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

[Slava Zakharin 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``.
----------------
vzakhari wrote:

> However, I think the reverse is also true. Suppose in your outer loop you set llvm.loop.vectorize.reassociate_fpreductions.enable to 0, that should override any inner loop that sets it to 1 for consistency.

Hmm, that does not sound right to me.  If the inner loop computes a different reduction than the outer loop, then the metadata should probably not apply to the inner loop, e.g.:
```
double s1 = 0.0;
for (...) {
  double s2 = 0.0;
  for (...) {
    s2 += ...;
  }
  s1 += ...;
}
```

Do you think it will be more consistent to propagate the metadata's "enable" effect to the whole loop-nest regardless of which loop it is set on?

P.S. I am on vacation for 1.5 weeks, and I won't be able to reply to the comments during my absence. Sorry for the inconvenience.

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