Re: [patch] Fix PR middle-end/65958

[Richard Biener <richard.guenther@gmail.com>]

On Wed, Nov 18, 2015 at 12:12 PM, Eric Botcazou <ebotcazou@adacore.com> wrote:
>> Ok.  I wonder if we document GCCs VLA implementation somewhere so we can add
>> a note on the interaction with alloca.
>
> I found this in 10.7.2 Blocks:
>
> "Variable-length arrays (VLAs) complicate this process, as their size
> often refers to variables initialized earlier in the block.  To handle
> this, we currently split the block at that point, and move the VLA into
> a new, inner `BIND_EXPR'.  This strategy may change in the future."
>
> which sounds totally obsolete to me.  Proposed change:

Looks good to me.  I also found the Arrays of Variable Length section
in extend.texi which also refers to alloca as doing the same.  We may
want to add a note there that you should not mix both and that only
VLAs (when not mixed with alloca) are freed at scope boundary.

Richard.

> Index: doc/generic.texi
> ===================================================================
> --- doc/generic.texi    (revision 230453)
> +++ doc/generic.texi    (working copy)
> @@ -1950,11 +1950,15 @@ this initialization replaces the @code{D
>  will never require cleanups.  The scope of these variables is just the
>  body
>
> -Variable-length arrays (VLAs) complicate this process, as their
> -size often refers to variables initialized earlier in the block.
> -To handle this, we currently split the block at that point, and
> -move the VLA into a new, inner @code{BIND_EXPR}.  This strategy
> -may change in the future.
> +Variable-length arrays (VLAs) complicate this process, as their size
> +often refers to variables initialized earlier in the block and their
> +initialization involves an explicit stack allocation.  To handle this,
> +we add an indirection and replace them with a pointer to stack space
> +allocated by means of @code{alloca}.  In most cases, we also arrange
> +for this space to be reclaimed when the enclosing @code{BIND_EXPR} is
> +exited, the exception to this being when there is an explicit call to
> +@code{alloca} in the source code, in which case the stack is left
> +depressed on exit of the @code{BIND_EXPR}.
>
>  A C++ program will usually contain more @code{BIND_EXPR}s than
>  there are syntactic blocks in the source code, since several C++
>
>
> --
> Eric Botcazou