Faster BCD into integer parsing

Author: nielsdos

ext/bcmath/libbcmath/src/recmul.c

@@ -48,12 +48,68 @@
 
 /* Multiply utility routines */
 
+/* This is based on the technique described in https://kholdstare.github.io/technical/2020/05/26/faster-integer-parsing.html.
+ * This function transforms AABBCCDD into 1000 * AA + 100 * BB + 10 * CC + DD,
+ * with the caveat that all components must be in the interval [0, 25] to prevent overflow
+ * due to the multiplication by power of 10 (10 * 25 = 250 is the largest number that fits in a byte).
+ * The advantage of this method instead of using shifts + 3 multiplications is that this is cheaper
+ * due to its divide-and-conquer nature.
+ */
+#if SIZEOF_SIZE_T == 4
+static uint32_t bc_parse_chunk_chars(const char *str)
+{
+	uint32_t tmp;
+	memcpy(&tmp, str, sizeof(tmp));
+#if !BC_LITTLE_ENDIAN
+	tmp = BC_BSWAP(tmp);
+#endif
+
+	uint32_t lower_digits = (tmp & 0x0f000f00) >> 8;
+	uint32_t upper_digits = (tmp & 0x000f000f) * 10;
+
+	tmp = lower_digits + upper_digits;
+
+	lower_digits = (tmp & 0x00ff0000) >> 16;
+	upper_digits = (tmp & 0x000000ff) * 100;
+
+	return lower_digits + upper_digits;
+}
+#elif SIZEOF_SIZE_T == 8
+static uint64_t bc_parse_chunk_chars(const char *str)
+{
+	uint64_t tmp;
+	memcpy(&tmp, str, sizeof(tmp));
+#if !BC_LITTLE_ENDIAN
+	tmp = BC_BSWAP(tmp);
+#endif
+
+	uint64_t lower_digits = (tmp & 0x0f000f000f000f00) >> 8;
+	uint64_t upper_digits = (tmp & 0x000f000f000f000f) * 10;
+
+	tmp = lower_digits + upper_digits;
+
+	lower_digits = (tmp & 0x00ff000000ff0000) >> 16;
+	upper_digits = (tmp & 0x000000ff000000ff) * 100;
+
+	tmp = lower_digits + upper_digits;
+
+	lower_digits = (tmp & 0x0000ffff00000000) >> 32;
+	upper_digits = (tmp & 0x000000000000ffff) * 10000;
+
+	return lower_digits + upper_digits;
+}
+#endif
+
 /*
  * Converts BCD to uint, going backwards from pointer n by the number of
  * characters specified by len.
  */
 static inline BC_UINT_T bc_partial_convert_to_uint(const char *n, size_t len)
 {
+	if (len == BC_MUL_UINT_DIGITS) {
+		return bc_parse_chunk_chars(n - BC_MUL_UINT_DIGITS + 1);
+	}
+
 	BC_UINT_T num = 0;
 	BC_UINT_T base = 1;