llvm dev - Sep 2010 - [LLVMdev] using GCC LTO files as a frontend to dragonegg?

Hello,

With GCC, it is possible to compile GIMPLE from an object file back to 
assembly.  With Dragonegg, it seems to be a NOP.  (See below for a 
comparison.)

This appears it could be a nice way to mix the strengths of GCC and 
LLVM.  If it worked..  Should it?

Thanks,

Marcus

[mdaniels at dn002 dragonegg]$ cat hello.c
#include <stdio.h>
int hello () { printf ("Hello World\n"); }
[mdaniels at dn002 dragonegg]$ gcc-4.5 -flto -c hello.c[mdaniels at dn002 
dragonegg]$ gcc-4.5 -S -x lto hello.o
[mdaniels at dn002 dragonegg]$ cat hello.s
     .file    "hello.o"
     .section    .rodata
.LC0:
     .string    "Hello World"
     .text
.globl hello
     .type    hello, @function
hello:
.LFB0:
     .cfi_startproc
     pushq    %rbp
     .cfi_def_cfa_offset 16
     movq    %rsp, %rbp
     .cfi_offset 6, -16
     .cfi_def_cfa_register 6
     movl    $.LC0, %edi
     call    puts
     leave
     .cfi_def_cfa 7, 8
     ret
     .cfi_endproc
.LFE0:
     .size    hello, .-hello
     .ident    "GCC: (GNU) 4.5.1"
     .section    .note.GNU-stack,"", at progbits
[mdaniels at dn002 dragonegg]$ cat hello.c
#include <stdio.h>
int hello () { printf ("Hello World\n"); }
[mdaniels at dn002 dragonegg]$ gcc-4.5 -flto -c hello.c
[mdaniels at dn002 dragonegg]$ rm -f hello.s
[mdaniels at dn002 dragonegg]$ gcc-4.5 -O0 -c -x lto -S hello.o
[mdaniels at dn002 dragonegg]$ cat hello.s
     .file    "hello.o"
     .section    .rodata
.LC0:
     .string    "Hello World"
     .text
.globl hello
     .type    hello, @function
hello:
.LFB0:
     .cfi_startproc
     pushq    %rbp
     .cfi_def_cfa_offset 16
     movq    %rsp, %rbp
     .cfi_offset 6, -16
     .cfi_def_cfa_register 6
     movl    $.LC0, %edi
     call    puts
     leave
     .cfi_def_cfa 7, 8
     ret
     .cfi_endproc
.LFE0:
     .size    hello, .-hello
     .ident    "GCC: (GNU) 4.5.1"
     .section    .note.GNU-stack,"", at progbits
[mdaniels at dn002 dragonegg]$ rm -f hello.s
[mdaniels at dn002 dragonegg]$ gcc-4.5 -fplugin=./dragonegg.so -c -x lto 
-flto -S hello.o
[mdaniels at dn002 dragonegg]$ cat hello.s
     .file    "hello.o"

     .ident    "GCC: (GNU) 4.5.1 LLVM: 113541"


     .section    .note.GNU-stack,"", at progbits

On 10 September 2010 14:37, Marcus Daniels <mdaniels at lanl.gov>
wrote:
>  Hello,
>
> With GCC, it is possible to compile GIMPLE from an object file back to
> assembly.  With Dragonegg, it seems to be a NOP.  (See below for a
> comparison.)
>
> This appears it could be a nice way to mix the strengths of GCC and
> LLVM.  If it worked..  Should it?
I can't remember if lot1 (gcc's lto compiler) can load plugins, but if
it can, dragonegg can probably be made to work with it.

But if that is the case, I also don't see much difference in the
result you would get from using dragonegg with the original FE. I
think it plugs itself in the same spot the regular gcc writes its IL
to disk.
> Thanks,
>
> Marcus
Cheers,
-- 
Rafael Ávila de Espíndola

On 9/10/10 12:45 PM, Rafael Espindola wrote:
> I can't remember if lot1 (gcc's lto compiler) can load plugins, but
if
> it can, dragonegg can probably be made to work with it.
I see now it works if you ask for the GCC optimizations.. (the last flag)

gcc-4.5 -fplugin=./dragonegg.so -c -x lto -flto 
-fplugin-arg-dragonegg-emit-ir -S 
-fplugin-arg-dragonegg-enable-gcc-optzns  hello.o

Thanks,

Marcus

Hi Marcus,
> With GCC, it is possible to compile GIMPLE from an object file back to
> assembly.  With Dragonegg, it seems to be a NOP.  (See below for a
> comparison.)
>
> This appears it could be a nice way to mix the strengths of GCC and
> LLVM.  If it worked..  Should it?
I didn't have time to work on LTO with dragonegg yet, but it's on my
list
of things to do.  What I would like is: when compiling code with -flto and
dragonegg, LLVM bitcode is recorded alongside the assembler in special ELF
sections, just like gcc stores gimple when -flto is used without dragonegg.
When reading such files with the lto frontend and dragonegg, any gimple gets
turned into LLVM bitcode, and gets combined with any LLVM bitcode read-in
from ELF sections; the combined module is then optimized using LLVM's link
time optimizations.  Hopefully this is feasible, as I said I didn't work on
it yet.

Ciao,

Duncan.

On Mon, Sep 13, 2010 at 04:27, Duncan Sands <baldrick at free.fr> wrote:
> Hopefully this is feasible, as I said I didn't work on
> it yet.
It sounds doable, but I'm not sure why would you want to convert the
gimple into LLVM bitcode, if you are already saving LLVM bitcode in
the file.  Wouldn't you be just duplicating code?


Diego.