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

[Yingwei Zheng via llvm-commits]

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@@ -7579,6 +7579,23 @@ static Value *simplifyIntrinsic(CallBase *Call, ArrayRef<Value *> Args,
     }
     return nullptr;
   }
+  case Intrinsic::ct_select: {
+    // Only fold on a literal IR-constant condition or identical arms; these
+    // are the only two folds LangRef permits for ct.select. Folding through
+    // ValueTracking-derived known bits would defeat the constant-time
+    // contract on conditions the user wants kept opaque.
+    Value *Cond = Args[0], *TrueVal = Args[1], *FalseVal = Args[2];
+    if (auto *CI = dyn_cast<ConstantInt>(Cond)) {
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dtcxzyw wrote:

What is the benefit of this simplification, as the intrinsic cannot be DCEd?



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