[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
> 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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