[Bug c/60660] New: alloca function for inlines that allocates on caller stack

[Bug c/60660] New: alloca function for inlines that allocates on caller stack

[josh at joshtriplett dot org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60660

            Bug ID: 60660
           Summary: alloca function for inlines that allocates on caller
                    stack
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: josh at joshtriplett dot org

To implement certain helper functions, it would help greatly to have the
ability to perform stack allocations in the parent function from an inline
function.  If the function is guaranteed to be inline (for instance, with
always_inline), this seems straightforward.  A function like
__builtin_alloca_caller() would make it possible to write something like
alloca_sprintf safely, without multiple evaluation of the variable arguments as
a macro would.

[Bug c/60660] alloca function for inlines that allocates on caller stack

[pinskia at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60660

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
This is much harder than you think really.  since you have to move all the
arguments of the callee function

[Bug c/60660] alloca function for inlines that allocates on caller stack

[pinskia at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60660

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
>without multiple evaluation of the variable arguments as a macro would.

You can use statement expressions with typeof extension to get around the issue
of multiple evaluation issue of macros already.

[Bug c/60660] alloca function for inlines that allocates on caller stack

[mpolacek at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60660

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |mpolacek at gcc dot gnu.org

--- Comment #3 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Note that even functions marked as always_inline might not be in fact
inlineable.

[Bug c/60660] alloca function for inlines that allocates on caller stack

[josh at joshtriplett dot org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60660

--- Comment #4 from Josh Triplett <josh at joshtriplett dot org> ---
(In reply to Andrew Pinski from comment #1)
> This is much harder than you think really.  since you have to move all the
> arguments of the callee function

That I'm aware of, but it should be less of an issue with an always inline
function, where you shouldn't necessarily be passing stack arguments in the
first place.

(In reply to Andrew Pinski from comment #2)
> >without multiple evaluation of the variable arguments as a macro would.
> 
> You can use statement expressions with typeof extension to get around the
> issue of multiple evaluation issue of macros already.

Not with a variable argument list, due to limitations in the C preprocessor.

(In reply to Marek Polacek from comment #3)
> Note that even functions marked as always_inline might not be in fact
> inlineable.

This seems like exactly the same limitation that __builtin_va_arg_pack has.