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

[David Sherwood via llvm-commits]

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@@ -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:

Well, I guess that depends upon how you want this metadata to behave and what you want to achieve. My point really was that it should be consistent in my opinion - it would seem odd to permit `llvm.loop.vectorize.reassociate_fpreductions.enable=1` to override inner loops, but not permit `llvm.loop.vectorize.reassociate_fpreductions.enable=0` given something has gone to the effort of explicitly adding it. Of course if the metadata is completely missing from the outer loop (surely the common case?), then it cannot override any metadata on inner loops anyway. I think whatever behaviour we decide upon should be documented explicitly in the LangRef to avoid confusion.

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