[llvm] [LV] Avoid low-trip-count vectorization for countable early exits (PR #225635)

[David Sherwood via llvm-commits]

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@@ -3090,7 +3090,8 @@ LoopVectorizationCostModel::computeMaxVF(ElementCount UserVF, unsigned UserIC) {
     unsigned MaxVFForTC = llvm::bit_floor(TC.getFixedValue());
     if (TC.getFixedValue() - MaxVFForTC <= 1 && MaxVFForTC / EffectiveIC > 1 &&
         MaxVFForTC <= (MaxFactors.FixedVF.getFixedValue() * EffectiveIC) &&
-        !Config.OptForSize) {
+        !Config.OptForSize &&
+        TheLoop->getExitingBlock() == TheLoop->getLoopLatch()) {
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david-arm wrote:

As mentioned separately, this needs to be a stricter test to handle loops where there is a single exiting block (the latch) with combined exit conditions, that is an early exit. It's better to query the LoopVectorizationLegality function `hasUncountableEarlyExit` for an accurate answer.

https://github.com/llvm/llvm-project/pull/225635