[LLVMdev] TargetData::getPreferredAlignment(const GlobalVariable *GV) is strange ...

[Chris Lattner]

On Mar 9, 2011, at 8:40 AM, Duncan Sands wrote:

> Hi Fabian,
> 
>> I am somewhat confused by the following method within the LLVM,
>> especially the lines
>> "confusion starts" ->  "confusion ends" are hard to follow.
> 
> yes, this seems like a wart.  It has been there ever since Chris added the
> getPreferredAlignmentLog method in commit 25978.  Maybe he can comment on
> whether the code to bump up the alignment for big objects is still needed.

Ah, I really vaguely remember this.  IIRC, there was some fortran benchmark (that was running through f2c) which had a large array of doubles.  On X86, double is only 4-byte aligned, and the huge array was getting put at a "mod 16=4" offset.  This caused really really awful performance.

A reasonable solution to this was to bump up the alignment of stuff proactively, but you don't want to do this for everything, because this ends up wasting lots of memory.  GCC has a similar policy IIRC.

Is there some problem that this is causing?

-Chris


> Maybe the
>> idea is that if there
>> are such big data structures one does not waste much memory anyway if
>> they are aligned
>>  to a 16-byte boundary. However, my assembler complains here because
>> it only supports
>>  1-, 2-, 4- and 8-byte boundaries :-(
>> 
>> I checked the svn log but I didn't find any explanation, the doxygen
>> docu is not very helpful
>> here, too. So, any help on this issue is highly appreciated.
>> 
>> Thanks in advance!
>> 
>> Ciao, Fabian
>> 
>> unsigned TargetData::getPreferredAlignment(const GlobalVariable *GV) const {
>>   const Type *ElemType = GV->getType()->getElementType();
>>   unsigned Alignment = getPrefTypeAlignment(ElemType);
>>   if (GV->getAlignment()>  Alignment)
>>     Alignment = GV->getAlignment();
>> 
>> ==================== confusion starts ========================
>>   if (GV->hasInitializer()) {
>>     if (Alignment<  16) {
>>       // If the global is not external, see if it is large.  If so, give it a
>>       // larger alignment.
>>       if (getTypeSizeInBits(ElemType)>  128)
>>         Alignment = 16;    // 16-byte alignment.
>>     }
>>   }
>> ==================== confusion ends =========================
>> 
>>   return Alignment;
>> }
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