8355719: Reduce memory consumption of BigInteger.pow()

src/java.base/share/classes/java/math/BigInteger.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 1996, 2024, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 1996, 2025, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -1246,6 +1246,16 @@ else if (val < 0 && val >= -MAX_CONSTANT)
         return new BigInteger(val);
     }
 
+    /**
+     * Constructs a BigInteger with magnitude specified by the long,
+     * which may not be zero, and the signum specified by the int.
+     */
+    private BigInteger(long mag, int signum) {
+        assert mag != 0 && signum != 0;
+        this.signum = signum;
+        this.mag = toMagArray(mag);
+    }
+
     /**
      * Constructs a BigInteger with the specified value, which may not be zero.
      */
@@ -1256,16 +1266,14 @@ private BigInteger(long val) {
         } else {
             signum = 1;
         }
+        mag = toMagArray(val);
+    }
 
-        int highWord = (int)(val >>> 32);
-        if (highWord == 0) {
-            mag = new int[1];
-            mag[0] = (int)val;
-        } else {
-            mag = new int[2];
-            mag[0] = highWord;
-            mag[1] = (int)val;
-        }
+    private static int[] toMagArray(long mag) {
+        int highWord = (int) (mag >>> 32);
+        return highWord == 0
+                ? new int[] { (int) mag }
+                : new int[] { highWord, (int) mag };
     }
 
     /**
@@ -2589,116 +2597,101 @@ public BigInteger pow(int exponent) {
         if (exponent < 0) {
             throw new ArithmeticException("Negative exponent");
         }
-        if (signum == 0) {
-            return (exponent == 0 ? ONE : this);
-        }
+        if (exponent == 0 || this.equals(ONE))
+            return ONE;
+
+        if (signum == 0 || exponent == 1)
+            return this;
 
-        BigInteger partToSquare = this.abs();
+        BigInteger base = this.abs();
+        final boolean negative = signum < 0 && (exponent & 1) == 1;
 
         // Factor out powers of two from the base, as the exponentiation of
         // these can be done by left shifts only.
         // The remaining part can then be exponentiated faster.  The
         // powers of two will be multiplied back at the end.
-        int powersOfTwo = partToSquare.getLowestSetBit();
-        long bitsToShiftLong = (long)powersOfTwo * exponent;
-        if (bitsToShiftLong > Integer.MAX_VALUE) {
+        final int powersOfTwo = base.getLowestSetBit();
+        final long bitsToShiftLong = (long) powersOfTwo * exponent;
+        final int bitsToShift = (int) bitsToShiftLong;
+        if (bitsToShift != bitsToShiftLong) {
             reportOverflow();
         }
-        int bitsToShift = (int)bitsToShiftLong;
 
-        int remainingBits;
-
-        // Factor the powers of two out quickly by shifting right, if needed.
-        if (powersOfTwo > 0) {
-            partToSquare = partToSquare.shiftRight(powersOfTwo);
-            remainingBits = partToSquare.bitLength();
-            if (remainingBits == 1) {  // Nothing left but +/- 1?
-                if (signum < 0 && (exponent&1) == 1) {
-                    return NEGATIVE_ONE.shiftLeft(bitsToShift);
-                } else {
-                    return ONE.shiftLeft(bitsToShift);
-                }
-            }
-        } else {
-            remainingBits = partToSquare.bitLength();
-            if (remainingBits == 1) { // Nothing left but +/- 1?
-                if (signum < 0  && (exponent&1) == 1) {
-                    return NEGATIVE_ONE;
-                } else {
-                    return ONE;
-                }
-            }
-        }
+        // Factor the powers of two out quickly by shifting right.
+        base = base.shiftRight(powersOfTwo);
+        final int remainingBits = base.bitLength();
+        if (remainingBits == 1) // Nothing left but +/- 1?
+            return (negative ? NEGATIVE_ONE : ONE).shiftLeft(bitsToShift);
 
         // This is a quick way to approximate the size of the result,
         // similar to doing log2[n] * exponent.  This will give an upper bound
         // of how big the result can be, and which algorithm to use.
-        long scaleFactor = (long)remainingBits * exponent;
+        final long scaleFactor = (long) remainingBits * exponent;
 
