[Patch, libfortran] RFC: Shared vtables, constification

[Jerry DeLisle]

On 03/15/2012 11:42 AM, Janne Blomqvist wrote:
> PING! (At this point, obviously for trunk only)
>

Yes, OK for trunk.

> On Mon, Feb 13, 2012 at 20:20, Janne Blomqvist
> <blomqvist.janne@gmail.com>  wrote:
>> Hi,
>>
>> the attached patch changes the low-level libgfortran IO dispatching
>> mechanism to use shared vtables for each stream type, instead of all
>> the function pointers being replicated for each unit. This is similar
>> to e.g. how the C++ frontend implements vtables. The benefits are:
>>
>> - Slightly smaller heap memory overhead for each unit as only the
>> vtable pointer needs to be stored, and slightly faster unit
>> initialization as only the vtable pointer needs to be setup instead of
>> all the function pointers in the stream struct.
>>
>> - Looking at unix.o with readelf, one sees
>>
>> Relocation section '.rela.data.rel.ro.local.mem_vtable' at offset
>> 0x15550 contains 8 entries:
>>
>> and similarly for the other vtables; according to
>> http://www.airs.com/blog/archives/189 this means that after relocation
>> the page where this data resides may be marked read-only.
>>
>> The downside is that the sizes of the .text and .data sections are
>> increased. Before:
>>
>>    text    data     bss     dec     hex filename
>> 1116991    6664     592 1124247  112797
>> ./x86_64-unknown-linux-gnu/libgfortran/.libs/libgfortran.so
>>
>> After:
>>
>>    text    data     bss     dec     hex filename
>> 1117487    6936     592 1125015  112a97
>> ./x86_64-unknown-linux-gnu/libgfortran/.libs/libgfortran.so
>>
>>
>> The data section increase is due to the vtables, the text increase is,
>> I guess, due to the extra pointer dereference when calling the IO
>> functions.
>>
>> Regtested on x86_64-unknown-linux-gnu, Ok for trunk, or 4.8?
>>
>> 2012-02-13  Janne Blomqvist<jb@gcc.gnu.org>
>>
>>         * io/unix.h (struct stream): Rename to stream_vtable.
>>         (struct stream): New struct definition.
>>         (sread): Dereference vtable pointer.
>>         (swrite): Likewise.
>>         (sseek): Likewise.
>>         (struncate): Likewise.
>>         (sflush): Likewise.
>>         (sclose): Likewise.
>>         * io/unix.c (raw_vtable): New variable.
>>         (buf_vtable): Likewise.
>>         (mem_vtable): Likewise.
>>         (mem4_vtable): Likewise.
>>         (raw_init): Assign vtable pointer.
>>         (buf_init): Likewise.
>>         (open_internal): Likewise.
>>         (open_internal4): Likewise.
>>
>>
>>
>> --
>> Janne Blomqvist
>
>
>