[LLVMbugs] [Bug 16226] New: [-cxx-abi microsoft] C++ objects passed by value are constructed in place on the argument stack slots

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=16226

            Bug ID: 16226
           Summary: [-cxx-abi microsoft] C++ objects passed by value are
                    constructed in place on the argument stack slots
           Product: clang
           Version: unspecified
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: normal
          Priority: P
         Component: C++
          Assignee: unassignedclangbugs at nondot.org
          Reporter: reid.kleckner at gmail.com
                CC: dgregor at apple.com, llvmbugs at cs.uiuc.edu
    Classification: Unclassified

This is kind of a big problem, since I don't know how to represent this in LLVM
IR.  I'm also not sure how to file this, since it cuts across the ABI lowering
in Clang and LLVM.

MSVC lowers this code like this:

struct S {
  S();
  ~S();
  int a, b;
};

void foo(S s) {
  // ~S() is called before return  
}

void bar() {
  S t;
  foo(t);  // S(const S &) is called on t.
}

LLVM arguments are always passed by value, and there's no way to talk about the
stack space that they may or may not use depending on target architecture.

Would it make sense to invent some kind of special alloca to handle this case?

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