[llvm] [LV] Count cost of middle block if TC <= VF. (PR #168949)

[David Sherwood via llvm-commits]

================
@@ -9283,6 +9283,14 @@ static bool isOutsideLoopWorkProfitable(GeneratedRTChecks &Checks,
   // one exists.
   TotalCost += calculateEarlyExitCost(CostCtx, Plan, VF.Width);
 
+  // If the expected trip count is less than the VF, the vector loop will only
+  // execute a single iteration. Then the middle block is executed the same
+  // number of times as the vector region.
+  // TODO: Extend logic to always account for the cost of the middle block.
+  auto ExpectedTC = getSmallBestKnownTC(PSE, L);
+  if (ExpectedTC && ElementCount::isKnownLE(*ExpectedTC, VF.Width))
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david-arm wrote:

This code now seems inconsistent with the sum of `calculateEarlyExitCost` and `Checks.getCost()`, which is always adding on the cost of work in the early exits regardless of trip count. Also, the code below already seems to have proper logic to handle this more complelely, i.e.

```
  if (auto ExpectedTC = getSmallBestKnownTC(PSE, L)) {
    if (ElementCount::isKnownLT(*ExpectedTC, VF.MinProfitableTripCount)) {
```

I don't know why we shouldn't just always add on the cost to be consistent with the existing code that expects it? It feels really odd to treat the middle block differently to the runtime check block and early exit block code.

https://github.com/llvm/llvm-project/pull/168949