sdiv in array subscript

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1

Hi llvm-dev,

Looks like currently ScalarEvolution will give up if there is a sdiv in array subscript, e.g.

int i;

A[i * 64 / 2]

in this case ScalarEvolution will just return an unknown for (i * 64 / 2).

For this case, InstCombine will do the jobs, but in general, is there a pass to deal with the sdiv here? like replace sdiv by udiv based on the range of “i”?

Thanks
Hongbin

2

Instcombine will replace sdiv with udiv if it can prove the sign bit is zero, but it isn't very good at analyzing induction variables; we might be able to improve that.

There are also some related transforms in lib/Transforms/Utils/SimplifyIndVar.cpp.

-Eli

3

Hi Eli,

Thanks. Do you mean ideally we should extend SimplifyIndVar to do the sdiv->udiv replacement?

Thanks
Hongbin

4

I haven't really looked into it closely, but it seems to make sense.

-Eli

5

Hi Eli,

Thanks. Do you mean ideally we should extend SimplifyIndVar to do the
sdiv->udiv replacement?

I haven't really looked into it closely, but it seems to make sense.

Ok.

Once I extend SimplifyIndVar, e.g. the simplifyUsersOfIV function. How I
can add a regression test for the extension? simplifyUsersOfIV is not a
pass.

Thanks
Hongbin

6

It gets called by -indvars. -Eli

7

Hi Eli,

I create a patch at https://reviews.llvm.org/D31488

Thanks
Hongbin