[llvm-dev] SCEV determines the inner loop induction variable to be loop-invariant at the scope of the outer loop

[congzhe cao via llvm-dev]

Hi Michael,

Thanks for the reply! As you've seen, my purpose is indeed to use
getSCEVAtScope(V, L) with V being an instruction/value in the inner loop
and L being the outer loop.

- I can somewhat see that if V is the induction variable for the inner
loop, then getSCEVAtScope(V, L) tries to derive the backedgeTakenCount of
the inner loop. Is it always correct?

- I'm also wondering if V is not the induction variable for the inner loop
but some other value inside the inner loop (so this is a more general
situation), what would be the expected behavior of getSCEVAtScope(V, L)? It
would be ideal if you could let me know the logic in SCEV regarding this
situation.

Thanks again,
Congzhe

On Wed, Feb 3, 2021 at 12:54 PM Michael Kruse <llvmdev at meinersbur.de> wrote:

> %j.018 is variant in the innermost loop (for.body4), but after
> existing that loop, it will have the value before leaving the loop. If
> you intent to use %j.018 in the innermost loop, you need to call
> getSCEVAtScope() with the innermost loop as the scope. getSCEVAtScope
> with a scope outside the loop (or NULL) will try to derive the exit
> value.
>
> Michael
>
>
>
> Am Mi., 3. Feb. 2021 um 11:11 Uhr schrieb congzhe cao via llvm-dev
> <llvm-dev at lists.llvm.org>:
> >
> > Dear all,
> >
> >
> >
> > For the following IR which is essentially a doubly nested loop, if we
> get the SCEV expression for the inner loop induction variable, i.e.,
> %j.018, at the scope of the outer loop using getSCEVAtScope(), the result
> is: 9. That is a constant, or loop-invariant.
> >
> >
> > However, %j.018 does keep changing within the scope of the outer loop
> since it is the induction variable of the inner loop, so it is not
> straightforward to me why %j.018 is considered a loop-invariant. Something
> like “{0,+,1}<nuw><nsw><%for.body4>” would make more sense to me.
> >
> >
> >
> > I’m wondering if I can get any comments on that?
> >
> >
> >
> > Best regards,
> >
> > Congzhe
> >
> >
> >
> > ***************************************************************
> >
> > define dso_local i32 @main() {
> >
> > entry:
> >
> >   br label %for.cond1.preheader
> >
> >
> >
> > for.cond1.preheader:                              ; preds =
> %for.cond.cleanup3, %entry
> >
> >   %i.020 = phi i32 [ 0, %entry ], [ %inc8, %for.cond.cleanup3 ]
> >
> >   %x.019 = phi i32 [ 17, %entry ], [ %add, %for.cond.cleanup3 ]
> >
> >   br label %for.body4
> >
> >
> >
> > for.cond.cleanup:                                 ; preds =
> %for.cond.cleanup3
> >
> >   ret i32 %add
> >
> >
> >
> > for.cond.cleanup3:                                ; preds = %for.body4
> >
> >   %inc8 = add nuw nsw i32 %i.020, 1
> >
> >   %exitcond21 = icmp eq i32 %inc8, 10
> >
> >   br i1 %exitcond21, label %for.cond.cleanup, label %for.cond1.preheader
> >
> >
> >
> > for.body4:                                        ; preds = %for.body4,
> %for.cond1.preheader
> >
> >   %j.018 = phi i64 [ 0, %for.cond1.preheader ], [ %inc, %for.body4 ]
> >
> >   %x.117 = phi i32 [ %x.019, %for.cond1.preheader ], [ %add, %for.body4 ]
> >
> >   %cmp5 = icmp eq i64 %j.018, 9
> >
> >   %conv = zext i1 %cmp5 to i32
> >
> >   %add = add nsw i32 %x.117, %conv
> >
> >   call void @_Z3foov()
> >
> >   %inc = add nuw nsw i64 %j.018, 1
> >
> >   %exitcond = icmp eq i64 %inc, 10
> >
> >   br i1 %exitcond, label %for.cond.cleanup3, label %for.body4
> >
> > }
> >
> >
> >
> > declare dso_local void @_Z3foov() local_unnamed_addr #1
> >
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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