On 2015.04.13 at 18:23 +0200, Markus Trippelsdorf wrote:
> On 2015.04.12 at 12:14 +0200, Ingo Molnar wrote:
> > In my (past) experience the main win from -flto is not due to better 
> > hot/cold decisions, but simply due to more aggressive dead code 
> > elimination. -flto has less of an effect on code that is actually 
> > being executed.
> > 
> > Which isn't to be sneered at, but it's far less of a direct effect as 
> > branch probabilities are, which cut to the core of most hotpaths in 
> > the kernel.
> 
> I did some measurements with gcc-5.1-RC on X86_64 using Andi's latest
> LTO kernel patch for 4.0. With my simple monolithic .config the code
> size savings are below 1%. That is lower than I've expected.

I must have made a measurement mistake above, because the actual code
size savings are roughly 5%:

   text    data     bss     dec     filename
   8746230  970072  802816 10519118 ./vmlinux gcc-5 (lto) 
   9202488  978512  811008 10992008 ./vmlinux gcc-5
   8686246 1009104  811008 10506358 ./vmlinux gcc-4.9 (lto)
   9228994  992976  815104 11037074 ./vmlinux gcc-4.9 

-- 
Markus