[llvm-dev] [RFC] Annotating global functions and variables to prevent ICF during linking
James Y Knight via llvm-dev
llvm-dev at lists.llvm.org
Wed Apr 7 12:26:18 PDT 2021
Three things:
1. if chrome already has a mechanism/hack to make functions distinct in the
face of icf=all, do we really need to add a compiler feature?
2. Are we sure functions aren't ending up in the addrsig table spuriously
(and that the addrsig table isn't getting lost)? IOW: are the additional
functions that get emitted with icf=safe really needed (as far as
reasonable compiler analysis can determine), or should/could they have been
removed?
3. If we _do_ need to implement something here (which I'm skeptical of) it
would be better to add a compiler flag which adjusts how the contents of
the addrsig table are computed, vs adding a new linker mechanism.
You'd continue to link with icf=safe, but the compiler option would allow
more functions to be merged, by putting fewer into addrsig.
This would also allow the "unsafe" semantics to be per-TU instead of global
for the entire link, allowing you not to break other libraries that may be
linked into the same binary.
On Wed, Apr 7, 2021, 1:32 PM Reid Kleckner <rnk at google.com> wrote:
> Thanks for gathering the data, Zequan. For more context, I'll add that
> chrome.dll is around 163MB, so saving 6MB is around 3.5% of binary size. I
> feel like this is a compelling use case for aggressive ICF.
>
> Chrome has historically used MSVC ICF, which is aggressive, and it can
> make debugging challenging. We're looking for a way that we can selectively
> power down ICF for functions that developers are interested in. I think it
> would be reasonable to have a compiler flag that emits an extra section
> with the same format as llvm_addrsig, with the semantics that it blocks ICF
> even in the aggressive mode.
>
> For the user interface, we can either invent a new attribute, (noicf,
> blockicf, disable_icf), document that it only works with linkers that
> support the new section (LLD), and go that way. Alternatively we can
> overload an existing attribute like __attribute__((used)). I think I favor
> the new attribute. It makes it easier to search and find the relevant
> documentation.
>
> Does that seem reasonable?
>
> On Wed, Apr 7, 2021 at 10:06 AM Zequan Wu <zequanwu at google.com> wrote:
>
>> I got some data for the size increase from safe icf to all icf. The 14 MB
>> I initially found was due to .addrsig not emitted on COFF thinlto.
>> After enabling it, the size of chrome.dll is increased by 6 MB when
>> switching from all icf to safe icf on windows and 5 MB increase on linux.
>>
>> Here is a breakdown of chrome.dll section size increase on Windows.
>> 5574656 bytes increase in .text
>> 643072 bytes increase in .rdata
>> 573440 bytes increase in .pdata
>> 57344 bytes increase in .reloc
>> 407152 bytes increase in rvatable
>>
>> I dumped the sum of section size removed and selected/removed sections
>> count by icf in ELF and COFF as below.
>> all-icf.win.txt:
>> total size removed: 10763454 selected: 19712 removed: 153544
>> safe-icf.win.txt:
>> total size removed: 6122783 selected: 8653 removed: 48999
>> all-icf.linux.txt:
>> total size removed: 10423699 selected: 22254 removed: 158806
>> safe-icf.linux.txt:
>> total size removed: 6165470 selected: 9313 removed: 53981
>>
>> There is not much difference. So, safe icf is performing the same on
>> linux and windows. I think we have enough motivation to add a new section
>> for disabling icf.
>>
>> On Thu, Mar 25, 2021 at 10:57 AM Reid Kleckner <rnk at google.com> wrote:
>>
>>> I should add, though, that I agree with the final direction here: if the
>>> cost of safe ICF is low, it is much better to just use it instead of adding
>>> a whole new feature requiring documentation, etc. We need new measurements
>>> to show if the feature is justified.
>>>
>>> On Thu, Mar 25, 2021 at 10:55 AM Reid Kleckner <rnk at google.com> wrote:
>>>
>>>> This is a bit of a hot take, but I'd rather make aggressive ICF (fold
>>>> any identical code) the norm and scrap the C++ language guarantee about
>>>> function pointer identity. I don't participate in C++ standardization, so
>>>> my opinion doesn't carry much weight, but I lean towards giving the
>>>> optimizer a free hand here. There are a number of other ways in which
>>>> function pointer identity breaks down. Consider
>>>> -fvisibility-inlines=hidden, which if we are being real, is the only way to
>>>> compile big C++ projects, even those with explicit visibility annotations.
