[LoopVectorize] Vectorize the reduction pattern of integer min/max with index.

[Mel-Chen]

Summary

Consider the following loops:

int idx = ii;
int max = mm;
for (int i = 0; i < n; ++i) {
  int x = a[i];
  if (max < x) {
    max = x;
    idx = i;
  }
}

and

int idx = ii;
int max = mm;
for (int i = 0; i < n; ++i) {
  int x = a[i];
  if (max <= x) {
    max = x;
    idx = i;
  }
}

Changes:

• New recurrence kinds: SelectIVICmp and SelectIVFCmp. This is temporary. Eventually, SelectICmp/SelectFCmp should be able to fully support conditional scalar assignment. At that time, SelectIVICmp/SelectIVFCmp will be removed.
• New recurrence Kinds: MinMaxFirstIdx and MinMaxLastIdx. This kind is not directly generated by function AddReductionVar, but converted from SelectIVICmp/SelectIVFCmp.

TODOs:

• Now have not support that the min/max recurrence without exit instruction. Refer to test case smax_idx_max_no_exit_user.
• Support the min/max recurrence in select(cmp()). Refer to test case smax_idx_select_cmp.
• Support FP min/max recurrence.

[Mel-Chen]

Rebase and update the command in test case.

[Mel-Chen]

Changes:

• Fix interleave code generation for SelectIVICmp and SelectIVFCmp.
• Fix the internal compiler error for MinMaxFirstIdx and MinMaxLastIdx when -force-vector-width=1.
• Update test cases. Add more run command lines.

[Mel-Chen]

Changes:

• Rebase
• Split the patch of test case and implementation
• Update FIXME

[Mel-Chen]

Changes:

• Remove function createInductionSelectCmpTargetReduction
• Add function createSentinelValueHandling
• Fix the start value of SelectCmp and MinMaxIdx

And then I don't know why all FIXME has been fixed, will confirm.

[Mel-Chen]

Rebase and fix the check prefix in test cases.

[Mel-Chen]

Changes:

• Confirm the operands of intrinsic and cmp. Fixed test case @smax_idx_not_vec_1.
• Format code.

[Mel-Chen]

Rebase.

[Mel-Chen]

@fhahn Ping. What do you think of my approach? I am looking forward to your reply.

[Mel-Chen]

Changes:

• Rebase
• Fix the bug of predicate normalization

[Mel-Chen]

Rebase and update test case result.

[fhahn]

llvm/include/llvm/Analysis/IVDescriptors.h

379

It would be helpful to document how the new system of recurrences depending on other recurrences would work I think, possibly also with an explanation of the whole approach in the patch description.

llvm/lib/Analysis/IVDescriptors.cpp

733

nit: Variables should start with upper case

also, move definition to use?

758

The naming here is a bit confusing now, NonPhi can be an increasing loop induction? In that case it would be a phi, right?

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

777

We are in the process of removing those kinds of global maps that are used to carry information used during codegen and later. Ideally the combination of values would be modeled explicitly in the exit block of the plan, but we are not there yet. This is the main reason for D132063 doing things the way it does.

3805

this would need documenting.

llvm/test/Transforms/LoopVectorize/smax-idx.ll

1

Could you add new tests as a separate patch?

[Mel-Chen]

llvm/include/llvm/Analysis/IVDescriptors.h

379

Sure. I will document the whole approach. and update in the summary tomorrow.
Quickly explain the function of UserRecurPhi . The purpose of UserRecurPhi is to allow the recurrence to be used in the loop (loop internal use), and to ensure that the user is also a recurrence. UserRecurPhi will record the candidate user recurrence phi, and UserRecurKind will recored the excepted user recurrence kind. Currently I'm limiting candidates to one, but it should be possible to have more than one.

llvm/lib/Analysis/IVDescriptors.cpp

758

Yes, it's a little confusing here. It could be better to replace NonPhi with NonRecurPhi. By the way, are you interested in supporting full functional SelectCmp pattern? I think min max with index pattern really needs to depend on the SelectCmp to be safe.

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

777

I see., but DependRecurrenceMasks exists for a reason. Consider the following case:

int idx = ii;
int foo = jj;
int max = mm;
for (int i = 0; i < n; ++i) {
  int x = a[i];
  if (max < x) {
    max = x;
    idx = i;
    foo = b[i];
  }
}

That mask has the chance to be reused, and I try to keep that flexibility. Of course, we can recalculate the mask for each reduction that needs a mask, but currently using the global maps to preserve the mask is a relatively simple method that I think of.
I have heard that VPlan is going to be extended to other blocks, could you share the relevant discussion links?

3805

Sure. Quick explanation: min/max recurrence should be done earlier than min max idx recurrence, because idx recurrence depends on the mask produced by min max recurrence. Here is to ensure that the recurrence dependencies are correct.

llvm/test/Transforms/LoopVectorize/smax-idx.ll

1

Of course. I will split an NFC patch tomorrow.

[Mel-Chen]

Split test case into parent revision.

[Mel-Chen]

llvm/include/llvm/Analysis/IVDescriptors.h

379

Too busy today. The document will be available in next week.