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https://github.com/fluencelabs/redis
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Faster ll2string() implementation.
Based on ideas documented in this blog post: https://www.facebook.com/notes/facebook-engineering/three-optimization-tips-for-c/10151361643253920 The original code was modified to handle signed integers, reformetted to fit inside the Redis code base, and was stress-tested with a program in order to validate the implementation against snprintf(). Redis was measured to be measurably faster from the point of view of clients in real-world operations because of this change, since sometimes number to string conversion is used extensively (for example every time a GET results into an integer encoded object to be returned to the user).
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101
src/util.c
101
src/util.c
@ -214,28 +214,87 @@ long long memtoll(const char *p, int *err) {
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return val*mul;
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}
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/* Convert a long long into a string. Returns the number of
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* characters needed to represent the number, that can be shorter if passed
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* buffer length is not enough to store the whole number. */
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int ll2string(char *s, size_t len, long long value) {
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char buf[32], *p;
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unsigned long long v;
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size_t l;
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/* Return the number of digits of 'v' when converted to string in radix 10.
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* See ll2string() for more information. */
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uint32_t digits10(uint64_t v) {
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if (v < 10) return 1;
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if (v < 100) return 2;
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if (v < 1000) return 3;
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if (v < 1000000000000UL) {
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if (v < 100000000UL) {
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if (v < 1000000) {
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if (v < 10000) return 4;
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return 5 + (v >= 100000);
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}
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return 7 + (v >= 10000000UL);
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}
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if (v < 10000000000UL) {
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return 9 + (v >= 1000000000UL);
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}
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return 11 + (v >= 100000000000UL);
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}
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return 12 + digits10(v / 1000000000000UL);
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}
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if (len == 0) return 0;
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v = (value < 0) ? -value : value;
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p = buf+31; /* point to the last character */
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do {
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*p-- = '0'+(v%10);
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v /= 10;
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} while(v);
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if (value < 0) *p-- = '-';
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p++;
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l = 32-(p-buf);
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if (l+1 > len) l = len-1; /* Make sure it fits, including the nul term */
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memcpy(s,p,l);
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s[l] = '\0';
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return l;
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/* Convert a long long into a string. Returns the number of
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* characters needed to represent the number.
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* If the buffer is not big enough to store the string, 0 is returned.
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*
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* Based on the following article (that apparently does not provide a
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* novel approach but only publicizes an already used technique):
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*
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* https://www.facebook.com/notes/facebook-engineering/three-optimization-tips-for-c/10151361643253920
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*
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* Modified in order to handle signed integers since the original code was
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* designed for unsigned integers. */
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int ll2string(char* dst, size_t dstlen, long long svalue) {
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static const char digits[201] =
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"0001020304050607080910111213141516171819"
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"2021222324252627282930313233343536373839"
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"4041424344454647484950515253545556575859"
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"6061626364656667686970717273747576777879"
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"8081828384858687888990919293949596979899";
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int negative;
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unsigned long long value;
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/* The main loop works with 64bit unsigned integers for simplicity, so
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* we convert the number here and remember if it is negative. */
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if (svalue < 0) {
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value = -svalue;
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negative = 1;
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} else {
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value = svalue;
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negative = 0;
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}
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/* Check length. */
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uint32_t const length = digits10(value)+negative;
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if (length >= dstlen) return 0;
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/* Null term. */
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uint32_t next = length;
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dst[next] = '\0';
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next--;
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while (value >= 100) {
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int const i = (value % 100) * 2;
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value /= 100;
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dst[next] = digits[i + 1];
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dst[next - 1] = digits[i];
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next -= 2;
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}
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/* Handle last 1-2 digits. */
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if (value < 10) {
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dst[next] = '0' + (uint32_t) value;
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} else {
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int i = (uint32_t) value * 2;
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dst[next] = digits[i + 1];
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dst[next - 1] = digits[i];
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}
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/* Add sign. */
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if (negative) dst[0] = '-';
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return length;
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}
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/* Convert a string into a long long. Returns 1 if the string could be parsed
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