[llvm] [LLVM] New NoDivergenceSource function attribute (PR #111832)

[Tim Renouf via llvm-commits]

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@@ -2082,6 +2082,16 @@ example:
     function call, use of ``longjmp``, or other means. It is a compiler hint that
     is used at module level to improve dataflow analysis, dropped during linking,
     and has no effect on functions defined in the current module.
+``nodivergencesource``
+    A call to this function is not a source of divergence. In uniformity
+    analysis, a *source of divergence* is an instruction that generates
+    divergence even if its inputs are uniform. A call with no further information
+    would normally be considered a source of divergence; setting this attribute
+    on a function means that a call to it is not a source of divergence.
+
+    This is useful where known-name calls are being used as an extension
+    mechanism for instructions, as for example in `llvm-dialects
+    <https://github.com/GPUOpen-Drivers/llvm-dialects>`.
----------------
trenouf wrote:

OK I'll drop it. But here is some further explanation:

LLVM IR does not let you add your own new instructions, unlike MLIR. llvm-dialects is a scheme for adding your own new instructions, somewhat like MLIR dialects, using similar tablegen declarations. A dialect op is implemented as a call to a function declaration with a known name, e.g. `@lgc.load.descriptor` for the `LoadDescriptorOp` in dialect lgc. We want to be able to configure such known-name declarations to not be a source of divergence (same as you can use attributes to configure how it interacts with memory, etc).

Another known-name-calls extension mechanism for instructions is the `@dx.op.*` calls in DXIL. That is just an example; it is not our goal to use this attribute with DXIL ops.

https://github.com/llvm/llvm-project/pull/111832