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

[Florian Hahn via llvm-commits]

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@@ -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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fhahn wrote:

Yep, this is intentional in the initial patch (from the description: `Note that isOutsideLoopWorkProfitable already scales the cost of blocks outside the vector region, but the patch restricts accounting for the middle block to cases where VF <= ExpectedTC, to initially catch some worst cases and avoid regressions.`)

The TODO is to enable it unconditionally, but that potentially has much higher impact, which is why I'd prefer to do this as a separate commit, to make it slightly easier to narrow down regressions.

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