constexpr TimeDelta::operator/

The mul/div operators are tricky. operator/ works when |a| is small
enough (in absolute value) to deterministically not risk overflow.

operator*() on the other hand doesn't because of a limitation in
__builtin_mul_overflow. We could do some template hacking to make
operator*(a) == operator/(1.0/a) when |a| is a constant expression.

Bug: 761570
Change-Id: I9906edfc049017ad19872e4a586b2ec675404850
Reviewed-on: https://chromium-review.googlesource.com/886344Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Yuri Wiitala <miu@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532947}

base/time/time.h

@@ -218,7 +218,9 @@ class BASE_EXPORT TimeDelta {
   }
   constexpr TimeDelta operator-() const { return TimeDelta(-delta_); }
 
-  // Computations with numeric types.
+  // Computations with numeric types. operator*() isn't constexpr because of a
+  // limitation around __builtin_mul_overflow (but operator/(1.0/a) works for
+  // |a|'s of "reasonable" size -- i.e. that don't risk overflow).
   template <typename T>
   TimeDelta operator*(T a) const {
     CheckedNumeric<int64_t> rv(delta_);
@@ -231,7 +233,7 @@ class BASE_EXPORT TimeDelta {
     return TimeDelta(std::numeric_limits<int64_t>::max());
   }
   template <typename T>
-  TimeDelta operator/(T a) const {
+  constexpr TimeDelta operator/(T a) const {
     CheckedNumeric<int64_t> rv(delta_);
     rv /= a;
     if (rv.IsValid())
@@ -247,7 +249,7 @@ class BASE_EXPORT TimeDelta {
     return *this = (*this * a);
   }
   template <typename T>
-  TimeDelta& operator/=(T a) {
+  constexpr TimeDelta& operator/=(T a) {
     return *this = (*this / a);
   }
 

base/time/time_unittest.cc

@@ -1322,70 +1322,82 @@ TEST(TimeDelta, NumericOperators) {
   constexpr double d = 0.5;
   EXPECT_EQ(TimeDelta::FromMilliseconds(500),
             (TimeDelta::FromMilliseconds(1000) * d));
-  EXPECT_EQ(TimeDelta::FromMilliseconds(2000),
-            (TimeDelta::FromMilliseconds(1000) / d));
+  static_assert(TimeDelta::FromMilliseconds(2000) ==
+                    (TimeDelta::FromMilliseconds(1000) / d),
+                "");
   EXPECT_EQ(TimeDelta::FromMilliseconds(500),
             (TimeDelta::FromMilliseconds(1000) *= d));
-  EXPECT_EQ(TimeDelta::FromMilliseconds(2000),
-            (TimeDelta::FromMilliseconds(1000) /= d));
+  static_assert(TimeDelta::FromMilliseconds(2000) ==
+                    (TimeDelta::FromMilliseconds(1000) /= d),
+                "");
   EXPECT_EQ(TimeDelta::FromMilliseconds(500),
             (d * TimeDelta::FromMilliseconds(1000)));
 
   constexpr float f = 0.5;
   EXPECT_EQ(TimeDelta::FromMilliseconds(500),
             (TimeDelta::FromMilliseconds(1000) * f));
-  EXPECT_EQ(TimeDelta::FromMilliseconds(2000),
-            (TimeDelta::FromMilliseconds(1000) / f));
+  static_assert(TimeDelta::FromMilliseconds(2000) ==
+                    (TimeDelta::FromMilliseconds(1000) / f),
+                "");
   EXPECT_EQ(TimeDelta::FromMilliseconds(500),
             (TimeDelta::FromMilliseconds(1000) *= f));
-  EXPECT_EQ(TimeDelta::FromMilliseconds(2000),
-            (TimeDelta::FromMilliseconds(1000) /= f));
+  static_assert(TimeDelta::FromMilliseconds(2000) ==
+                    (TimeDelta::FromMilliseconds(1000) /= f),
+                "");
   EXPECT_EQ(TimeDelta::FromMilliseconds(500),
             (f * TimeDelta::FromMilliseconds(1000)));
 
