Enable link time optimisation in builds

[mithro]

Link time optimisation seems to be resulting in much smaller binaries as any unused elements can be totally removed.

There are however some issues to make this work;

• Have to use a modern gcc
• Use GCC for linking
• Have to mark a couple of functions like the ISR as actually used.
• Have to link against libgcc

[mithro]

I have a WIP branch at https://github.com/mithro/litex/tree/enable-lto?files=1

[mithro]

It is unclear why we have to link against libgcc when the symbols should be provided by libcompiler_rt. It seems to be because GCC is generating function calls itself (rather than being in the code) which cause lto to not understand that the symbols are used.

[enjoy-digital]

This has been integrated with #401. For now it's enabled with for all processors except LM32. If this is causing issue with others configurations, we'll add a parameter be able to disable LTO when needed.

[enjoy-digital]

LTO has been reverted in 979f98e since seems to cause subtle issues on some configurations. We'll have to test it more before enabling it.

[enjoy-digital]

cc @mithro @mateusz-holenko.

[ewenmcneill]

In case it helps, the link issue I had (in #417) with litex-buildenv was with the "stub firmware" in litex-buildenv, which uses the auto-generated litex compiler/linker/flags/etc, but has its own Makefile. So (copying from my comment on #417 (comment)), " updating the definition of $(LD) in common.mak instead of changing it locally in just the BIOS Makefile" might make these changes more robust (/consistent) for anything else that uses the litex generated compiler/linker/flags, etc. (It looks like that Makefile suffered from trying to use ld with flags intended for gcc, via $(CPUFLAGS), on lm32....)

Also (as I said in the same comment) maybe having a build time option to "force LTO off" would help anyone thinking their target is suffering from bad LTO handling.

In general I think it'd be a great idea to have LTO available as a build option, and probably even on by default.

Ewen

/cc @mithro @mateusz-holenko

[mithro]

Having LTO on by default with a way to easily turn it off seems like a good plan. If LTO is off by default, it will just end up bit rotting until it no longer works.

[enjoy-digital]

I'm also ok with that, we just need to work a bit more on it before integrating it.

[DurandA]

I just found this issue, should I close #682 which is a duplicate of this?

* [ ]  Have to link against libgcc

This is not necessary. It was required because libcompiler_rt was optimized out.

[ewenmcneill]

should I close #682 which is a duplicate of this?

@DurandA It looks to me like #682 is much more active now. I'd probably lean towards closing this issue (which was already closed once, then reopened when the patch was reverted), with a comment like "LTO (re-)implementation work moved to #682"...

(Looks like @mithro or @enjoy-digital digital would have to be the ones to close this specific older issue.)

Ewen

[mithro]

Let's keep #682 and close this one.