>>>> Function pointer identity is just fragile and easy to break, and I don't
>>>> encourage people to rely on it. If you need some kind of globally unique
>>>> identifier, it's pretty straightforward to use data instead of code.
>>>>
>>>> On Tue, Mar 23, 2021 at 2:55 PM James Y Knight via llvm-dev <
>>>> llvm-dev at lists.llvm.org> wrote:
>>>>
>>>>> In general, I don't like the "icf=all" mode. It's not nice to have the
>>>>> linker break semantics of the program, and I think we should not encourage
>>>>> its use. (And, in particular, adding new features just for it seems like a
>>>>> bad idea.) Instead, we should make sure that icf=safe works as well as it
>>>>> possibly can, and encourage its use.
>>>>>
>>>>> It's not possible to conclude anything based only on the size results,
>>>>> but it does indicate that some further investigation may be a good idea, to
>>>>> look into *why* icf=safe is apparently less-effective on the windows
>>>>> binary vs the linux one.
>>>>>
>>>>>
>>>>> On Tue, Mar 23, 2021 at 2:50 PM Fangrui Song via llvm-dev <
>>>>> llvm-dev at lists.llvm.org> wrote:
>>>>>
>>>>>>
>>>>>> On 2021-03-23, Zequan Wu wrote:
>>>>>> >The size increase of chrome on Linux by switching from all ICF to
>>>>>> safe ICF
>>>>>> >is small.
>>>>>> >All ICF:
>>>>>> > text data bss dec hex filename
>>>>>> >169314343 8472660 2368965 180155968 abcf640 chrome
>>>>>> >Safe ICF:
>>>>>> > text data bss dec hex filename
>>>>>> >174521550 8497604 2368965 185388119 b0ccc57 chrome
>>>>>> >
>>>>>> >On Windows, chrome.dll increases size by around 14 MB (12MB
>>>>>> increases in
>>>>>> >.text section).
>>>>>> >All ICF:
>>>>>> >Size of out\Default\chrome.dll is 170.715648 MB
>>>>>> > name: mem size , disk size
>>>>>> > .text: 141.701417 MB
>>>>>> > .rdata: 22.458476 MB
>>>>>> > .data: 3.093948 MB, 0.523264 MB
>>>>>> > .pdata: 4.412364 MB
>>>>>> > .00cfg: 0.000040 MB
>>>>>> > .gehcont: 0.000132 MB
>>>>>> > .retplne: 0.000108 MB
>>>>>> > .rodata: 0.004544 MB
>>>>>> > .tls: 0.000561 MB
>>>>>> > CPADinfo: 0.000056 MB
>>>>>> > _RDATA: 0.000244 MB
>>>>>> > .rsrc: 0.285232 MB
>>>>>> > .reloc: 1.324196 MB
>>>>>> >Safe ICF:
>>>>>> >Size of out\icf-safe\chrome.dll is 184.499712 MB
>>>>>> > name: mem size , disk size
>>>>>> > .text: 153.809529 MB
>>>>>> > .rdata: 23.123628 MB
>>>>>> > .data: 3.093948 MB, 0.523264 MB
>>>>>> > .pdata: 5.367396 MB
>>>>>> > .00cfg: 0.000040 MB
>>>>>> > .gehcont: 0.000132 MB
>>>>>> > .retplne: 0.000108 MB
>>>>>> > .rodata: 0.004544 MB
>>>>>> > .tls: 0.000561 MB
>>>>>> > CPADinfo: 0.000056 MB
>>>>>> > _RDATA: 0.000244 MB
>>>>>> > .rsrc: 0.285232 MB
>>>>>> > .reloc: 1.379364 MB
>>>>>> >
>>>>>> >If an attribute is used and it affects unnamed_addr of a symbol, it
>>>>>> >determines whether the symbols should show up in the .addrsig table.
>>>>>> >All-ICF mode in ld.lld and lld-link ignore symbols in the .addrsig
>>>>>> table,
>>>>>> >if they belong to code sections. So, it won't have an effect on
>>>>>> disabling
>>>>>> >ICF.
>>>>>>
>>>>>> Is there something which can be improved on lld-link? Note also that
>>>>>> the size difference between gold --icf={safe,all} is smaller than the
>>>>>> difference between ld.lld --icf={safe,all}. Some may be due to gold
>>>>>> performing "unsafe" safe ICF, some may suggest places where clang
>>>>>> fails
>>>>>> to add {,local_}unnamed_addr.
