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

[Argyrios Kyrtzidis]

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 ?

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

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