From 923519f31d44095eeb662d2e26af689e66633716 Mon Sep 17 00:00:00 2001
From: Stiopa Koltsov <stepan.koltsov@gmail.com>
Date: Mon, 7 Dec 2020 08:09:22 -0800
Subject: [PATCH] Starlark: optimize StarlarkInt.Big comparison to
 StarlarkInt.Int{32,64}

Perform comparison without conversion of smaller integers to
`BigInteger`.  `StarlarkInt.compareTo` does not allocate now.

For this benchmark:

```
def test():
    x = 17 << 77
    for i in range(10):
        print(i)
        for j in range(10000000):
            x > 1

test()
```

```
A: n=27 mean=4.262 std=0.203 se=0.039 min=4.036 med=4.193
B: n=27 mean=4.113 std=0.172 se=0.033 min=3.859 med=4.049
B/A: 0.965 0.941..0.990 (95% conf)
```

Speed up is about 7% when comparing to an integer outside of
`BigInteger` cached range (-16..16).

Finally, `StarlarkInt.Big` to `StarlarkInt.Big` comparison performance
seems to stay the same (within 95% confidence interval after 100
test iterations).

Closes #12638.

PiperOrigin-RevId: 346095040
---
 .../net/starlark/java/eval/StarlarkInt.java   | 33 ++++++++++---------
 1 file changed, 17 insertions(+), 16 deletions(-)

diff --git a/src/main/java/net/starlark/java/eval/StarlarkInt.java b/src/main/java/net/starlark/java/eval/StarlarkInt.java
index d63186ba92cf29..d810c6b2e15770 100644
--- a/src/main/java/net/starlark/java/eval/StarlarkInt.java
+++ b/src/main/java/net/starlark/java/eval/StarlarkInt.java
@@ -427,26 +427,27 @@ public int compareTo(StarlarkInt x) {
 
   // binary operators
 
-  // In the common case, both operands are int32, so the operations
-  // can be done using longs with minimal fuss around overflows.
-  // All other combinations are promoted to BigInteger.
-  // TODO(adonovan): use long x long operations even if one or both
-  // operands are Int64; promote to Big x Big only upon overflow.
-  // (See the revision history for the necessary overflow checks.)
-
-  /** Returns a value whose signum is equal to x - y. */
+  /** Returns signum(x - y). */
   public static int compare(StarlarkInt x, StarlarkInt y) {
-    if (x instanceof Int32 && y instanceof Int32) {
-      return Integer.compare(((Int32) x).v, ((Int32) y).v);
-    }
-
+    // If both arguments are big, we compare BigIntegers.
+    // If neither argument is big, we compare longs.
+    // If only one argument is big, its magnitude is greater
+    // than the other operand, so only its sign matters.
+    //
+    // We avoid unnecessary branches.
     try {
-      return Long.compare(x.toLongFast(), y.toLongFast());
+      long xl = x.toLongFast();
+      try {
+        long yl = y.toLongFast();
+        return Long.compare(xl, yl); // (long, long)
+      } catch (Overflow unused) {
+        return -((Big) y).v.signum(); // (long, big)
+      }
     } catch (Overflow unused) {
-      /* fall through */
+      return y instanceof Big
+          ? ((Big) x).v.compareTo(((Big) y).v) // (big, big)
+          : ((Big) x).v.signum(); // (big, long)
     }
-
-    return x.toBigInteger().compareTo(y.toBigInteger());
   }
 
   /** Returns x + y. */