[cfe-commits] r83816 - in /cfe/trunk: lib/CodeGen/CGCXX.cpp test/CodeGenCXX/virtual-function-calls.cpp

[Douglas Gregor]

On Oct 11, 2009, at 4:55 PM, Anders Carlsson wrote:

> Author: andersca
> Date: Sun Oct 11 18:55:52 2009
> New Revision: 83816
>
> URL: http://llvm.org/viewvc/llvm-project?rev=83816&view=rev
> Log:
> If the base type of a member call is a record type we don't need to  
> emit a virtual call.

This doesn't look right... details below.

> Modified:
>    cfe/trunk/lib/CodeGen/CGCXX.cpp
>    cfe/trunk/test/CodeGenCXX/virtual-function-calls.cpp
>
> Modified: cfe/trunk/lib/CodeGen/CGCXX.cpp
> URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/CodeGen/CGCXX.cpp?rev=83816&r1=83815&r2=83816&view=diff
>
> = 
> = 
> = 
> = 
> = 
> = 
> = 
> = 
> ======================================================================
> --- cfe/trunk/lib/CodeGen/CGCXX.cpp (original)
> +++ cfe/trunk/lib/CodeGen/CGCXX.cpp Sun Oct 11 18:55:52 2009
> @@ -230,10 +230,13 @@
>   // C++ [class.virtual]p12:
>   //   Explicit qualification with the scope operator (5.1)  
> suppresses the
>   //   virtual call mechanism.
> +  //
> +  // We also don't emit a virtual call if the base expression has a  
> record type
> +  // because then we know what the type is.
>   llvm::Value *Callee;
> -  if (MD->isVirtual() && !ME->hasQualifier())
> -    // FIXME: push getCanonicalDecl as a conversion using the  
> static type system (CanCXXMethodDecl).
> -    Callee = BuildVirtualCall(MD->getCanonicalDecl(), This, Ty);
> +  if (MD->isVirtual() && !ME->hasQualifier() &&
> +      !ME->getBase()->getType()->isRecordType())
> +    Callee = BuildVirtualCall(MD, This, Ty);

The type of the expression could be a record type even if the dynamic  
type of the object isn't known at compile time. If I tweak your  
example like this:

void f(A &a) {
   a.f('c');
}

I now get a direct call, when I should get a virtual call. To do this  
devirtualization, I suggest abstracting the "find the dynamic type of  
this expression" logic into a separate function. Then, we can teach it  
about common cases, such as referring to a VarDecl of (non-reference)  
type, constructing a temporary of a specific type, etc. I expect that  
this logic will get smarter over time, as we think of more analyses  
that can help make this distinction.

	- Doug