Refactor FixedWidthInteger init (swiftlang#36485)

Author: meg-gupta

stdlib/public/core/Integers.swift

@@ -3160,20 +3160,25 @@ extension FixedWidthInteger {
       self = Self(_truncatingBits: source._lowWord)
     }
     else {
-      let neg = source < (0 as T)
-      var result: Self = neg ? ~0 : 0
-      var shift: Self = 0
-      let width = Self(_truncatingBits: Self.bitWidth._lowWord)
-      for word in source.words {
-        guard shift < width else { break }
-        // Masking shift is OK here because we have already ensured
-        // that shift < Self.bitWidth. Not masking results in
-        // infinite recursion.
-        result ^= Self(_truncatingBits: neg ? ~word : word) &<< shift
-        shift += Self(_truncatingBits: Int.bitWidth._lowWord)
-      }
-      self = result
+      self = Self._truncatingInit(source)
+    }
+  }
+
+  @_alwaysEmitIntoClient
+  internal static func _truncatingInit<T: BinaryInteger>(_ source: T) -> Self {
+    let neg = source < (0 as T)
+    var result: Self = neg ? ~0 : 0
+    var shift: Self = 0
+    let width = Self(_truncatingBits: Self.bitWidth._lowWord)
+    for word in source.words {
+      guard shift < width else { break }
+      // Masking shift is OK here because we have already ensured
+      // that shift < Self.bitWidth. Not masking results in
+      // infinite recursion.
+      result ^= Self(_truncatingBits: neg ? ~word : word) &<< shift
+      shift += Self(_truncatingBits: Int.bitWidth._lowWord)
     }
+    return result
   }
 
   @_transparent

test/SILOptimizer/jumpthreadtest.swift

@@ -0,0 +1,17 @@
+// RUN: %target-swift-frontend -O -parse-as-library -emit-sil -enable-ossa-modules %s | %FileCheck %s
+// REQUIRES: PTRSIZE=32,swift_stdlib_asserts
+
+import Swift
+
+// CHECK-LABEL: sil [noinline] @$s14jumpthreadtest3fooys6UInt64Vs5UInt8VF :
+// CHECK: bb0
+// CHECK:  [[FUNC:%.*]] = function_ref @$ss17FixedWidthIntegerPsE15_truncatingInityxqd__SzRd__lFZs6UInt64V_s5UInt8VTgq5Tf4nd_n : 
+// CHECK: apply [[FUNC]]
+// CHECK-NOT: bb1
+// CHECK-LABEL: } // end sil function '$s14jumpthreadtest3fooys6UInt64Vs5UInt8VF'
+@inlinable
+@inline(never)
+public func foo(_ p:UInt8) -> UInt64 {
+  let q = UInt64(truncatingIfNeeded: p)
+  return q
+}