RFC: Idea for code size reduction

[Richard Guenther]

On Fri, Mar 7, 2008 at 11:02 AM, Philipp Marek <philipp@marek.priv.at> wrote:
> Hello everybody,
>
>  I have a feature request.
>
>
>  I'd like to (manually) define some byte blocks, eg. as functions with an
>  identifier.  Then, if GCC would emit exactly these bytes, it puts a JMP
>  identifier there instead.
>
>  This would help by sharing many identical (text/code) byte sequences, to
>  make a smaller image - like -fmerge-constants does for data - and therefor
>  helps for cache locality, too.
>
>
>  I wrote some perl scripts to test this.  I took a "alldefconfig" i686
>  kernel, let objdump disassemble it, and on "iret", "ret", "ljmp" or "jmp"
>  with a 4 byte address I store the last few bytes.  Another script goes
>  through these block-ends, and estimates the number of bytes saved, if
>  identical sequences get changed into a 5 byte opcode (jump with 32bit
>  address).
>
>
>  The original kernel has
>   $ size vmlinux
>      text    data     bss     dec     hex  filename
>    4802166  446554  606208 5854928  5956d0 vmlinux
>
>  and my scripts emit space savings of about 30kB.  Now that's not
>  that much, but for embedded systems it means another userspace
>  binary more.
>   [ If anyone's interested, I can post my scripts here ... they're not
>     that large. ]
>
>  And, of course, the same can be used for other binaries too - which could
>  save other space as well.  (blender-bin, text size 8664025, gives 97KB
>  savings)
>
>
>  I'd imagine the use that such:
>  - Built the binary
>  - Run my scripts, to find some common code sequences
>  - Give them to gcc
>  - Rebuild, using the optimizations
>
>  I don't know whether some new optimize switch could be used, to do all
>  that internally in the compiler and linker.
>
>
>  What do you think about that?
>  [ Please keep at least *me* cc'ed - don't know whether that's that
>   interesting for linux-tiny. ]

Sounds like what -frtl-abstract-sequences is trying to do.

Richard.