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

[Julius Alexandre 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.
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wizardengineer wrote:

Rewrote that comment in 64613ed31ef6. It now explains what the node does and the rules it has to follow, timing independent of the condition, both arms always evaluated, combines have to preserve that, instead of just repeating the name.

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