[LLVMbugs] [Bug 3100] New: llvm-g++ is unable to devirtualize a function call

[bugzilla-daemon at cs.uiuc.edu]

http://llvm.org/bugs/show_bug.cgi?id=3100

           Summary: llvm-g++ is unable to devirtualize a function call
           Product: tools
           Version: trunk
          Platform: PC
        OS/Version: Linux
            Status: NEW
          Severity: normal
          Priority: P2
         Component: llvm-g++
        AssignedTo: unassignedbugs at nondot.org
        ReportedBy: rafael.espindola at gmail.com
                CC: llvmbugs at cs.uiuc.edu


Testcase from the g++ testsuite
-----------------------------------------
struct S { S(); virtual void xyzzy(); };
inline void foo(S *s) { s->xyzzy(); }
void bar() { S s; foo(&s); }
----------------------------------------

With gcc we got

call    _ZN1S5xyzzyEv

with llvm-g++ we got

call    *%rax

there rax contains the pointer fetched from the vtable. The problem is that
llvm-g++ knows what the vtable looks like, but doesn't communicate that to
llvm.

A simpler testcase also fails with llvm:

----------------------------------------
struct S { S() {}; virtual void xyzzy(); };
inline void foo(S *s) { s->xyzzy(); }
void bar() { S s; foo(&s); }
----------------------------------------

Now we have the constructor body and we do a bit better:
----------------------------
call    *_ZTV1S+16
---------------------------

but llvm-g++ only says

@_ZTV1S = external constant [3 x i32 (...)*]

so there is no way for llvm to const propagate the fetch.


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