[cfe-dev] C++ AST Question

[Chris Lattner]

On Jun 27, 2010, at 1:36 AM, Eli Friedman wrote:
>> After diving into it, the memcpy is being made by CodeGenFunction::EmitCXXConstructorCall, so I came up with this patch:
> 
> Stop right there; the behavior here is exactly the same in C mode,
> where the CXXConstructorCall clearly doesn't exist.  You're not going
> to get anywhere examining it...

Good point, that occurred to me in the middle of the night :)

> Take a look at what happens in CodeGenFunction::EmitCall (the version
> in CGExpr.cpp).  The copy already exists after EmitCallArgs runs:
> since it isn't aware that the ABI is going to pass the struct by
> value, and you can't just pass the pointer D into foo(), it
> immediately make a copy to ensure correct behavior.

Ok, right.  Specifically, we end up down in the bowels of:

RValue CodeGenFunction::EmitCallArg(const Expr *E, QualType ArgType) {
  if (ArgType->isReferenceType())
    return EmitReferenceBindingToExpr(E, /*InitializedDecl=*/0);

  return EmitAnyExprToTemp(E);
}

EmitAnyExprToTemp(E) is emitting the extra copy.  The fix is to emit aggregates as l-values, but this won't work for the CallArgList data structure that Daniel has set up.  Daniel, what do you think is the right approach here?  Here's a simple C example:

struct DeclGroup {
  long NumDecls;
  long Y;
};

long foo(struct DeclGroup D);
void bar(struct DeclGroup *D) {
  foo(*D);
}

Compiles to:

define void @bar(%struct.DeclGroup* %D) {
entry:
  %D.addr = alloca %struct.DeclGroup*, align 8    ; <%struct.DeclGroup**> [#uses=2]
  %agg.tmp = alloca %struct.DeclGroup, align 8    ; <%struct.DeclGroup*> [#uses=1]
  %allocapt = bitcast i32 undef to i32            ; <i32> [#uses=0]
  store %struct.DeclGroup* %D, %struct.DeclGroup** %D.addr
  %tmp = load %struct.DeclGroup** %D.addr         ; <%struct.DeclGroup*> [#uses=1]
  %tmp1 = bitcast %struct.DeclGroup* %agg.tmp to i8* ; <i8*> [#uses=1]
  %tmp2 = bitcast %struct.DeclGroup* %tmp to i8*  ; <i8*> [#uses=1]
  call void @llvm.memcpy.p0i8.p0i8.i64(i8* %tmp1, i8* %tmp2, i64 16, i32 8, i1 false)
}

I want to kill off the extra temporary + memcpy.

-Chris