[llvm] [LoopVectorize] Don't scalarize predicated instruction with optsize (PR #129265)

[David Sherwood via llvm-commits]

================
@@ -5660,6 +5662,13 @@ LoopVectorizationCostModel::getMemInstScalarizationCost(Instruction *I,
   if (VF.isScalable())
     return InstructionCost::getInvalid();
 
+  // Don't scalarize predicated instructions when optimizing for size unless
+  // we're forced to.
+  if (isPredicatedInst(I) && OptForSize &&
+      !ForceTailFoldingStyle.getNumOccurrences() &&
----------------
david-arm wrote:

I think this needs to be

```
  !(ForceTailFoldingStyle.getNumOccurrences() && ForceTailFoldingStyle != TailFoldingStyle::None)
```

because otherwise the user could explicitly disable tail-folding for a loop that has control flow (leading to predicated instructions), and we will still scalarise. Or since the default is none anyway, you could also just write

```
  ForceTailFoldingStyle != TailFoldingStyle::None &&
```

https://github.com/llvm/llvm-project/pull/129265