COFF: Stop requiring comdat sections to have an external leader to participate in ICF.

[pcc]

Summary

This requirement was added in r254578 to fix pr25686. However, it
appears to have originated from a misdiagnosis of the problem: link.exe
refused to merge the two sections because they are non-executable,
not because they have internal leaders. If I set up a similar scenario
with functions instead of globals I see that link.exe merges them.

[ruiu]

LGTM

lld/trunk/COFF/ICF.cpp

 // identical .xdata sections, because the address of unwind information is
 // insignificant to the user program and the Visual C++ linker does this.
 bool ICF::isEligible(SectionChunk *C) {
   // Non-comdat chunks, dead chunks, and writable chunks are not elegible.
   bool Writable = C->getPermissions() & llvm::COFF::IMAGE_SCN_MEM_WRITE;
   if (!C->isCOMDAT() || !C->isLive() || Writable)
     return false;
 
-  // Code sections with external symbols are eligible.
-  bool Global = C->Sym && C->Sym->isExternal();
-  bool Executable = C->getPermissions() & llvm::COFF::IMAGE_SCN_MEM_EXECUTE;
-  if (Global && Executable)
+  // Code sections are eligible.
+  if (C->getPermissions() & llvm::COFF::IMAGE_SCN_MEM_EXECUTE)
     return true;
 
   // .xdata unwind info sections are eligble.
   return C->getSectionName().split('$').first == ".xdata";
 }
 
 // Split an equivalence class into smaller classes.
 void ICF::segregate(size_t Begin, size_t End, bool Constant) {

lld/trunk/test/COFF/icf-executable.s

+# RUN: llvm-mc -triple=x86_64-windows-msvc %s -filetype=obj -o %t.obj
+# RUN: lld-link -entry:main %t.obj -out:%t.exe -verbose 2>&1 | FileCheck %s
+
+# CHECK: Selected internal
+# CHECK:   Removed f2
+
+.section .text,"xr",one_only,internal
+internal:
+.globl main
+main:
+call f2
+ret
+
+.section .text,"xr",one_only,f2
+.globl f2
+f2:
+call main
+ret