[LLVMbugs] [Bug 20116] New: Miscompilation when devirtualization is aborted due to covariant return types

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=20116

            Bug ID: 20116
           Summary: Miscompilation when devirtualization is aborted due to
                    covariant return types
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: LLVM Codegen
          Assignee: unassignedclangbugs at nondot.org
          Reporter: izbyshev at ispras.ru
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

Created attachment 12699
  --> http://llvm.org/bugs/attachment.cgi?id=12699&action=edit
A failing test case

When compiling the attached code, clang doesn't emit 'this' adjustment code for
'static_cast<A*>(z)' expression and ends up using a wrong vtable for the
virtual call. As a result, B.g() is called instead of Z.f().

Inspection of CodeGenFunction::EmitCXXMemberCallExpr() code shows that clang
can stop devirtualization in the middle if it finds out that return types are
covariant. At that moment, 'Base' expression that is later used for emission of
'this' pointer has already been assigned to 'Inner' expression (with casts
stripped), so no 'this' adjustment code is generated.

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