         // Use slightly different algorithms for small and large operands.
-        // See if the result will safely fit into a long. (Largest 2^63-1)
-        if (partToSquare.mag.length == 1 && scaleFactor <= 62) {
-            // Small number algorithm.  Everything fits into a long.
-            int newSign = (signum <0  && (exponent&1) == 1 ? -1 : 1);
-            long result = 1;
-            long baseToPow2 = partToSquare.mag[0] & LONG_MASK;
-
-            int workingExponent = exponent;
-
-            // Perform exponentiation using repeated squaring trick
-            while (workingExponent != 0) {
-                if ((workingExponent & 1) == 1) {
-                    result = result * baseToPow2;
-                }
-
-                if ((workingExponent >>>= 1) != 0) {
-                    baseToPow2 = baseToPow2 * baseToPow2;
-                }
-            }
+        // See if the result will safely fit into an unsigned long. (Largest 2^64-1)
+        if (scaleFactor <= Long.SIZE) {
+            // Small number algorithm.  Everything fits into an unsigned long.
+            final int newSign = negative ? -1 : 1;
+            final long result = unsignedLongPow(base.mag[0] & LONG_MASK, exponent);
 
             // Multiply back the powers of two (quickly, by shifting left)
-            if (powersOfTwo > 0) {
-                if (bitsToShift + scaleFactor <= 62) { // Fits in long?
-                    return valueOf((result << bitsToShift) * newSign);
-                } else {
-                    return valueOf(result*newSign).shiftLeft(bitsToShift);
-                }
-            } else {
-                return valueOf(result*newSign);
-            }
-        } else {
-            if ((long)bitLength() * exponent / Integer.SIZE > MAX_MAG_LENGTH) {
-                reportOverflow();
-            }
+            return bitsToShift + scaleFactor <= Long.SIZE  // Fits in long?
+                    ? new BigInteger(result << bitsToShift, newSign)
+                    : new BigInteger(result, newSign).shiftLeft(bitsToShift);
+        }
 
-            // Large number algorithm.  This is basically identical to
-            // the algorithm above, but calls multiply() and square()
-            // which may use more efficient algorithms for large numbers.
-            BigInteger answer = ONE;
+        if ((bitLength() - 1L) * exponent >= Integer.MAX_VALUE) {
+            reportOverflow();
+        }
 
-            int workingExponent = exponent;
-            // Perform exponentiation using repeated squaring trick
-            while (workingExponent != 0) {
-                if ((workingExponent & 1) == 1) {
-                    answer = answer.multiply(partToSquare);
-                }
+        // Large number algorithm.  This is basically identical to
+        // the algorithm above, but calls multiply()
+        // which may use more efficient algorithms for large numbers.
+        BigInteger answer = ONE;
 
-                if ((workingExponent >>>= 1) != 0) {
-                    partToSquare = partToSquare.square();
-                }
-            }
-            // Multiply back the (exponentiated) powers of two (quickly,
-            // by shifting left)
-            if (powersOfTwo > 0) {
-                answer = answer.shiftLeft(bitsToShift);
-            }
+        final int expZeros = Integer.numberOfLeadingZeros(exponent);
+        int workingExp = exponent << expZeros;
+        // Perform exponentiation using repeated squaring trick
+        // The loop relies on this invariant:
+        // base^exponent == answer^(2^expLen) * base^(workingExp >>> (32-expLen))
+        for (int expLen = Integer.SIZE - expZeros; expLen > 0; expLen--) {
+            answer = answer.multiply(answer);
+            if (workingExp < 0) // leading bit is set
+                answer = answer.multiply(base);
 
-            if (signum < 0 && (exponent&1) == 1) {
-                return answer.negate();
-            } else {
-                return answer;
-            }
+            workingExp <<= 1;
+        }
+
+        // Multiply back the (exponentiated) powers of two (quickly,
+        // by shifting left)
+        answer = answer.shiftLeft(bitsToShift);
+        return negative ? answer.negate() : answer;
+    }
+
+    /**
+     * Computes {@code x^n} using repeated squaring trick.
+     * Assumes {@code x != 0 && x^n < 2^Long.SIZE}.
+     */
+    static long unsignedLongPow(long x, int n) {
+        if (x == 1L || n == 0)
+            return 1L;
+
+        if (x == 2L)
+            return 1L << n;
+
+        /*
+         * The method assumption means that n <= 40 here.
+         * Thus, the loop body executes at most 5 times.
+         */
+        long pow = 1L;
+        for (; n != 1; n >>>= 1) {
+            if ((n & 1) != 0)
+                pow *= x;
+
+            x *= x;
         }
+        return pow * x;
     }
 
     /**