Using Link Time Optimization (LTO)

[Johannes Pfau]

Am Sun, 23 Mar 2014 02:14:20 +0000
schrieb "Mike" <none at none.com>:

> Hello,
> 
> I have some code generating the following assembly:
> {OnReset}:
>   8000010:       b508            push    {r3, lr}
>   8000012:       20ff            movs    r0, #255        ; 0xff
>   8000014:       f000 f828       bl      8000068 <{MyFunction}>
>   8000018:       e7fe            b.n     8000018 <{OnReset}+0x8>
>   800001a:       bf00            nop
> 
> 08000068
> {MyFunction}:
>   8000068:       f44f 5380       mov.w   r3, #4096       ; 0x1000
>   800006c:       f2c2 0300       movt    r3, #8192       ; 0x2000
>   8000070:       7018            strb    r0, [r3, #0]
>   8000072:       4770            bx      lr
> 
> "MyFunction" and "OnReset" are in different source files and 
> therefore compiled to different object files.  I would like to 
> get "MyFunction" fully inlined to "OnReset" to remove the extra 
> branch instructions (bl and bx).
> 
> It's my understanding that because the two functions are compiled 
> into separate object files, this must be done using LTO.  If I 
> compile them into the same object file, I get the full inlining 
> I'm looking for, but that's not going to scale well for my 
> project.
> 
> ** Beautiful, isn't it? **
> {OnReset}:
>   8000010:       f44f 5380       mov.w   r3, #4096       ; 0x1000
>   8000014:       f2c2 0300       movt    r3, #8192       ; 0x2000
>   8000018:       22ff            movs    r2, #255        ; 0xff
>   800001a:       701a            strb    r2, [r3, #0]
>   800001c:       e7fe            b.n     800001c <{OnReset}+0xc>
>   800001e:       bf00            nop
> 
> 
> I've tried adding -flto to my compiler and linker flags and a 
> number of other things without success.  The compiler seems to 
> generate extra information in my object files, but the linker 
> doesn't seem to do the optimization.  I don't get any ICEs, 
> however, as stated in Bug 61 and 88.  I just don't get the result 
> I'm after.
> 
> Here are my compiler commands:
> arm-none-eabi-gdc -mthumb -mcpu=cortex-m4 -fno-emit-moduleinfo 
> -ffunction-sections -fdata-sections -O3 -c -flto ...
> arm-none-eabi-ld -T link/link.ld -Map binary/memory.map 
> --gc-sections -flto ...
> 
> I'm using my arm-none-eabi cross toolchain built from the GDC 4.8 
> branch.  I tried adding --enable-lto to my toolchain's configure, 
> but that had no effect.  It's my understanding that it's enabled 
> by default anyway.
> 
> Does anyone know how I can get this level of inlining without 
> compiling all my source into one object file?
> 
> Thanks for any help,
> Mike

Some time ago LTO was only supported by the gold linker, so you might
need to configure binutils with --enable-gold --enable-plugins
--enable-lto

GCC should also be compiled with --enable-gold --enable-plugins
--enable-lto

http://gcc.gnu.org/onlinedocs/gcc-4.8.0/gcc/Optimize-Options.html
also says if you link manually you must use gcc to link, not ld and
pass -flto when linking as well:
gcc -o myprog -flto -O2 foo.o bar.o

You can also try passing -fuse-linker-plugin to all gcc commands.

I never used LTO though, so I'm not sure if this will actually help :-)