[llvm-dev] ScalarEvolution class returns no valid loop exit count

[Philip Pfaffe via llvm-dev]

Your Loop *L is from a different LoopInfo object than the one you're giving
to ScalarEvolution. So your SE doesn't know your loop, and can't tell you
anything about it.

Cheers,
Philip

On Wed, Nov 28, 2018 at 2:26 PM benedikt steinmeier via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hi Sanjoy,
>
> the LLVM IR is parsed with following expression:
> .......
> string file = "....";
> static LLVMContext cont;
> SMDiagnostic diag;
> unique_ptr<Module> module = parseIRFile(file, diag, cont);
> .......
>
> I mixed up the LLVM loop exit block with the loop exiting block in my
> analyses code. But after
> resolving this issue, the exiting block is successfully determined:
> ......
> BB1:                                              ; preds = %BB3, %BB0
>   %3 = load i32, i32* %2, align 4
>   %4 = icmp slt i32 %3, 10
>   br i1 %4, label %BB2, label %BB4
> .......
>
> Nevertheless the loop exiting count can not be calculated with the
> ScalarEvolution class. I will try to
> do the loop optimizations.
>
> Many greetings
> Benedikt
>
>
>
>
>
> Am Di., 27. Nov. 2018 um 22:25 Uhr schrieb Sanjoy Das <
> sanjoy at playingwithpointers.com>:
>
>> Are you sure exitingblock is not null?  This should probably be an
>> assert in getExitCount itself.
>>
>> Other than that, SCEV normally expects that the loops have been
>> canonicalized and some basic optimizations have been performed.  How
>> are you obtaining the IR you're passing to analysis?
>>
>> -- Sanjoy
>> On Tue, Nov 27, 2018 at 1:14 PM benedikt steinmeier via llvm-dev
>> <llvm-dev at lists.llvm.org> wrote:
>> >
>> > Hi,
>> >
>> > I have problems to estimate the loop exit count of a simple loop with
>> the ScalarEvolution class.
>> >
>> > simple loop:
>> > ......
>> >     int a = 0;
>> >     for(int i; i < 10; ++i){
>> >         a = a + 1;
>> >     };
>> > ......
>> >
>> > For the loop analyzation I use the ScalarEvolution class with the
>> following initialization:
>> > ......
>> > void analysis(Function* func)
>> >     DominatorTree DT = DominatorTree();
>> >     DT.recalculate(*func);
>> >     DT.updateDFSNumbers();
>> >
>> >     LoopInfoBase<BasicBlock, Loop> LIB;
>> >     LIB.releaseMemory();
>> >     LIB.analyze(DT);
>> >
>> >     for(auto&bb :*func){
>> >         Loop * L = LIB.getLoopFor(&bb);
>> >         if(L != nullptr){
>> >             AssumptionCache AC = AssumptionCache(*bb.getParent());
>> >             Triple MT(llvm::sys::getDefaultTargetTriple());
>> >             TargetLibraryInfoImpl TLII(MT);
>> >             TargetLibraryInfoWrapperPass TLI =
>> TargetLibraryInfoWrapperPass(TLII);
>> >
>> >             LoopInfo LI = LoopInfo(DT);
>> >             LI.analyze(DT);
>> >
>> >             ScalarEvolution SE = ScalarEvolution(*func,
>> TLI.getTLI(),AC, DT, LI);
>> >             BasicBlock * exitingblock = L->getUniqueExitBlock();
>> >             const SCEV * exitingcount = SE.getExitCount (L,exitblock );
>> >         }
>> >     }
>> > }
>> > ......
>> >
>> > Unfortunately I get this result by printing the SCEV exitingcount
>> variable:
>> > ***COULDNOTCOMPUTE***
>> >
>> > It has been tested that the exitingblock of the loop was calculated
>> successfully.
>> > The LLVM IR is parsed and analysed without the usual pass framework.
>> For this reason
>> > the use of the getAnalysis<ScalarEvolution>() is not possible. The
>> question is wether there
>> > is something I`m doing wrong during the initialization of the
>> ScalarEvolution class? Or is
>> > the use of LLVM analysis classes without the passmanager framework not
>> possible?
>> >
>> > Best regards!
>> > Benedikt
>> >
>> >
>> > _______________________________________________
>> > LLVM Developers mailing list
>> > llvm-dev at lists.llvm.org
>> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
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