[llvm] r266509 - [DebugInfo] Reduce size of DILocalVariable from 40 to 32 bytes.

[Duncan P. N. Exon Smith via llvm-commits]

> On 2016-Apr-15, at 20:14, Davide Italiano <davide at freebsd.org> wrote:
> 
> On Fri, Apr 15, 2016 at 7:57 PM, Duncan P. N. Exon Smith
> <dexonsmith at apple.com> wrote:
>> 
>>> On 2016-Apr-15, at 19:27, Davide Italiano via llvm-commits <llvm-commits at lists.llvm.org> wrote:
>>> 
>>> Author: davide
>>> Date: Fri Apr 15 21:27:56 2016
>>> New Revision: 266509
>>> 
>>> URL: http://llvm.org/viewvc/llvm-project?rev=266509&view=rev
>>> Log:
>>> [DebugInfo] Reduce size of DILocalVariable from 40 to 32 bytes.
>>> 
>>> This significantly contributes to peak memory usage during a
>>> LTO Release+DebugInfo build of clang. In my profile the peak usage
>>> is around 164MB before this change and ~130MB after.
>>> 
>>> Modified:
>>>   llvm/trunk/include/llvm/IR/DebugInfoMetadata.h
>>> 
>>> Modified: llvm/trunk/include/llvm/IR/DebugInfoMetadata.h
>>> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/IR/DebugInfoMetadata.h?rev=266509&r1=266508&r2=266509&view=diff
>>> ==============================================================================
>>> --- llvm/trunk/include/llvm/IR/DebugInfoMetadata.h (original)
>>> +++ llvm/trunk/include/llvm/IR/DebugInfoMetadata.h Fri Apr 15 21:27:56 2016
>>> @@ -1868,13 +1868,16 @@ class DILocalVariable : public DIVariabl
>>>  friend class LLVMContextImpl;
>>>  friend class MDNode;
>>> 
>>> -  unsigned Arg;
>>> -  unsigned Flags;
>>> +  unsigned Arg : 16;
>>> +  unsigned Flags : 16;
>> 
>> How does this save memory?  Are you on a 32-bit platform?
>> 
> 
> No, x86_64.

Yup, I missed the 32-bits in DIVariable.

You may be able to shave some bits off of MDNode::NumOperands and
MDNode::NumUnresolved.  I'm not sure you can get enough to help, but
maybe.

> 
>> BTW, I think there was some use case where someone needed an insane
>> number of arguments.  I don't remember what it was though.  Might
>> explain the bot failures?
>> 
> 
> Sorry, looking at the failures now.
> 
>>> 
>>>  DILocalVariable(LLVMContext &C, StorageType Storage, unsigned Line,
>>>                  unsigned Arg, unsigned Flags, ArrayRef<Metadata *> Ops)
>>>      : DIVariable(C, DILocalVariableKind, Storage, Line, Ops), Arg(Arg),
>>> -        Flags(Flags) {}
>>> +        Flags(Flags) {
>>> +    assert(Flags < ((1 << 16) - 1) && "DILocalVariable: Flags out of range");
>>> +    assert(Arg < ((1 << 16) - 1) && "DILocalVariable: Arg out of range");
>>> +  }
>>>  ~DILocalVariable() = default;
>>> 
>>>  static DILocalVariable *getImpl(LLVMContext &Context, DIScope *Scope,
>>> 
>>> 
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>> 
> 
> 
> 
> -- 
> Davide
> 
> "There are no solved problems; there are only problems that are more
> or less solved" -- Henri Poincare