[LLVMdev] sinking address computing in CodeGenPrepare

[Evan Cheng]

On Nov 12, 2013, at 11:24 AM, Junbum Lim <junbums at gmail.com> wrote:

> 
> I wonder why CodeGenPrepare breaks GEP into integer calculations (ptrtoin/add/inttopt) instead of directly sinking the address calculation using GEP into user's block.

I believe it's primary for address mode matching where only part of the GEP can be folded (depending on the instruction set).

Evan

> 
> Thanks,
> Jun
> 
> 
> On Nov 12, 2013, at 12:07 PM, Evan Cheng <evan.cheng at apple.com> wrote:
> 
>> The reason for this is to allow folding of address computation into loads and stores. A lot of modern arch, e.g. X86 and arm, have complex addressing mode. 
>> 
>> Evan
>> 
>> Sent from my iPad
>> 
>>> On Nov 12, 2013, at 8:39 AM, Junbum Lim <junbums at gmail.com> wrote:
>>> 
>>> Hi All,
>>> 
>>> In CodeGenPrepare pass,  OptimizeMemoryInst() try to sink  address computing into users' block by converting GET to integers? It appear that it have impacts on ISel's result, but I'm not clear about the main purpose of the transformation. 
>>> 
>>> FROM :
>>> for.body.lr.ph:
>>>              %zzz = getelementptr inbounds %struct.SS* %a2, i32 0, i32 35    
>>> 
>>> for.body:
>>>              %4 = load double* %zzz, align 8, !tbaa !0
>>> 
>>> TO :
>>> for.body:
>>> %sunkaddr27 = ptrtoint %struct.SS* %a2 to i32       <----- sink address computing into user's block
>>> %sunkaddr28 = add i32 %sunkaddr27, 272
>>> %sunkaddr29 = inttoptr i32 %sunkaddr28 to double*
>>> %4 = load double* %sunkaddr29, align 8, !tbaa !8
>>> 
>>> 
>>> From what I observed, this transformation can cause poor alias analysis results without using GEP.  So, I want to see there is any way to avoid this conversion.
>>> 
>>> My question is : 
>>> 1. Why do we need to sink address computing into users' block? What is the benefit of this conversion ?  
>>> 2. Can we directly use GEP instead of breaking it into integer calculations ? 
>>> 
>>> Thanks,
>>> Jun
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>