Optimize bignum * 2^n case

ChangeLog

@@ -1,3 +1,8 @@
+2023-10-26  Shiro Kawai  <shiro@acm.org>
+
+	* src/bignum.c (bignum_mul_si): Optimize special case when bignum
+	  muliplied by y=2^n (y is fixnum).
+
 2023-10-24  Shiro Kawai  <shiro@acm.org>
 
 	* lib/r7rs-setup.scm (open-binary-*-file): Even though Gauche

src/bignum.c

@@ -814,13 +814,19 @@ static ScmBignum *bignum_mul_si(const ScmBignum *bx, long y)
         br->sign = -br->sign;
         return br;
     }
-    /* TODO: optimize for 2^n case !*/
     ScmBignum *br;
     br = make_bignum(bx->size + 1); /* TODO: more accurate estimation */
-    u_long yabs = (y<0)? -y:y;
-    br->sign = bx->sign;
-    bignum_mul_word(br, bx, yabs, 0);
-    if (y<0) br->sign = -br->sign;
+    if (y & (y-1)) {
+        /* general case */
+        u_long yabs = (y<0)? -y:y;
+        br->sign = bx->sign;
+        bignum_mul_word(br, bx, yabs, 0);
+        if (y<0) br->sign = -br->sign;
+    } else {
+        /* fast path when y is a power of two (note that y != 0). */
+        int n = Scm__LowestBitNumber((u_long)y);
+        bignum_lshift(br, bx, n);
+    }
     return br;
 }
 

test/number.scm

@@ -1488,6 +1488,17 @@
   (test* "0.0 * bignum"  0.0 (Apply * `(0.0 ,big)) eqv?)
   (test* "bignum * 1.0"  1.0e20 (Apply * `(,big 1.0)) eqv?)
   (test* "1.0 * bignum"  1.0e20 (Apply * `(1.0 ,big)) eqv?)
+
+  ;; 2^n shortcut
+  (dotimes [n 64]
+    (test* (format "bignum * 2^~a" n)
+           (ash big n)
+           (Apply * `(,big ,(expt 2 n)))
+           eqv?)
+    (test* (format "2^~a & bignum" n)
+           (ash big n)
+           (Apply * `(,(expt 2 n) ,big))
+           eqv?))
   )
 
 (test* "ratnum * 0"  0 (Apply * '(1/2 0)) eqv?)