbol and -trace-symbol=symbol
 
 # RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t
 # RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux \
 # RUN: %p/Inputs/trace-symbols-foo-weak.s -o %t1
 # RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux \
 # RUN: %p/Inputs/trace-symbols-foo-strong.s -o %t2
 # RUN: ld.lld -shared %t1 -o %t1.so
 # RUN: ld.lld -shared %t2 -o %t2.so
 # RUN: rm -f %t1.a
 # RUN: llvm-ar rcs %t1.a %t1
 # RUN: rm -f %t2.a
 # RUN: llvm-ar rcs %t2.a %t2
 
+# RUN: ld.lld -y foo -shared %t1 %t1.so -o /dev/null | \
+# RUN:   FileCheck --check-prefix=PREEMPT %s --implicit-check-not=foo
+# PREEMPT:      trace-symbols.s.tmp1: definition of foo
+# PREEMPT-NEXT: trace-symbols.s.tmp1.so: shared definition of foo
+
 # RUN: ld.lld -y foo -trace-symbol common -trace-symbol=hsymbol \
 # RUN:   %t %t1 %t2 -o %t3 | FileCheck -check-prefix=OBJECTRFOO %s
 # OBJECTRFOO: trace-symbols.s.tmp: reference to foo
 
 # RUN: ld.lld -y foo -trace-symbol=common -trace-symbol=hsymbol \
 # RUN:   %t %t1 %t2 -o %t3 | FileCheck -check-prefix=OBJECTDCOMMON %s
 # OBJECTDCOMMON: trace-symbols.s.tmp1: common definition of common