[Stdlib performance] Make integer conversion operations transparent

The integer conversion operations were inlinable, but aren't getting
inlined in debug builds, which results in unreasonably poor
performance. Mark them as transparent so we don't end up with
unspecialized generic code in the hot path.

Fixes #78501

Author: DougGregor

stdlib/public/core/Integers.swift

@@ -3040,8 +3040,7 @@ extension UnsignedInteger where Self: FixedWidthInteger {
   /// - Parameter source: A value to convert to this type of integer. The value
   ///   passed as `source` must be representable in this type.
   @_semantics("optimize.sil.specialize.generic.partial.never")
-  @inlinable // FIXME(inline-always)
-  @inline(__always)
+  @_transparent
   public init<T: BinaryInteger>(_ source: T) {
     // This check is potentially removable by the optimizer
     if T.isSigned {
@@ -3056,8 +3055,7 @@ extension UnsignedInteger where Self: FixedWidthInteger {
   }
 
   @_semantics("optimize.sil.specialize.generic.partial.never")
-  @inlinable // FIXME(inline-always)
-  @inline(__always)
+  @_transparent
   public init?<T: BinaryInteger>(exactly source: T) {
     // This check is potentially removable by the optimizer
     if T.isSigned && source < (0 as T) {
@@ -3255,8 +3253,7 @@ extension SignedInteger where Self: FixedWidthInteger {
   /// - Parameter source: A value to convert to this type of integer. The value
   ///   passed as `source` must be representable in this type.
   @_semantics("optimize.sil.specialize.generic.partial.never")
-  @inlinable // FIXME(inline-always)
-  @inline(__always)
+  @_transparent
   public init<T: BinaryInteger>(_ source: T) {
     // This check is potentially removable by the optimizer
     if T.isSigned && source.bitWidth > Self.bitWidth {
@@ -3273,8 +3270,7 @@ extension SignedInteger where Self: FixedWidthInteger {
   }
 
   @_semantics("optimize.sil.specialize.generic.partial.never")
-  @inlinable // FIXME(inline-always)
-  @inline(__always)
+  @_transparent
   public init?<T: BinaryInteger>(exactly source: T) {
     // This check is potentially removable by the optimizer
     if T.isSigned && source.bitWidth > Self.bitWidth && source < Self.min {

test/IRGen/integer_conversion.swift

@@ -0,0 +1,26 @@
+// RUN: %target-swift-frontend -primary-file %s -emit-ir | %FileCheck %s
+// RUN: %target-swift-frontend -primary-file %s -O -emit-ir | %FileCheck %s
+// REQUIRES: CPU=x86_64 || CPU=arm64
+
+// https://github.com/swiftlang/swift/issues/78501
+public struct PcgRandom {
+  private var state: UInt64 = 0;
+
+  // CHECK-LABEL: define{{.*}}swiftcc i32 @"$s18integer_conversion9PcgRandomV6next32s6UInt32VyF"
+  public mutating func next32() -> UInt32 {
+    // CHECK-NOT: sSUss17FixedWidthIntegerRzrlEyxqd__cSzRd__lufC
+    // CHECK-NOT: sSZss17FixedWidthIntegerRzrlEyxqd__cSzRd__lufC
+    // CHECK: ret i32
+    let oldstate : UInt64 = state
+    state = oldstate &* 6364136223846793005 &+ 1;
+    let shifted = oldstate >> 18
+    let xor = shifted ^ oldstate
+    let xorshifted64 = xor >> 27
+    let xorshifted = UInt32((xorshifted64 << 32) >> 32)
+    let rot : UInt32 = UInt32(oldstate >> 59)
+    let nrot : UInt32 = UInt32(bitPattern: -Int32(rot))
+    return (xorshifted >> rot) | (xorshifted << (nrot & 31))
+  }
+
+  init() {}
+}