[llvm] [LV][EVL] Attach a new metadata on EVL vectorized loops (PR #131000)

[Min-Yih Hsu via llvm-commits]

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@@ -121,6 +125,12 @@ class LoopVectorizeHints {
     SK_PreferScalable = 1
   };
 
+  enum TailFoldingKind {
+    TFK_Unspecified = -1,
+    /// Tail folding with explicit vector length intrinsics.
+    TFK_EVL = 0
+  };
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mshockwave wrote:

> Does that mean it would also be possible to set the tail-folding style with the hints?
> Might not be a good idea to open a two-way street.

Currently LV doesn't use this metadata to make any decision w.r.t. tail-folding style, but I agree it does make such impression. 
While I can move the logics to LoopVectorize.cpp as you pointed out, I'm afraid the raw metadata form -- `llvm.loop.isvectorized.tailfoldingstyle = 0` -- is not really self-described (and that's one of the reasons why I wrapped it with `LoopVectorizeHints::isEVLVectorized`). IMHO, there are two potential ways to move forward:
  1. Use a string value to indicate tail-folding style in the metadata: `llvm.loop.isvectorized.tailfoldingstyle = "evl"`. Which was what Luke originally suggested
  2. Use boolean-value metadata so that it's more intuitive when accessing it. Which was my original approach.

Of course, I'm open for other suggestions. What do you think @lukel97 @fhahn ?

https://github.com/llvm/llvm-project/pull/131000