[LLVMbugs] [Bug 22285] New: comdats cause uncalled linkonce_odr functions to be retained at -O0/-O1

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=22285

            Bug ID: 22285
           Summary: comdats cause uncalled linkonce_odr functions to be
                    retained at -O0/-O1
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: Interprocedural Optimizations
          Assignee: david.majnemer at gmail.com
          Reporter: dblaikie at gmail.com
                CC: llvmbugs at cs.uiuc.edu, rafael.espindola at gmail.com
    Classification: Unclassified

$ cat inl.ll
  $f1 = comdat any
  define void @f2() {
  entry:
    call void @f1()
    ret void
  }

  define linkonce_odr void @f1() comdat {
  entry:
    ret void
  }
  $ opt -S -inline inl.ll -o - | grep define
  define void @f2() {
  define linkonce_odr void @f1() comdat {
  $ cat inl2.ll
  define void @f2() {
  entry:
    call void @f1()
    ret void
  }
  define linkonce_odr void @f1() {
  entry:
    ret void
  }
  $ opt inl2.ll -S -inline -o - | grep define
  define void @f2() {
  $

The problem seems to be at lib/Transforms/IPO/Inliner.cpp:678:
    // It is unsafe to drop a function with discardable linkage from a COMDAT
    // without also dropping the other members of the COMDAT.
    // The inliner doesn't visit non-function entities which are in COMDAT
    // groups so it is unsafe to do so *unless* the linkage is local.
    if (!F->hasLocalLinkage() && F->hasComdat())
      continue;

Which is correct, but very conservative. At higher optimization levels some
other pass (I'll check which one in a moment) must kick in and still manage to
cleanup these dead comdats.

Since a comdat doesn't refer to its members, it's not trivial to just ask "is
this a comdat with only one element" and then drop the function (& comdat)
anyway. Maybe it needs a separate pass specifically looking for comdats whos
elements are all unused linkonce_odr elements?

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