Re: [Vala] how to *not* automagically copy arrays passed to methods?

[Luca Bruno <lethalman88 gmail com>]

On Wed, Dec 21, 2011 at 8:54 AM, Matto Marjanovic <maddog mir com> wrote:

> You're right... it is not copied in the example I gave above.  However,
> it is copied if the method body produces a closure --- I didn't realize
> that that detail changed the behavior (until just now).
>
> Compiling this with "valac -C":
>
>    void foo(uint8[] x) {
>      x[0] = 7;
>    }
>        delegate void Callback();
>        void baz(Callback callback) {
>      callback();
>    }
>        void bar(uint8[] buffer) {
>      buffer[0] = 77;
>      baz(() => { buffer[0] = 23; });
>    }
>
> yields the following C code:
>    ...
>    void foo (guint8* x, int x_length1) {
>        guint8* _tmp0_;
>        gint _tmp0__length1;
>        guint8 _tmp1_;
>        _tmp0_ = x;
>        _tmp0__length1 = x_length1;
>        _tmp0_[0] = (guint8) 7;
>        _tmp1_ = _tmp0_[0];
>    }
>    ...
>    void bar (guint8* x, int x_length1) {
>        Block1Data* _data1_;
>        guint8* _tmp0_;
>        gint _tmp0__length1;
>        guint8* _tmp1_;
>        gint _tmp1__length1;
>        guint8* _tmp2_;
>        gint _tmp2__length1;
>        guint8 _tmp3_;
>        _data1_ = g_slice_new0 (Block1Data);
>        _data1_->_ref_count_ = 1;
>        _tmp0_ = x;
>        _tmp0__length1 = x_length1;
>        _tmp1_ = (_tmp0_ != NULL) ? _vala_array_dup1 (_tmp0_,
> _tmp0__length1) : ((gpointer) _tmp0_);
>        _tmp1__length1 = _tmp0__length1;
>        _data1_->x = _tmp1_;
>        _data1_->x_length1 = _tmp1__length1;
>        _tmp2_ = _data1_->x;
>        _tmp2__length1 = _data1_->x_length1;
>        _tmp2_[0] = (guint8) 7;
>        _tmp3_ = _tmp2_[0];
>        baz (___lambda0__callback, _data1_);
>        block1_data_unref (_data1_);
>        _data1_ = NULL;
>    }
>
> It looks like the presence of the closure is causing the array to get
> dup'ed, and it's being dup'd even before the assignment that occurs
> before the closure is constructed.

Yes, that's an unfortunate well-known issue that happens with async methods
too, and it's unfortunately hard to fix due to backward-compatibility. You
can workaround it by assigning the array to an unowned local variable
(impossible with async methods).

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