[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 19:57, Duncan P. N. Exon Smith via llvm-commits <llvm-commits at lists.llvm.org> 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?

Oh, I see: DIVariable adds 32 bits, so this rounds it down.

> 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?
> 
>> 
>>  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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