Drop __real_ symbol from the output symbol table.

[ruiu]

Summary

In D34993, we discussed and concluded that we should drop real_
symbol from the symbol table, but I did the opposite in D50569.
This patch is to drop real_ symbol.

[chrisjackson]

@ruiu

Hello, LGTM.

[grimar]

My apologies here, I did not see this patch for some reason. I believe it is ok, so LGTM.

[MaskRay]

@thakis This fixed the --wrap bug mentioned in https://bugs.chromium.org/p/chromium/issues/detail?id=1086636

It seems that this patch had never been committed. I commited it with a few adjustment to the comment and wrap-no-real.s

Note that omitting __real_ is a correctness matter if __real_ is undefined:

// Now renaming is complete, and no one refers to real. We drop real from
// .symtab and .dynsym. If real is undefined, it is important that we don't
// leave it in .dynsym, because otherwise it might lead to an undefined symbol
// error in a subsequent link. If real is defined, we could emit real as an
// alias for sym, but that could degrade the user experience of some tools
// that can print out only one symbol for each location: sym is a preferred
// name than real, but they might print out real instead.

lld/ELF/SymbolTable.cpp

void SymbolTable::wrap(Symbol *Sym, Symbol *Real, Symbol *Wrap) {
   // Swap symbols as instructed by -wrap.
   int &Idx1 = Symtab->SymMap[CachedHashStringRef(Sym->getName())];
   int &Idx2 = Symtab->SymMap[CachedHashStringRef(Real->getName())];
   int &Idx3 = Symtab->SymMap[CachedHashStringRef(Wrap->getName())];
 
   Idx2 = Idx1;
   Idx1 = Idx3;
 
-  // Now renaming is complete. No one refers Real symbol. We could leave
-  // Real as-is, but if Real is written to the symbol table, that may
-  // contain irrelevant values. So, we copy all values from Sym to Real.
-  StringRef S = Real->getName();
-  memcpy(Real, Sym, sizeof(SymbolUnion));
-  Real->setName(S);
+  // Now renaming is complete, and no one refers Real symbol. We drop Real
+  // from the symbol table because it's useless in the output binary.
+  // Alternatively, we could emit Real as an alias for Sym, but that could
+  // degrade the user experience of some tools that can print out only one
+  // symbol for each location; Sym is a preferred name than Real, but they
+  // might print out Real instead.
+  Real->IsUsedInRegularObj = false;
 }
 
 static uint8_t getMinVisibility(uint8_t VA, uint8_t VB) {
   if (VA == STV_DEFAULT)
     return VB;
   if (VB == STV_DEFAULT)
     return VA;
   return std::min(VA, VB);

lld/test/ELF/lto/wrap-2.ll

 ; THIN-NEXT: popq  %rax
 ; THIN-NEXT: jmp{{.*}}<bar>
 
 ; Check that bar and __wrap_bar retain their original binding.
 ; BIND:      Name: bar
 ; BIND-NEXT: Value:
 ; BIND-NEXT: Size:
 ; BIND-NEXT: Binding: Local
-; BIND:      Name: __real_bar
-; BIND-NEXT: Value:
-; BIND-NEXT: Size:
-; BIND-NEXT: Binding: Local
 ; BIND:      Name: __wrap_bar
 ; BIND-NEXT: Value:
 ; BIND-NEXT: Size:
 ; BIND-NEXT: Binding: Local
 
 target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64-unknown-linux-gnu"
 
 declare void @bar()
 declare void @__real_bar()
 
 define void @foo() {
   call void @bar()
   call void @__real_bar()
   ret void
 }

lld/test/ELF/wrap-no-real.s

 // CHECK-NEXT: movl $0x11010, %edx
 // CHECK-NEXT: movl $0x11000, %edx
 
 // RUN: llvm-objdump -t %t | FileCheck -check-prefix=SYM %s
 
 
 // SYM:      0000000000000000  *UND*     00000000
 // SYM-NEXT: 0000000000202000  .dynamic  00000000 .hidden _DYNAMIC
-// SYM-NEXT: 0000000000011000  *ABS*     00000000 __real_foo
 // SYM-NEXT: 0000000000011010  *ABS*     00000000 __wrap_foo
 // SYM-NEXT: 0000000000201000  .text     00000000 _start
 // SYM-NEXT: 0000000000011000  *ABS*     00000000 foo
 
 .global _start
 _start:
   movl $foo, %edx
   movl $__wrap_foo, %edx
   movl $__real_foo, %edx

lld/test/ELF/wrap.s

 // SYM2:      Name: __wrap_foo
 // SYM2-NEXT: Value: 0x11010
 // SYM2-NEXT: Size:
 // SYM2-NEXT: Binding: Weak
 // SYM2-NEXT: Type:    None
 // SYM2-NEXT: Other [
 // SYM2-NEXT:   STV_PROTECTED
 // SYM2-NEXT: ]
-// SYM3:      Name: __real_foo
-// SYM3-NEXT: Value: 0x11000
-// SYM3-NEXT: Size:
-// SYM3-NEXT: Binding: Global
-// SYM3-NEXT: Type:    None
-// SYM3-NEXT: Other:   0
+// SYM3-NOT:  Name: __real_foo
 
 .global _start
 _start:
   movl $foo, %edx
   movl $__wrap_foo, %edx
   movl $__real_foo, %edx