Re: [PATCH v2 3/4] kbuild: re-implement CONFIG_TRIM_UNUSED_KSYMS to make it work in one-pass

[Rasmus Villemoes]

On 09/03/2021 20.54, Nicolas Pitre wrote:
> On Wed, 10 Mar 2021, Masahiro Yamada wrote:
> 

>>> I'm not sure I do understand every detail here, especially since it is
>>> so far away from the version that I originally contributed. But the
>>> concept looks good.
>>>
>>> I still think that there is no way around a recursive approach to get
>>> the maximum effect with LTO, but given that true LTO still isn't applied
>>> to mainline after all those years, the recursive approach brings
>>> nothing. Maybe that could be revisited if true LTO ever makes it into
>>> mainline, and the desire to reduce the binary size is still relevant
>>> enough to justify it.
>>
>> Hmm, I am confused.
>>
>> Does this patch change the behavior in the
>> combination with the "true LTO"?
>>
>> Please let me borrow this sentence from your article:
>> "But what LTO does is more like getting rid of branches that simply
>> float in the air without being connected to anything or which have
>> become loose due to optimization."
>> (https://lwn.net/Articles/746780/)
>>
>> This patch throws unneeded EXPORT_SYMBOL metadata
>> into the /DISCARD/ section of the linker script.
>>
>> The approach is different (preprocessor vs linker), but
>> we will still get the same result; the unneeded
>> EXPORT_SYMBOLs are disconnected from the main trunk.
>>
>> Then, the true LTO will remove branches floating in the air,
>> right?
>>
>> So, what will be lost by this patch?
> 
> Let's say you have this in module_foo:
> 
> int foo(int x)
> {
> 	return 2 + bar(x);
> }
> EXPORT_SYMBOL(foo);
> 
> And module_bar:
> 
> int bar(int y)
> {
> 	return 3 * baz(y);
> }
> EXPORT_SYMBOL(bar);
> 
> And this in the main kernel image:
> 
> int baz(int z)
> {
> 	return plonk(z);
> }
> EXPORT_SYMBOLbaz);
> 
> Now we build the kernel and modules. Then we realize that nothing 
> references symbol "foo". We can trim the "foo" export. But it would be 
> necessary to recompile module_foo with LTO (especially if there is 
> some other code in that module) to realize that nothing 
> references foo() any longer and optimize away the reference to bar(). 

But, does LTO even do that to modules? Sure, the export metadata for foo
vanishes, so there's no function pointer reference to foo, but given
that modules are just -r links, the compiler/linker can't really assume
that the generated object won't later be linked with something that does
require foo? At least for the simpler case of --gc-sections, ld docs say:

'--gc-sections'
...

    This option can be set when doing a partial link (enabled with
     option '-r').  In this case the root of symbols kept must be
     explicitly specified either by one of the options '--entry',
     '--undefined', or '--gc-keep-exported' or by a 'ENTRY' command in
     the linker script.

and I would assume that for LTO, --gc-keep-exported would be the only
sane semantics (keep any external symbol with default visibility).

Can you point me at a tree/set of LTO patches and a toolchain where the
previous implementation would actually eventually eliminate bar() from
module_bar?

Rasmus