On Wed, Mar 16, 2022 at 10:15:17AM +1300, Simon Glass wrote: > Hi Tom, > > On Mon, 14 Mar 2022 at 16:42, Tom Rini <tr...@konsulko.com> wrote: > > > > On Mon, Mar 14, 2022 at 03:43:05PM -0600, Simon Glass wrote: > > > Hi Tom, > > > > > > On Mon, 14 Mar 2022 at 14:23, Tom Rini <tr...@konsulko.com> wrote: > > > > > > > > On Mon, Mar 14, 2022 at 02:18:14PM -0600, Simon Glass wrote: > > > > > Hi Tom, > > > > > > > > > > On Mon, 14 Mar 2022 at 13:45, Tom Rini <tr...@konsulko.com> wrote: > > > > > > > > > > > > On Mon, Mar 14, 2022 at 01:21:02PM -0600, Simon Glass wrote: > > > > > > > Hi Tom, > > > > > > > > > > > > > > On Mon, 14 Mar 2022 at 12:29, Tom Rini <tr...@konsulko.com> wrote: > > > > > > > > > > > > > > > > On Mon, Mar 14, 2022 at 12:24:42PM -0600, Simon Glass wrote: > > > > > > > > > Hi Tom, > > > > > > > > > > > > > > > > > > On Mon, 14 Mar 2022 at 06:49, Tom Rini <tr...@konsulko.com> > > > > > > > > > wrote: > > > > > > > > > > > > > > > > > > > > On Sat, Mar 12, 2022 at 10:58:44AM -0700, Simon Glass wrote: > > > > > > > > > > > Hi Tom, > > > > > > > > > > > > > > > > > > > > > > On Mon, 7 Mar 2022 at 07:33, Tom Rini > > > > > > > > > > > <tr...@konsulko.com> wrote: > > > > > > > > > > > > > > > > > > > > > > > > On Fri, Mar 04, 2022 at 08:42:57AM -0700, Simon Glass > > > > > > > > > > > > wrote: > > > > > > > > > > > > > > > > > > > > > > > > > LTO (Link-Time Optimisation) is an very useful > > > > > > > > > > > > > feature which can > > > > > > > > > > > > > significantly reduce the size of U-Boot binaries. So > > > > > > > > > > > > > far it has been > > > > > > > > > > > > > made available for selected ARM boards and sandbox. > > > > > > > > > > > > > > > > > > > > > > > > > > However, incremental builds are much slower when LTO > > > > > > > > > > > > > is used. For example, > > > > > > > > > > > > > an incremental build of sandbox takes 2.1 seconds on > > > > > > > > > > > > > my machine, but 6.7 > > > > > > > > > > > > > seconds with LTO enabled. > > > > > > > > > > > > > > > > > > > > > > > > > > Add a LTO_BUILD=n parameter to the build, so it can > > > > > > > > > > > > > be disabled during > > > > > > > > > > > > > development if needed, for faster builds. > > > > > > > > > > > > > > > > > > > > > > > > > > Add some documentation about LTO while we are here. > > > > > > > > > > > > > > > > > > > > > > > > > > Signed-off-by: Simon Glass <s...@chromium.org> > > > > > > > > > > > > > > > > > > > > > > > > We don't need this since you can do: > > > > > > > > > > > > make EXTRA_CFLAGS="-fno-lto" EXTRA_LDFLAGS="-fno-lto" > > > > > > > > > > > > to pass -fno-lto to compile/linking and disable lto and > > > > > > > > > > > > per > > > > > > > > > > > > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=46905 this > > > > > > > > > > > > has been working > > > > > > > > > > > > for some time. > > > > > > > > > > > > > > > > > > > > > > Thanks for that, it is a big pain point for me, picking > > > > > > > > > > > up this patch > > > > > > > > > > > for every series I write. The incremental build time for > > > > > > > > > > > sandbox goes > > > > > > > > > > > from 3 seconds to 27 seconds on my laptop with LTO, which > > > > > > > > > > > is > > > > > > > > > > > intolerable. > > > > > > > > > > > > > > > > > > > > Yeah, I noticed it's visible on my laptop, but not at all > > > > > > > > > > on my desktop > > > > > > > > > > (i7 vs Ryzen 7). > > > > > > > > > > > > > > > > > > > > > The EXTRA_CFLAGS says it is for 'Backward compatibility' > > > > > > > > > > > and it still > > > > > > > > > > > does the various LTO things (i.e. it changes the build > > > > > > > > > > > logic). It > > > > > > > > > > > > > > > > > > > > We're