[llvm] [LoopVectorizer] Add a -force-vscale-for-tuning override option. (PR #156916)

[David Sherwood via llvm-commits]

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@@ -1473,6 +1477,11 @@ class LoopVectorizationCostModel {
   /// vscale_range.min == vscale_range.max then return vscale_range.max, else
   /// return the value returned by the corresponding TTI method.
   void initializeVScaleForTuning() {
+    if (VScaleForTuningOpt.getNumOccurrences()) {
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david-arm wrote:

Thanks for this! I like the idea of having a flag to force the choice of vscale, but should this be sanitised according to vscale_range on the function or at least emit a warning that it's architecturally unsupported? For example, if the function has the `vscale_range(1, 16)` attribute then only power-of-2 vscale values are permitted leading to choices of 1,2,4,8 or 16.

https://github.com/llvm/llvm-project/pull/156916