[llvm] [RFC][llvm] Added llvm.loop.vectorize.reassociate_fpreductions.enable metadata. (PR #141685)

[David Sherwood via llvm-commits]

================
@@ -7593,6 +7593,36 @@ Note that setting ``llvm.loop.interleave.count`` to 1 disables interleaving
 multiple iterations of the loop. If ``llvm.loop.interleave.count`` is set to 0
 then the interleave count will be determined automatically.
 
+'``llvm.loop.vectorize.reassociate_fpreductions.enable``' Metadata
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+This metadata selectively allows or disallows reassociating floating-point
+reductions, which otherwise may be unsafe to reassociate, during loop
+vectorization. For example, a floating point ``ADD`` reduction without
+``reassoc`` fast-math flags may be vectorized provided that this metadata
+allows it. The first operand is the string
+``llvm.loop.vectorize.reassociate_fpreductions.enable``
+and the second operand is a bit. If the bit operand value is 1 unsafe
+reduction reassociations are enabled. A value of 0 disables unsafe
+reduction reassociations.
+
+Note that the reassociation of floating point reductions that is allowed
+by other means is considered safe, so this metadata is a no-op
+in such cases.
+
+For example, reassociation of floating point reduction
+in a loop with ``!{!"llvm.loop.vectorize.enable", i1 1}`` metadata is allowed
+regardless of the value of
+``llvm.loop.vectorize.reassociate_fpreductions.enable``.
----------------
david-arm wrote:

Just a quick thought - what would you expect to happen for nested loops where the reduction variable is used at all levels of the loop? For example,

```
  float v = 0;
  for (...) {
    for (...) {
      for (...) {
        v += ...;
      }
      v += ...;
    }
    v += ...;
  }
```

Suppose the metadata is only added to the outer loop, but not the inner loops. It's possible that the inner loops get fully unrolled such that only the outer loop remains by the time we run the loop vectoriser. Is it valid to still reassociate? If so, that implies all inner loops must inherit the property from the outer loop. Would you consider it a bug to add it to the outer loop, but not the inner loops? Alternatively, if the inner loops do not get unrolled, would it be legal for the vectoriser to walk up to the outermost loop and use the metadata on the outermost loop to reassociate reductions on the innermost, etc?

https://github.com/llvm/llvm-project/pull/141685