[llvm-dev] Loop Invariants Detection questions

[Thomas RUBIANO via llvm-dev]

On Wed, Jan 18, 2017 at 8:05 PM, Friedman, Eli <efriedma at codeaurora.org>
wrote:

> On 1/18/2017 2:56 AM, Thomas RUBIANO wrote:
>
> Ty Eli for your answer.
>
> On Tue, Jan 17, 2017 at 8:11 PM, Friedman, Eli <efriedma at codeaurora.org>
> wrote:
>
>> On 1/17/2017 7:12 AM, Thomas RUBIANO via llvm-dev wrote:
>>
>>> Hi all!
>>>
>>> I'm new here, and would like to implement my own Loop Invariant
>>> Detection adding some more information on Quasi-Invariants.
>>>
>>> First, is there anything about Quasi-Invariants detection in LLVM I
>>> would missed?
>>>
>>> I've seen LICM using LoopInfo::isLoopInvariant for finding invariants.
>>> It seems that this method considers a Value invariant if:
>>> - it's an Instruction not presents in the current loop (what does it
>>> mean? There is no dependence analysis on In and Out "variables" of all
>>> instructions in the loop?)
>>>
>>
>> isLoopInvariant just checks whether the definition of a value is an
>> instruction inside the loop.
>>
>
> Ok, the term "definition" makes it clear. Do, if here it's:
> %1 = load i32, i32* %fact, align 4
> %mul = mul nsw i32 %1, %2
>
>
> load i32, i32* %fact, align 4 is the def of %1 and it's inside the loop
> then it's not invariant…
>
>
> Exactly.  If LICM can prove the load is in fact invariant, it will move it
> out of the loop.
>
>
>
>> - this Value is not an Instruction (then a Constant I guess…).
>>>
>>
>> Or an function argument, or a few other obscure things which don't really
>> matter in this context.
>>
>> I've seen LoopAccessAnalysis using it too. What does this analysis do
>>> exactly on loop invariant address?
>>>
>>
>> ScalarEvolution::isLoopInvariant works on SCEV expressions instead of
>> Values, but it's essentially the same thing.
>>
>
> What can offer this SCEV expression more than Values?
>
>
> See the comment at the beginning of lib/Analysis/ScalarEvolution.cpp for
> a brief description and some references.
>
>
>>
>>> Also DependenceAnalysis seems to give dependence information on memory
>>> refs. But it seems to not be used by LICM…
>>>
>>
>> Yes, the current LICM uses alias analysis in a more direct manner (look
>> for AliasSetTracker).
>>
>>
>>> Also MemoryDependenceAnalysis "determines, for a given memory operation,
>>> what preceding memory operations it depends on".
>>>
>>> My question is: Where is really done this dependence analysis. The one
>>> which figures out which Instructions depends on others?
>>>
>>> Simply if I have:
>>> %0 = load i32, i32* %coucou, align 4
>>> %1 = load i32, i32* %fact, align 4
>>> %2 = load i32, i32* %i, align 4
>>> %mul = mul nsw i32 %1, %2
>>>
>>> mul instruction will depends on the two precedents loads because it uses
>>> their results %1 and %2 but not the first one.
>>>
>>> I guess this is done somewhere, and there is a special representation of
>>> this information but I didn't find, I'm a bit lost ^^'
>>>
>>
>> If you call "operands()" on the Instruction, it will return %1 and %2.
>> Please keep in mind that LLVM IR is SSA.
>>
>
> Yes but if I understand well, the AliasSetTracker can tell me which load
> it corresponds to…
> How can I use this AliasSetTracker to disambiguate the registers?
> Here it's just having the correspondence %1 → %fact and %2 → %i
>
> It would be just perfect for me to have the "real" In and Out of each
> Instruction.
> Maybe I should work on another level or with another object representation?
>
> Ty again :)
>
>
> Can you show me the complete function you're looking at?  Have you run
> mem2reg on your IR?
>

I was looking the LoopInvariantCodeMotion::runOnLoop.

No I didn't run mem2reg on my IR…
Is it necessary?

I finally use the address of the operands and instruction to have a kind of
ID of each %<num>
It seems that it refers well what I want…

Let be the Instruction I:
%0 = mul nsw i32 %1, %2

&I = 0x3d9f850 → %0
operands(0)→get() = 0x3d9f7bc → %1
operands(1)→get() = 0x3d9f80c → %2

then it should be ok if I take this for finding my dependencies between
Instructions?


>
> -Eli
>
> --
> Employee of Qualcomm Innovation Center, Inc.
> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project
>
>


-- 
 RUBIANO Thomas
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