[cfe-commits] r78267 - in /cfe/trunk: lib/CodeGen/CGCXX.cpp test/CodeGenCXX/copy-constructor-elim.cpp

[Douglas Gregor]

On Aug 5, 2009, at 6:03 PM, Fariborz Jahanian wrote:

> Author: fjahanian
> Date: Wed Aug  5 20:02:49 2009
> New Revision: 78267
>
> URL: http://llvm.org/viewvc/llvm-project?rev=78267&view=rev
> Log:
> Patch to optimize away copy constructor call when
> appropriate.
>
> Added:
>    cfe/trunk/test/CodeGenCXX/copy-constructor-elim.cpp
> Modified:
>    cfe/trunk/lib/CodeGen/CGCXX.cpp
>
> Modified: cfe/trunk/lib/CodeGen/CGCXX.cpp
> URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/CodeGen/CGCXX.cpp?rev=78267&r1=78266&r2=78267&view=diff
>
> =
> =
> =
> =
> =
> =
> =
> =
> ======================================================================
> --- cfe/trunk/lib/CodeGen/CGCXX.cpp (original)
> +++ cfe/trunk/lib/CodeGen/CGCXX.cpp Wed Aug  5 20:02:49 2009
> @@ -255,7 +255,20 @@
>   cast<CXXRecordDecl>(E->getType()->getAs<RecordType>()->getDecl());
>   if (RD->hasTrivialConstructor())
>     return;
> -
> +
> +  // Code gen optimization to eliminate copy constructor and return
> +  // its first argument instead.
> +  const CXXConstructorDecl *CDecl = E->getConstructor();
> +  if (E->getNumArgs() == 1 &&
> +      CDecl->isCopyConstructor(getContext())) {
> +    CXXConstructExpr::const_arg_iterator i = E->arg_begin();
> +    const Expr *SubExpr = (*i);
> +    // FIXME. Any other cases can be optimized away?
> +    if (isa<CallExpr>(SubExpr) || isa<CXXTemporaryObjectExpr> 
> (SubExpr)) {
> +      EmitAggExpr(SubExpr, Dest, false);
> +      return;
> +    }
> +  }


The intent of CXXConstructorExpr::isElidable() was to have Sema  
compute all of the cases where a copy constructor is elidable (based  
on its understanding of the language semantics). Then, CodeGen would  
just check isElidable() and elide in those cases. That way, CodeGen  
doesn't have to know anything about the language to do the right thing.

	- Doug