>>>>>>
>>>>>>
>>>>>> >On Mon, Mar 22, 2021 at 10:19 PM Fangrui Song <maskray at google.com>
>>>>>> wrote:
>>>>>> >
>>>>>> >>
>>>>>> >> On 2021-03-22, David Blaikie via llvm-dev wrote:
>>>>>> >> >ICF: Identical Code Folding
>>>>>> >> >
>>>>>> >> >Linker deduplicates functions by collapsing any identical
>>>>>> functions
>>>>>> >> >together - with icf=safe, the linker looks at a .addressing
>>>>>> section in the
>>>>>> >> >object file and any functions listed in that section are not
>>>>>> treated as
>>>>>> >> >collapsible (eg: because they need to meet C++'s "distinct
>>>>>> functions have
>>>>>> >> >distinct addresses" guarantee)
>>>>>> >>
>>>>>> >> The name originated from MSVC link.exe where icf stands for
>>>>>> "identical
>>>>>> >> COMDAT folding".
>>>>>> >> gold named it "identical code folding" - which makes some sense
>>>>>> because
>>>>>> >> gold does not fold readonly data.
>>>>>> >>
>>>>>> >> In LLD, the name is not accurate for two reasons: (1) the feature
>>>>>> can
>>>>>> >> apply to readonly data as well; (2) the folding is by section, not
>>>>>> by
>>>>>> >> function.
>>>>>> >>
>>>>>> >> We define identical sections as they have identical content and
>>>>>> their
>>>>>> >> outgoing relocation sets cannot be distinguished: they need to
>>>>>> have the
>>>>>> >> same number of relocations, with the same relative locations, with
>>>>>> the
>>>>>> >> referenced symbols indistinguishable.
>>>>>> >>
>>>>>> >> Then, ld.lld --icf={safe,all} works like this:
>>>>>> >>
>>>>>> >> For a set of identical sections, the linker picks one
>>>>>> representative and
>>>>>> >> drops the rest, then redirects references to the representative.
>>>>>> >>
>>>>>> >> Note: this can confuse debuggers/symbolizers/profilers easily.
>>>>>> >>
>>>>>> >> lld-link /opt:icf is different from ld.lld --icf but I haven't
>>>>>> looked
>>>>>> >> into it closely.
>>>>>> >>
>>>>>> >>
>>>>>> >> I find that the feature's saving is small given its downside
>>>>>> >> (also increaded link time: the current LLD's implementation is
>>>>>> inferior:
>>>>>> >> it performs a quadratic number of comparisons among an equality
>>>>>> class):
>>>>>> >>
>>>>>> >> This is the size differences for the 'lld' executable:
>>>>>> >>
>>>>>> >> % size lld.{none,safe,all}
>>>>>> >> text data bss dec hex filename
>>>>>> >> 96821040 7210504 550810 104582354 63bccd2 lld.none
>>>>>> >> 95217624 7167656 550810 102936090 622ae1a lld.safe
>>>>>> >> 94038808 7167144 550810 101756762 610af5a lld.all
>>>>>> >> % size gold.{none,safe,all}
>>>>>> >> text data bss dec hex filename
>>>>>> >> 96857302 7174792 550825 104582919 63bcf07 gold.none
>>>>>> >> 94469390 7174792 550825 102195007 6175f3f gold.safe
>>>>>> >> 94184430 7174792 550825 101910047 613061f gold.all
>>>>>> >>
>>>>>> >> Note that the --icf=all result caps the potential saving of the
>>>>>> proposed
>>>>>> >> annotation.
>>>>>> >>
>>>>>> >> Actually with some large internal targets I get even smaller
>>>>>> savings.
>>>>>> >>
>>>>>> >>
>>>>>> >> ld.lld --icf=safe is safer than gold --icf=safe but probably
>>>>>> misses some
>>>>>> >> opportunities.
>>>>>> >> It can be that clang codegen/optimizer fail to mark some cases as
>>>>>> >> {,local_}unnamed_addr.
>>>>>> >>
>>>>>> >> I know Chromium and the Windows world can be different:) But I'd
>>>>>> still
>>>>>> >> want to
>>>>>> >> get some numbers first.
