Merge pull request #12387 from ktopley-apple/dispatch-timebase-overflow

Fixes overflow trap when creating DispatchTime objects with large upt…

Author: ktopley-apple

stdlib/public/SDK/Dispatch/Time.swift

@@ -53,17 +53,26 @@ public struct DispatchTime : Comparable {
 	///               system sleep time), not zero nanoseconds since boot.
 	public init(uptimeNanoseconds: UInt64) {
 		var rawValue = uptimeNanoseconds
-		if (DispatchTime.timebaseInfo.numer != DispatchTime.timebaseInfo.denom) {
-			rawValue = (rawValue * UInt64(DispatchTime.timebaseInfo.denom) 
-				+ UInt64(DispatchTime.timebaseInfo.numer - 1)) / UInt64(DispatchTime.timebaseInfo.numer)
+
+		// UInt64.max means distantFuture. Do not try to scale it.
+		if rawValue != UInt64.max && DispatchTime.timebaseInfo.numer != DispatchTime.timebaseInfo.denom {
+			var (result, overflow) = rawValue.multipliedReportingOverflow(by: UInt64(DispatchTime.timebaseInfo.denom))
+			if !overflow {
+				(result, overflow) = result.addingReportingOverflow(UInt64(DispatchTime.timebaseInfo.numer - 1))
+			}
+			rawValue = overflow ? UInt64.max : result / UInt64(DispatchTime.timebaseInfo.numer)
 		}
 		self.rawValue = dispatch_time_t(rawValue)
 	}
 
 	public var uptimeNanoseconds: UInt64 {
 		var result = self.rawValue
-		if (DispatchTime.timebaseInfo.numer != DispatchTime.timebaseInfo.denom) {
-			result = result * UInt64(DispatchTime.timebaseInfo.numer) / UInt64(DispatchTime.timebaseInfo.denom)
+		var overflow: Bool
+
+		// UInt64.max means distantFuture. Do not try to scale it.
+		if rawValue != UInt64.max && DispatchTime.timebaseInfo.numer != DispatchTime.timebaseInfo.denom {
+			(result, overflow) = result.multipliedReportingOverflow(by: UInt64(DispatchTime.timebaseInfo.numer))
+			result = overflow ? UInt64.max : result / UInt64(DispatchTime.timebaseInfo.denom)
 		}
 		return result
 	}

test/stdlib/Dispatch.swift

@@ -9,10 +9,6 @@
 
 // REQUIRES: objc_interop
 
-// FIXME: rdar://34751238 DispatchTime.addSubtract test traps
-// XFAIL: CPU=armv7 || CPU=armv7k || CPU=armv7s || CPU=arm64
-
-
 import Dispatch
 import Foundation
 import StdlibUnittest
@@ -119,6 +115,31 @@ DispatchAPI.test("DispatchTime comparisons") {
     }
 }
 
+DispatchAPI.test("DispatchTime.create") {
+	var info = mach_timebase_info_data_t(numer: 1, denom: 1)
+	mach_timebase_info(&info)
+	let scales = info.numer != info.denom
+
+	// Simple tests for non-overflow behavior
+	var time = DispatchTime(uptimeNanoseconds: 0)
+	expectEqual(time.uptimeNanoseconds, 0)
+
+	time = DispatchTime(uptimeNanoseconds: 15 * NSEC_PER_SEC)
+	expectEqual(time.uptimeNanoseconds, 15 * NSEC_PER_SEC)
+
+	// On platforms where the timebase scale is not 1, the next two cases
+	// overflow and become DISPATCH_TIME_FOREVER (UInt64.max) instead of trapping.
+	time = DispatchTime(uptimeNanoseconds: UInt64.max - 1)
+	expectEqual(time.uptimeNanoseconds, scales ? UInt64.max : UInt64.max - UInt64(1))
+
+	time = DispatchTime(uptimeNanoseconds: UInt64.max / 2) 
+	expectEqual(time.uptimeNanoseconds, scales ? UInt64.max : UInt64.max / 2)
+
+	// UInt64.max must always be returned as UInt64.max.
+	time = DispatchTime(uptimeNanoseconds: UInt64.max)
+	expectEqual(time.uptimeNanoseconds, UInt64.max)
+}
+
 DispatchAPI.test("DispatchTime.addSubtract") {
 	var then = DispatchTime.now() + Double.infinity
 	expectEqual(DispatchTime.distantFuture, then)