[llvm] [ConstantTime][LLVM] Add llvm.ct.select intrinsic with generic SelectionDAG lowering (PR #166702)

[Simon Tatham via llvm-commits]

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@@ -783,6 +783,10 @@ enum NodeType {
   /// i1 then the high bits must conform to getBooleanContents.
   SELECT,
 
+  /// Constant-time Select, implemented with CMOV instruction. This is used to
+  /// implement constant-time select.
+  CTSELECT,
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statham-arm wrote:

Also, "Constant-time select [...] This is used to implement constant-time select" isn't really adding any value by repeating the same phrase again :slightly_smiling_face:

Perhaps better to restate the order of parameters (it's obvious _to you_ that it's the same as SELECT immediately above, but perhaps not to the next reader), and also, what conditions apply to the boolean – if it's not an i1, is it still expected to be an integer containing 0 or 1, or is it a bitmask containing 0 or ~0, or what?

https://github.com/llvm/llvm-project/pull/166702