[cfe-dev] C++ AST Question

[Eli Friedman]

On Sun, Jun 27, 2010 at 12:44 AM, Chris Lattner <clattner at apple.com> wrote:
> I'm trying to get this simple function to compile to decent code.  Lack of it is causing -O0 codegen to be monsterous:
>
> struct DeclGroup {
>  unsigned NumDecls;
> };
>
> int foo(DeclGroup D);
> void bar(DeclGroup *D) {
>  foo(*D);
> }
>
> The current issue that I'm fighting is that we get an extra pointless temporary and memcpy, which llvm-gcc isn't generating:
>
> $ clang t.cc -S -o - -emit-llvm
> ...
> define void @_Z3barP9DeclGroup(%struct.DeclGroup* %D) ssp {
> entry:
>  %D.addr = alloca %struct.DeclGroup*, align 8    ; <%struct.DeclGroup**> [#uses=2]
>  %agg.tmp = alloca %struct.DeclGroup, align 4    ; <%struct.DeclGroup*> [#uses=2]
>  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 4, i32 4, i1 false)
>  %coerce.dive = getelementptr %struct.DeclGroup* %agg.tmp, i32 0, i32 0 ; <i32*> [#uses=1]
>  %0 = load i32* %coerce.dive                     ; <i32> [#uses=1]
>  %coerce.val.ii = zext i32 %0 to i64             ; <i64> [#uses=1]
>  %call = call i32 @_Z3foo9DeclGroup(i64 %coerce.val.ii) ; <i32> [#uses=0]
>  ret void
> }
>
> 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...

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.

-Eli