>>>>>> >>
>>>>>> >>
>>>>>> >> Last, I have seen that Chromium has some code like
>>>>>> >>
>>>>>> >>
>>>>>> https://source.chromium.org/chromium/chromium/src/+/master:skia/ext/SkMemory_new_handler.cpp
>>>>>> >>
>>>>>> >> void sk_abort_no_print() {
>>>>>> >> // Linker's ICF feature may merge this function with other
>>>>>> >> functions with
>>>>>> >> // the same definition (e.g. any function whose sole job is
>>>>>> to call
>>>>>> >> abort())
>>>>>> >> // and it may confuse the crash report processing system.
>>>>>> >> // http://crbug.com/860850
>>>>>> >> static int static_variable_to_make_this_function_unique =
>>>>>> 0x736b;
>>>>>> >> // "sk"
>>>>>> >>
>>>>>> base::debug::Alias(&static_variable_to_make_this_function_unique);
>>>>>> >>
>>>>>> >> abort();
>>>>>> >> }
>>>>>> >>
>>>>>> >> If we want an approach to work with link.exe, I don't know what we
>>>>>> can
>>>>>> >> do...
>>>>>> >> If no desire for link.exe compatibility, I can see that having a
>>>>>> proper
>>>>>> >> way marking the function
>>>>>> >> can be useful... but in any case if an attribute is used, it
>>>>>> probably
>>>>>> >> should affect
>>>>>> >> unnamed_addr directly instead of being called *icf*.
>>>>>> >>
>>>>>> >>
>>>>>> >>
>>>>>> >> >On Mon, Mar 22, 2021 at 6:16 PM Philip Reames via llvm-dev <
>>>>>> >> >llvm-dev at lists.llvm.org> wrote:
>>>>>> >> >
>>>>>> >> >> Can you define ICF please? And give a bit of context?
>>>>>> >> >>
>>>>>> >> >> Philip
>>>>>> >> >> On 3/22/21 5:27 PM, Zequan Wu via llvm-dev wrote:
>>>>>> >> >>
>>>>>> >> >> Hi all,
>>>>>> >> >>
>>>>>> >> >> Background:
>>>>>> >> >> It's been a longstanding difficulty of debugging with ICF.
>>>>>> Programmers
>>>>>> >> >> don't have control over which sections should be folded by ICF,
>>>>>> which
>>>>>> >> >> sections shouldn't. The existing address significant table
>>>>>> won't have
>>>>>> >> >> effect for code sections during all ICF mode in both ld.lld and
>>>>>> >> lld-link.
>>>>>> >> >> By switching to safe ICF could mark code sections as unique,
>>>>>> but at a
>>>>>> >> cost
>>>>>> >> >> of increasing binary size out of control. So, it would be good
>>>>>> if
>>>>>> >> >> programmers could selectively disable ICF in source code by
>>>>>> annotating
>>>>>> >> >> global functions/variables with an attribute to improve
>>>>>> debugging
>>>>>> >> >> experience and have the control on the binary size increase.
>>>>>> >> >>
>>>>>> >> >> My plan is to add a new section table(`.no_icf`) to object
>>>>>> files.
>>>>>> >> Sections
>>>>>> >> >> of all symbols inside the table should not be folded by all ICF
>>>>>> mode.
>>>>>> >> And
>>>>>> >> >> symbols can only be added into the table by annotating global
>>>>>> >> >> functions/variables with a new attribute(`no_icf`) in source
>>>>>> code.
>>>>>> >> >>
>>>>>> >> >> What do you think about this approach?
>>>>>> >> >>
>>>>>> >> >> Thanks,
>>>>>> >> >> Zequan
>>>>>> >> >>
>>>>>> >> >>
>>>>>> >> >> _______________________________________________
>>>>>> >> >> LLVM Developers mailing listllvm-dev at lists.llvm.orghttps://
>>>>>> >> lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>>> >> >>
>>>>>> >> >> _______________________________________________
>>>>>> >> >> LLVM Developers mailing list
>>>>>> >> >> llvm-dev at lists.llvm.org
>>>>>> >> >> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>>> >> >>
>>>>>> >>
>>>>>> >> >_______________________________________________
>>>>>> >> >LLVM Developers mailing list
>>>>>> >> >llvm-dev at lists.llvm.org
>>>>>> >> >https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>>> >>
>>>>>> >>
>>>>>> _______________________________________________
>>>>>> LLVM Developers mailing list
>>>>>> llvm-dev at lists.llvm.org
>>>>>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>>>
>>>>> _______________________________________________
>>>>> LLVM Developers mailing list
>>>>> llvm-dev at lists.llvm.org
>>>>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>>
>>>>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20210407/cd779542/attachment.html>
More information about the llvm-dev
mailing list