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

Nico Weber thakis at chromium.org
Thu Jan 2 11:05:28 PST 2014

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
> 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;
> #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.
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