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

[Julius Alexandre via llvm-commits]

================
@@ -15989,6 +15989,155 @@ catch:
     ret void
 ```
 
+### Constant-Time Intrinsics
+
+These intrinsics are provided to support constant-time operations for
+security-sensitive code. Constant-time operations are designed to execute in
+time independent of secret data values, preventing timing side-channel leaks.
+
+(int_ct_select)=
+
+#### '`llvm.ct.select.*`' Intrinsic
+
+##### Syntax:
+
+This is an overloaded intrinsic. You can use `llvm.ct.select` on any integer,
+byte, floating-point, or pointer type, or on any vector of those types,
+including scalable vectors. The declarations below are a representative sample:
+
+```
+declare i8 @llvm.ct.select.i8(i1 <cond>, i8 <val1>, i8 <val2>)
+declare b8 @llvm.ct.select.b8(i1 <cond>, b8 <val1>, b8 <val2>)
+declare i32 @llvm.ct.select.i32(i1 <cond>, i32 <val1>, i32 <val2>)
+declare i64 @llvm.ct.select.i64(i1 <cond>, i64 <val1>, i64 <val2>)
+declare half @llvm.ct.select.f16(i1 <cond>, half <val1>, half <val2>)
+declare float @llvm.ct.select.f32(i1 <cond>, float <val1>, float <val2>)
+declare double @llvm.ct.select.f64(i1 <cond>, double <val1>, double <val2>)
+declare fp128 @llvm.ct.select.f128(i1 <cond>, fp128 <val1>, fp128 <val2>)
+declare ptr @llvm.ct.select.p0(i1 <cond>, ptr <val1>, ptr <val2>)
+declare <4 x i32> @llvm.ct.select.v4i32(i1 <cond>, <4 x i32> <val1>, <4 x i32> <val2>)
+declare <2 x double> @llvm.ct.select.v2f64(i1 <cond>, <2 x double> <val1>, <2 x double> <val2>)
+declare <2 x ptr> @llvm.ct.select.v2p0(i1 <cond>, <2 x ptr> <val1>, <2 x ptr> <val2>)
+declare <vscale x 4 x i32> @llvm.ct.select.nxv4i32(i1 <cond>, <vscale x 4 x i32> <val1>, <vscale x 4 x i32> <val2>)
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wizardengineer wrote:

Trimmed to four declarations.

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