[LLVMdev] sinking address computing in CodeGenPrepare

[Evan Cheng]

On Nov 20, 2013, at 5:38 PM, Hal Finkel <hfinkel at anl.gov> wrote:

> ----- Original Message -----
>> From: "Evan Cheng" <evan.cheng at apple.com>
>> To: "Junbum Lim" <junbums at gmail.com>
>> Cc: llvmdev at cs.uiuc.edu
>> Sent: Wednesday, November 20, 2013 7:01:49 PM
>> Subject: Re: [LLVMdev] sinking  address computing in CodeGenPrepare
>> 
>> 
>> On Nov 20, 2013, at 3:10 PM, Junbum Lim <junbums at gmail.com> wrote:
>> 
>>> 
>>> 
>>> When multiple GEPs or other operations are used for the address
>>> calculation, OptimizeMemoryInst() performs address matching and
>>> determines a final addressing expression as a simple form (e.g.,
>>> ptrtoint/add/inttoptr) and sinks it into user's block so that ISel
>>> could have better chance to fold address computation into LDRs and
>>> STRs. However, OptimizeMemoryInst() seems to do this
>>> transformation even when the address calculation derived from a
>>> single GEP, resulting in poor alias analysis because GEP is no
>>> longer used.
>> 
>> I don't follow your last statement. How does this impact AA?
>> CodeGenPrep is run late, after AA is done.
> 
> I don't know if this is relevant for Lim or not, but some targets use AA during CodeGen (instruction scheduling mostly, but SDAG too).

MachineSched uses AA to determine if something is loop invariant, which basically boils down to looking at machine operand and see it's pointing to constant memory. I don't see how that's impact by GEP vs. ADDS + MUL. Also, the analysis should have already been done and cached.

Evan

> 
> -Hal
> 
>> 
>> Evan
>> 
>>> 
>>> So, do you think it is a possible workaround to sink a GEP without
>>> converting it into a set of integer operations
>>> (ptrtoint/add/inttoptr) if the address mode is derived only from a
>>> single GEP.
>>> 
>>> Thanks,
>>> Jun
>>> 
>>> 
>>> On Nov 12, 2013, at 7:14 PM, Evan Cheng <evan.cheng at apple.com>
>>> wrote:
>>> 
>>>> 
>>>> 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
>>>>>>> _______________________________________________
>>>>>>> LLVM Developers mailing list
>>>>>>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>>>>>>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>>>>> 
>>>> 
>>> 
>> 
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> 
> -- 
> Hal Finkel
> Assistant Computational Scientist
> Leadership Computing Facility
> Argonne National Laboratory