   constexpr int i = 2;
   EXPECT_EQ(TimeDelta::FromMilliseconds(2000),
             (TimeDelta::FromMilliseconds(1000) * i));
-  EXPECT_EQ(TimeDelta::FromMilliseconds(500),
-            (TimeDelta::FromMilliseconds(1000) / i));
+  static_assert(TimeDelta::FromMilliseconds(500) ==
+                    (TimeDelta::FromMilliseconds(1000) / i),
+                "");
   EXPECT_EQ(TimeDelta::FromMilliseconds(2000),
             (TimeDelta::FromMilliseconds(1000) *= i));
-  EXPECT_EQ(TimeDelta::FromMilliseconds(500),
-            (TimeDelta::FromMilliseconds(1000) /= i));
+  static_assert(TimeDelta::FromMilliseconds(500) ==
+                    (TimeDelta::FromMilliseconds(1000) /= i),
+                "");
   EXPECT_EQ(TimeDelta::FromMilliseconds(2000),
             (i * TimeDelta::FromMilliseconds(1000)));
 
   constexpr int64_t i64 = 2;
   EXPECT_EQ(TimeDelta::FromMilliseconds(2000),
             (TimeDelta::FromMilliseconds(1000) * i64));
-  EXPECT_EQ(TimeDelta::FromMilliseconds(500),
-            (TimeDelta::FromMilliseconds(1000) / i64));
+  static_assert(TimeDelta::FromMilliseconds(500) ==
+                    (TimeDelta::FromMilliseconds(1000) / i64),
+                "");
   EXPECT_EQ(TimeDelta::FromMilliseconds(2000),
             (TimeDelta::FromMilliseconds(1000) *= i64));
-  EXPECT_EQ(TimeDelta::FromMilliseconds(500),
-            (TimeDelta::FromMilliseconds(1000) /= i64));
+  static_assert(TimeDelta::FromMilliseconds(500) ==
+                    (TimeDelta::FromMilliseconds(1000) /= i64),
+                "");
   EXPECT_EQ(TimeDelta::FromMilliseconds(2000),
             (i64 * TimeDelta::FromMilliseconds(1000)));
 
   EXPECT_EQ(TimeDelta::FromMilliseconds(500),
             (TimeDelta::FromMilliseconds(1000) * 0.5));
-  EXPECT_EQ(TimeDelta::FromMilliseconds(2000),
-            (TimeDelta::FromMilliseconds(1000) / 0.5));
+  static_assert(TimeDelta::FromMilliseconds(2000) ==
+                    (TimeDelta::FromMilliseconds(1000) / 0.5),
+                "");
   EXPECT_EQ(TimeDelta::FromMilliseconds(500),
             (TimeDelta::FromMilliseconds(1000) *= 0.5));
-  EXPECT_EQ(TimeDelta::FromMilliseconds(2000),
-            (TimeDelta::FromMilliseconds(1000) /= 0.5));
+  static_assert(TimeDelta::FromMilliseconds(2000) ==
+                    (TimeDelta::FromMilliseconds(1000) /= 0.5),
+                "");
   EXPECT_EQ(TimeDelta::FromMilliseconds(500),
             (0.5 * TimeDelta::FromMilliseconds(1000)));
 
   EXPECT_EQ(TimeDelta::FromMilliseconds(2000),
             (TimeDelta::FromMilliseconds(1000) * 2));
-  EXPECT_EQ(TimeDelta::FromMilliseconds(500),
-            (TimeDelta::FromMilliseconds(1000) / 2));
+  static_assert(TimeDelta::FromMilliseconds(500) ==
+                    (TimeDelta::FromMilliseconds(1000) / 2),
+                "");
   EXPECT_EQ(TimeDelta::FromMilliseconds(2000),
             (TimeDelta::FromMilliseconds(1000) *= 2));
-  EXPECT_EQ(TimeDelta::FromMilliseconds(500),
-            (TimeDelta::FromMilliseconds(1000) /= 2));
+  static_assert(TimeDelta::FromMilliseconds(500) ==
+                    (TimeDelta::FromMilliseconds(1000) /= 2),
+                "");
   EXPECT_EQ(TimeDelta::FromMilliseconds(2000),
             (2 * TimeDelta::FromMilliseconds(1000)));
 }