[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``.
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david-arm wrote:

That seems reasonable and thanks for explaining. I think it's worth explicitly stating this in the LangRef because once the metadata exists in LLVM it could be used by other frontends. For example, I can imagine in future someone may add a C level pragma that maps to this.

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