Miscompile with opt -passes=loop-reroll

[mikaelholmen]

LLVM commit: 41c6c75
Reproduce with:

opt -passes=loop-reroll bbi-77532.ll -S -o -

In the input the loop is executed 23 rounds

%i.021 = phi i16 [ 4, %entry ], [ %add45, %while.body ]
[...]
%add45 = add nsw i16 %i.021, 4
%sub = sub nsw i16 97, %add45
%cmp = icmp sge i16 %sub, 4
br i1 %cmp, label %while.body, label %while.end

and as seen above each round %add45 is incremented by 4:

%add45 = add nsw i16 %i.021, 4

After the loop %add45 is returned from the function:

%i.0.lcssa = phi i16 [ %add45, %while.body ]
ret i16 %i.0.lcssa

After loop-reroll, the loop executes 92 rounds

%indvar = phi i16 [ %indvar.next, %while.body ], [ 0, %entry ]
[...]
%indvar.next = add i16 %indvar, 1
%exitcond = icmp eq i16 %indvar, 91
br i1 %exitcond, label %while.end, label %while.body

but %add45 is still incremented by 4 each round

%add45 = add nsw i16 %i.021, 4

and it's still returned after the loop

%i.0.lcssa = phi i16 [ %add45, %while.body ]
ret i16 %i.0.lcssa

So the return value from the function is changed by the loop-reroll pass.
bbi-77532.ll.gz

[sun-jacobi]

@EugeneZelenko @mikaelholmen Hi, can I work on this issue?

[mikaelholmen]

@sun-jacobi : I don't mind, but I don't know if it makes sense either.

There are several tickets about loop-reroll miscompiles and e.g.
#68182
was closed since it was only reproducible with
"the unsupported / broken loop-reroll pass"

I don't know what the plan is for loop-reroll.

[fhahn]

FWIW see https://reviews.llvm.org/D150684 for a summary of the issues and a proposal to remove the pass (temporarily)

[nikic]

The LoopReroll pass has been removed by #80972.