[llvm-dev] Loop Invariants Detection questions

Friedman, Eli via llvm-dev llvm-dev at lists.llvm.org
Tue Jan 17 11:11:11 PST 2017

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.

> - 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.

> 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.


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