Ensure we can fold apply of a differentiable_function_inst.

lib/SILOptimizer/SILCombiner/SILCombinerApplyVisitors.cpp

@@ -1500,6 +1500,13 @@ SILInstruction *SILCombiner::visitApplyInst(ApplyInst *AI) {
     }
   }
 
+  // (apply (differentiable_function f)) to (apply f)
+  if (auto *DFI = dyn_cast<DifferentiableFunctionInst>(AI->getCallee())) {
+    return cloneFullApplySiteReplacingCallee(AI, DFI->getOperand(0),
+                                             Builder.getBuilderContext())
+      .getInstruction();
+  }
+
   // (apply (thin_to_thick_function f)) to (apply f)
   if (auto *TTTFI = dyn_cast<ThinToThickFunctionInst>(AI->getCallee())) {
     // We currently don't remove any possible retain associated with the thick

lib/SILOptimizer/SILCombiner/SILCombinerCastVisitors.cpp

@@ -1110,39 +1110,42 @@ SILInstruction *SILCombiner::visitConvertEscapeToNoEscapeInst(
   // %vjp' = convert_escape_to_noescape %vjp
   // %y = differentiable_function(%orig', %jvp', %vjp')
   if (auto *DFI = dyn_cast<DifferentiableFunctionInst>(Cvt->getOperand())) {
-    auto createConvertEscapeToNoEscape = [&](NormalDifferentiableFunctionTypeComponent extractee) {
-      if (!DFI->hasExtractee(extractee))
-        return SILValue();
+    if (DFI->hasOneUse()) {
+      auto createConvertEscapeToNoEscape =
+        [&](NormalDifferentiableFunctionTypeComponent extractee) {
+          if (!DFI->hasExtractee(extractee))
+            return SILValue();
 
-      auto operand = DFI->getExtractee(extractee);
-      auto fnType = operand->getType().castTo<SILFunctionType>();
-      auto noEscapeFnType =
-        fnType->getWithExtInfo(fnType->getExtInfo().withNoEscape());
-      auto noEscapeType = SILType::getPrimitiveObjectType(noEscapeFnType);
-      return Builder.createConvertEscapeToNoEscape(
-        operand.getLoc(), operand, noEscapeType, Cvt->isLifetimeGuaranteed())->getResult(0);
-    };
+          auto operand = DFI->getExtractee(extractee);
+          auto fnType = operand->getType().castTo<SILFunctionType>();
+          auto noEscapeFnType =
+            fnType->getWithExtInfo(fnType->getExtInfo().withNoEscape());
+          auto noEscapeType = SILType::getPrimitiveObjectType(noEscapeFnType);
+          return Builder.createConvertEscapeToNoEscape(
+            operand.getLoc(), operand, noEscapeType, Cvt->isLifetimeGuaranteed())->getResult(0);
+        };
 
-    SILValue originalNoEscape =
-      createConvertEscapeToNoEscape(NormalDifferentiableFunctionTypeComponent::Original);
-    SILValue convertedJVP = createConvertEscapeToNoEscape(
-      NormalDifferentiableFunctionTypeComponent::JVP);
-    SILValue convertedVJP = createConvertEscapeToNoEscape(
-      NormalDifferentiableFunctionTypeComponent::VJP);
-
-    llvm::Optional<std::pair<SILValue, SILValue>> derivativeFunctions;
-    if (convertedJVP && convertedVJP)
-      derivativeFunctions = std::make_pair(convertedJVP, convertedVJP);
-
-    auto *newDFI = Builder.createDifferentiableFunction(
-      DFI->getLoc(), DFI->getParameterIndices(), DFI->getResultIndices(),
-      originalNoEscape, derivativeFunctions);
-    assert(newDFI->getType() == Cvt->getType() &&
-           "New `@differentiable` function instruction should have same type "
-           "as the old `convert_escape_to_no_escape` instruction");
-    return newDFI;
-  }
+      SILValue originalNoEscape =
+        createConvertEscapeToNoEscape(NormalDifferentiableFunctionTypeComponent::Original);
+      SILValue convertedJVP = createConvertEscapeToNoEscape(
+        NormalDifferentiableFunctionTypeComponent::JVP);
+      SILValue convertedVJP = createConvertEscapeToNoEscape(
+        NormalDifferentiableFunctionTypeComponent::VJP);
+
+      llvm::Optional<std::pair<SILValue, SILValue>> derivativeFunctions;
+      if (convertedJVP && convertedVJP)
+        derivativeFunctions = std::make_pair(convertedJVP, convertedVJP);
 
+      auto *newDFI = Builder.createDifferentiableFunction(
+        DFI->getLoc(), DFI->getParameterIndices(), DFI->getResultIndices(),
+        originalNoEscape, derivativeFunctions);
+      assert(newDFI->getType() == Cvt->getType() &&
+             "New `@differentiable` function instruction should have same type "
+             "as the old `convert_escape_to_no_escape` instruction");
+      return newDFI;
+    }
+  }
+
   return nullptr;
 }
 

test/AutoDiff/SILOptimizer/differential_apply.swift

@@ -0,0 +1,48 @@
+// RUN: %target-swift-frontend -emit-sil -O %s | %FileCheck %s
+// REQUIRES: swift_in_compiler
+
+import _Differentiation
+
+@differentiable(reverse)
+@_silgen_name("test_f")
+// Check that (differentiable) closure apply is optimized out 
+// CHECK-LABEL: test_f : $@convention(thin) (@guaranteed Array<Double>) -> Double
+// CHECK-NOT: differentiable_function [parameters 0] [results 0]
+func f(array: [Double]) -> Double {
+  var array = array
+  array.update(at: 1,
+    byCalling: {
+      (element: inout Double) in
+      let initialElement = element;
+      element *= initialElement
+    }
+  )
+
+  return 0.0
+}
+
+public func valueWithPullback<T>(at x: T, of f: @differentiable(reverse) (inout T) -> Void) -> (value: Void, pullback: (inout T.TangentVector) -> Void) {fatalError()}
+public func pullback<T>(at x: T, of f: @differentiable(reverse) (inout T) -> Void) -> (inout T.TangentVector) -> Void {return valueWithPullback(at: x, of: f).pullback}
+
+public extension Array {
+  @differentiable(reverse)
+  mutating func update(at index: Int,
+    byCalling closure: @differentiable(reverse) (inout Element) -> Void) where Element: Differentiable {
+    closure(&self[index])
+  }
+}
+
+public extension Array where Element: Differentiable {
+    @derivative(of: update(at:byCalling:))
+    mutating func vjpUpdate(at index: Int, byCalling closure: @differentiable(reverse) (inout Element) -> Void) -> (value: Void, pullback: (inout Self.TangentVector) -> Void) {
+        let closurePullback = pullback(at: self[index], of: closure)
+        return (value: (), pullback: { closurePullback(&$0.base[index]) })
+    }
+}
+
+public struct D<I: Equatable, D> {
+  public subscript(_ index: I) -> D? {
+    get {fatalError()}
+    set {fatalError()}
+  }
+}