[AutoDiff] Adjoint buffer optimization for address projections.

Do not allocate adjoint buffers for address projections; they become
projections into their adjoint base buffer.

Author: dan-zheng

lib/SILOptimizer/Mandatory/Differentiation.cpp

@@ -4141,6 +4141,11 @@ class AdjointEmitter final : public SILInstructionVisitor<AdjointEmitter> {
       auto addActiveValue = [&](SILValue v) {
         if (visited.count(v))
           return;
+        // Skip address projections.
+        // Address projections do not need their own adjoint buffers; they
+        // become projections into their adjoint base buffer.
+        if (Projection::isAddressProjection(v))
+          return;
         visited.insert(v);
         bbActiveValues.push_back(v);
       };

test/AutoDiff/control_flow.swift

@@ -68,6 +68,22 @@ ControlFlowTests.test("Conditionals") {
   expectEqual((-20, 2), valueWithGradient(at: -10, in: cond_tuple))
   expectEqual((-2674, 2), valueWithGradient(at: -1337, in: cond_tuple))
 
+  func cond_tuple2(_ x: Float) -> Float {
+    // Convoluted function returning `x + x`.
+    let y: (Float, Float) = (x, x)
+    let y0 = y.0
+    if x > 0 {
+      let y1 = y.1
+      return y0 + y1
+    }
+    let y0_double = y0 + y.0
+    let y1 = y.1
+    return y0_double - y1 + y.0
+  }
+  expectEqual((8, 2), valueWithGradient(at: 4, in: cond_tuple2))
+  expectEqual((-20, 2), valueWithGradient(at: -10, in: cond_tuple2))
+  expectEqual((-2674, 2), valueWithGradient(at: -1337, in: cond_tuple2))
+
   func cond_tuple_var(_ x: Float) -> Float {
     // Convoluted function returning `x + x`.
     var y: (Float, Float) = (x, x)
@@ -135,6 +151,22 @@ ControlFlowTests.test("Conditionals") {
   expectEqual((-20, 2), valueWithGradient(at: -10, in: cond_struct))
   expectEqual((-2674, 2), valueWithGradient(at: -1337, in: cond_struct))
 
+  func cond_struct2(_ x: Float) -> Float {
+    // Convoluted function returning `x + x`.
+    let y = FloatPair(x, x)
+    let y0 = y.first
+    if x > 0 {
+      let y1 = y.second
+      return y0 + y1
+    }
+    let y0_double = y0 + y.first
+    let y1 = y.second
+    return y0_double - y1 + y.first
+  }
+  expectEqual((8, 2), valueWithGradient(at: 4, in: cond_struct2))
+  expectEqual((-20, 2), valueWithGradient(at: -10, in: cond_struct2))
+  expectEqual((-2674, 2), valueWithGradient(at: -1337, in: cond_struct2))
+
   func cond_struct_var(_ x: Float) -> Float {
     // Convoluted function returning `x + x`.
     var y = FloatPair(x, x)