ComputeSideEffects: fix a corner case with consuming indirect arguments in dead-end functions

Usually there _must_ be a read from a consuming in-argument, because the function has to consume the argument.
But in the special case if all control paths end up in an `unreachable`, the consuming read might have been dead-code eliminated.
Therefore make sure to add the read-effect in any case. Otherwise it can result in memory lifetime failures at a call site.

fixes a memory lifetime failure

rdar://134881045

Author: eeckstein

SwiftCompilerSources/Sources/Optimizer/FunctionPasses/ComputeSideEffects.swift

@@ -57,6 +57,7 @@ let computeSideEffects = FunctionPass(name: "compute-side-effects") {
   // instruction the argument might have escaped.
   for argument in function.arguments {
     collectedEffects.addEffectsForEscapingArgument(argument: argument)
+    collectedEffects.addEffectsForConsumingArgument(argument: argument)
   }
 
   // Don't modify the effects if they didn't change. This avoids sending a change notification
@@ -236,7 +237,17 @@ private struct CollectedEffects {
       addEffects(.destroy, to: argument)
     }
   }
-  
+
+  mutating func addEffectsForConsumingArgument(argument: FunctionArgument) {
+    if argument.convention == .indirectIn {
+      // Usually there _must_ be a read from a consuming in-argument, because the function has to consume the argument.
+      // But in the special case if all control paths end up in an `unreachable`, the consuming read might have been
+      // dead-code eliminated. Therefore make sure to add the read-effect in any case. Otherwise it can result
+      // in memory lifetime failures at a call site.
+      addEffects(.read, to: argument)
+    }
+  }
+
   private mutating func handleApply(_ apply: ApplySite) {
     let callees = calleeAnalysis.getCallees(callee: apply.callee)
     let args = apply.argumentOperands.lazy.map {

test/SILOptimizer/generic_specialization.swift

@@ -0,0 +1,19 @@
+// RUN: %target-swift-frontend -O -Xllvm -sil-disable-pass=function-signature-opts -emit-sil -primary-file %s | %FileCheck %s
+
+public struct S {
+  @inline(never)
+  init<T>(_ t: T) {
+    fatalError()
+  }
+}
+
+// check that this doesn't cause a memory lifetime failure
+
+// CHECK-LABEL: sil @$s22generic_specialization6testityAA1SVSSF :
+// CHECK:          [[F:%.*]] = function_ref @$s22generic_specialization1SVyACxclufCSS_Tt0g5 :
+// CHECK:            = apply [[F]](%0) : $@convention(thin) (@owned String) -> S
+// CHECK:       } // end sil function '$s22generic_specialization6testityAA1SVSSF'
+public func testit(_ s: String) -> S {
+  return S(s)
+}
+

test/SILOptimizer/side_effects.sil

@@ -1234,3 +1234,12 @@ bb0(%0 : $SP):
   %r = tuple ()
   return %r : $()
 }
+
+// CHECK-LABEL: sil @test_consuming_in_with_unreachable
+// CHECK-NEXT:  [%0: read v**]
+// CHECK-NEXT:  [global: ]
+// CHECK-NEXT:  {{^[^[]}}
+sil @test_consuming_in_with_unreachable : $@convention(thin) (@in X) -> () {
+bb0(%0 : $*X):
+  unreachable
+}