[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]

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
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