[llvm-dev] Canonicalize induction variables

[Ehsan Amiri via llvm-dev]

Does the fact that loop vectorizer supports arbitrary constant step size
suggest that this is by design?
https://reviews.llvm.org/rL227557

Apparently an older version of LLVM, canonicalized induction variables as
described above.
http://llvm.org/releases/2.6/docs/Passes.html#indvars



On Wed, Aug 24, 2016 at 9:48 PM, Yaoqing Gao via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> I just subscribed this group.  This is my first time to post a question
> (not sure if this is a right place for discussion) after I have a brief
> look at LLVM OPT (dev trunk).   I would expect loop simplification and
> induction variable canonicalization pass (IndVarSimplify pass) should be
> able to convert the following loops into a simple canonical form, i.e.,
> there is a canonical induction variable which starts at zero and steps by
> one,  getCanonicalInductionVariable() returns  the first PHI node in the
> loop header block.
>
> int test1 (int x[], int k, int s) {
>   int sum = 0;
>   for (int i = 0; i < k; i+=s) {
>     sum += x[i];
>   }
>   return sum;
> }
>
> int test2(int x[], int k, int s) {
>   int sum = 0;
>   for (int i = k; i > 0; i--) {
>     sum += x[i];
>   }
>   return sum;
> }
>
> Anyone can help explain why the current LLVM cannot canonicalize induction
> variables for the above loops (by design or a limitation to be fixed in the
> future)?  Thanks.
>
> --
> Best regards,
>
> Yaoqing Gao
>
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>
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