unlikely to move to newer Linux kernel kbuild logic > > > > > > > > > > so this isn't > > > > > > > > > > going away, and there's not much in the way of logic that's > > > > > > > > > > changed for > > > > > > > > > > LTO that I see. > > > > > > > > > > > > > > > > > > > > > seems odd to me to enable the option and then disable it > > > > > > > > > > > later in the > > > > > > > > > > > command line. It is therefore not quite equivalent. But > > > > > > > > > > > it seems to > > > > > > > > > > > work well enough for me fom a small amount of testing. If > > > > > > > > > > > you are > > > > > > > > > > > really set on not having a special option for it, I can > > > > > > > > > > > live with it > > > > > > > > > > > for now. I'm also not convinced that my patch entirely > > > > > > > > > > > removes the LTO > > > > > > > > > > > stuff in a consistent way. > > > > > > > > > > > > > > > > > > > > Yeah, I really don't want to go down the path of overriding > > > > > > > > > > CONFIG > > > > > > > > > > options via make/environment logic. I'm also open to > > > > > > > > > > turning off LTO on > > > > > > > > > > sandbox and on with qemu-* so it gets wider CI testing. > > > > > > > > > > > > > > > > > > Yes you did mention that, but the problem is that LTO is very > > > > > > > > > handy > > > > > > > > > with sandbox, to test the strange things that happen. For > > > > > > > > > example, I > > > > > > > > > found the bug where LTO was dropping a linker-list item, using > > > > > > > > > sandbox. We could perhaps make one of the sandbox builds not > > > > > > > > > use LTO, > > > > > > > > > e.g. sandbox_flattree ? > > > > > > > > > > > > > > > > Well, the big issue with LTO+sandbox is that it slows down your > > > > > > > > workflow, so I would think you want the inverse, one platform > > > > > > > > does > > > > > > > > enable it? > > > > > > > > > > > > > > It slows down all boards that use it, actually, so I normally > > > > > > > don't > > > > > > > want it enabled for my IDE. This is not sandbox-specific. > > > > > > > > > > > > The degree of slowdown depends on what you're building on. I see > > > > > > several seconds of link time on my laptop and nothing on my desktop > > > > > > (which isn't that high end either, it's not an EPYC or anything) > > > > > > when > > > > > > LTO is/isn't enabled. > > > > > > > > > > For me, on 16-core AMD desktop, incremental build > > > > > > > > > > LTO: real 0m7.744s > > > > > No LTO: real 0m3.172s > > > > > > > > > > On 4-core laptop: > > > > > > > > > > No LTO: real 0m2.429s > > > > > LTO: real 0m15.650s > > > > > > > > > > Perhaps the disconnect here is that you just don't see any difference? > > > > > What times are you seeing? > > > > > > > > My 16 core desktop is 14 seconds without, 15 seconds with (full build, > > > > not just the link time) and I don't recall what my laptop was but since > > > > it's longer than the desktop I'm always just ssh'd in there anyhow. > > > > > > Seems very slow. Are you sure that is an incremental build? > > > > No, it's a from-scratch build, which should be the worst case for this. > > OK. So I don't do fresh builds very often, mostly they are incremental > (for obvious reasons...I am developing things incrementally). So the > difference is quite dramatic, as you can see. > > Does my problem make more sense now?
Yes, I didn't realize incremental non-LTO link was so quick. But no, I don't want to open the door here to other "make iterative build quicker, override the config" options. Maybe just another note to the development docs and see if that leads to a chorus of people asking for this feature after all. -- Tom
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