[llvm] [LoopVectorize][NFC] Cache the result of getVScaleForTuning (PR #124732)

[David Sherwood via llvm-commits]

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@@ -3838,6 +3861,11 @@ FixedScalableVFPair LoopVectorizationCostModel::computeFeasibleMaxVF(
   if (!Legal->isSafeForAnyVectorWidth())
     this->MaxSafeElements = MaxSafeElements;
 
+  if (MaxSafeScalableVF != ElementCount::getScalable(0)) {
+    // Cache the value of vscale for tuning, since we'll need it.
+    initializeVScaleForTuning();
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david-arm wrote:

I'm not sure I follow. Are you worried about us initialising VScaleForTuning twice? I initially avoided doing this in the constructor because I thought then we'd be doing unnecessary work if the target doesn't support scalable vectors. However, I guess the attribute won't be present in that case so the check is cheap. I'm happy to move into a constructor in that case.

https://github.com/llvm/llvm-project/pull/124732