[cfe-dev] Should dump methods be LLVM_ATTRIBUTE_USED only in debug builds?

[Nico Weber]

On Thu, Jan 2, 2014 at 11:36 AM, Argyrios Kyrtzidis <akyrtzi at gmail.com>wrote:

>
> On Jan 2, 2014, at 11:05 AM, Nico Weber <thakis at chromium.org> wrote:
>
> On Fri, Dec 27, 2013 at 5:01 PM, Chandler Carruth <chandlerc at google.com>wrote:
>
>>
>> On Fri, Dec 27, 2013 at 7:46 PM, Nico Weber <thakis at chromium.org> wrote:
>>
>>> Hi,
>>>
>>> r151033 and r151037 marked a few dump() methods as LLVM_ATTRIBUTE_USED,
>>> and over time this inspired marking several other dump() methods to be
>>> marked as such too (see e.g. 178400, 182186, 159790, 173548).
>>>
>>> I understand that having the dump() methods available in the debugger is
>>> useful, but these annotations prevent the dump() methods from being
>>> dead-stripped, and they end up keeping lots of code alive. For example,
>>> clang-format depends on ASTDumper, TypePrinter, StmtVisitor and related
>>> stuff solely for these dump methods.
>>>
>>> Since binaries now get dead-stripped, this leads to measurable bloat:
>>> clang-format goes from 1.7 MB to 1.2 MB if I remove the
>>> LLVM_ATTRIBUTE_USEDs on the 17 dump methods in include/clang – an almost
>>> 30% reduction.
>>>
>>> Does it make sense to only mark dump methods as LLVM_ATTRIBUTE_USED if
>>> !NDEBUG?
>>>
>>
>> I think what makes sense is to not have these methods in !NDEBUG. This is
>> the way dump methods are being written in LLVM these days:
>>
>> #if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
>>   void some_helper_function_only_used_by_dump() const;
>>   void LLVM_ATTRIBUTE_NOINLINE LLVM_ATTRIBUTE_USED dump() const;
>> #endif
>>
>> At least, this is how SROA.cpp does it, and I think it has been suggested
>> to do it consistently in this way.
>>
>> My preference would be to:
>>
>> a) Make this pattern easier to reproduce where needed.
>> b) Try to make existing dump code use it more regularly throughout both
>> LLVM and Clang to fix just the problem you've identified.
>>
>
> Ah, good point bringing up dump() in llvm! Looking through ` ack -C 3 -i
> dump include/` in llvm, it looks like none of the dump methods in llvm are
> marked as LLVM_USED – maybe it's reasonable that people who want to call
> this from gdb during debugging insert that locally manually?
>
> Argyrios, would just removing all the LLVM_ATTRIBUTE_USEDs on dump methods
> in clang be fine with you? That sounds like the simples change, and it
> would be consistent with what's done in llvm.
>
>
> Are you suggesting that one should add ‘LLVM_ATTRIBUTE_USED’ locally and
> rebuild if s/he wants to dump something in the debugger ?
>

Yes.


> We should be consistent across LLVM but having the dump methods available
> at least for !NDEBUG, as Chandler suggested, makes sense for me.
>

With "available", do you mean "marked LLVM_ATTRIBUTE_USED"? If we remove
LLVM_ATTRIBUTE_USED, dump() could be compiled in both release and debug,
which is nice in a way (and it's unlikely someone will accidentally add a
call to them from live code).
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