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Try to improve low number itoa

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This commit is contained in:
Raúl Marín 2024-06-21 18:30:45 +02:00
parent c70977bf1d
commit df8341c447
1 changed files with 43 additions and 32 deletions
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75
base/base/itoa.cpp
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@ -3,6 +3,34 @@
#include <base/extended_types.h>
#include <base/itoa.h>
namespace
{
ALWAYS_INLINE inline char * outOneDigit(char * p, uint8_t value)
{
*p = '0' + value;
return p + 1;
}
// Using a lookup table to convert binary numbers from 0 to 99
// into ascii characters as described by Andrei Alexandrescu in
// https://www.facebook.com/notes/facebook-engineering/three-optimization-tips-for-c/10151361643253920/
const char digits[201] = "00010203040506070809"
"10111213141516171819"
"20212223242526272829"
"30313233343536373839"
"40414243444546474849"
"50515253545556575859"
"60616263646566676869"
"70717273747576777879"
"80818283848586878889"
"90919293949596979899";
ALWAYS_INLINE inline char * outTwoDigits(char * p, uint8_t value)
{
memcpy(p, &digits[value * 2], 2);
p += 2;
return p;
}
namespace jeaiii
{
/*
@ -84,43 +112,48 @@ template <class T>
inline ALWAYS_INLINE char * to_text_from_integer(char * b, T i)
{
constexpr auto q = sizeof(T);
using U = cond<q == 1, unsigned char, cond<q <= sizeof(short), unsigned short, cond<q <= sizeof(UInt32), UInt32, UInt64>>>;
using U = cond<q == 1, char8_t, cond<q <= sizeof(UInt16), UInt16, cond<q <= sizeof(UInt32), UInt32, UInt64>>>;
// convert bool to int before test with unary + to silence warning if T happens to be bool
U const n = +i < 0 ? *b++ = '-', U(0) - U(i) : U(i);
if (n < UInt32(1e2))
if (n < U(1e2))
{
*reinterpret_cast<pair *>(b) = digits.fd[n];
return n < 10 ? b + 1 : b + 2;
return n < 10 ? outOneDigit(b, n) : outTwoDigits(b, n);
}
if (n < UInt32(1e6))
{
if (n < UInt32(1e4))
if (sizeof(U) == 1 || n < U(1e4))
{
auto f0 = UInt32(10 * (1 << 24) / 1e3 + 1) * n;
*reinterpret_cast<pair *>(b) = digits.fd[f0 >> 24];
b -= n < UInt32(1e3);
if constexpr (sizeof(U) == 1)
b -= 1;
else
b -= n < U(1e3);
auto f2 = (f0 & mask24) * 100;
*reinterpret_cast<pair *>(b + 2) = digits.dd[f2 >> 24];
return b + 4;
}
auto f0 = UInt64(10 * (1ull << 32ull) / 1e5 + 1) * n;
*reinterpret_cast<pair *>(b) = digits.fd[f0 >> 32];
b -= n < UInt32(1e5);
if constexpr (sizeof(U) == 2)
b -= 1;
else
b -= n < U(1e5);
auto f2 = (f0 & mask32) * 100;
*reinterpret_cast<pair *>(b + 2) = digits.dd[f2 >> 32];
auto f4 = (f2 & mask32) * 100;
*reinterpret_cast<pair *>(b + 4) = digits.dd[f4 >> 32];
return b + 6;
}
if (n < UInt64(1ull << 32ull))
if (sizeof(U) == 4 || n < UInt64(1ull << 32ull))
{
if (n < UInt32(1e8))
if (n < U(1e8))
{
auto f0 = UInt64(10 * (1ull << 48ull) / 1e7 + 1) * n >> 16;
*reinterpret_cast<pair *>(b) = digits.fd[f0 >> 32];
b -= n < UInt32(1e7);
b -= n < U(1e7);
auto f2 = (f0 & mask32) * 100;
*reinterpret_cast<pair *>(b + 2) = digits.dd[f2 >> 32];
auto f4 = (f2 & mask32) * 100;
@ -248,28 +281,6 @@ inline ALWAYS_INLINE char * to_text_from_integer(char * b, T i)
}
}
namespace
{
// Using a lookup table to convert binary numbers from 0 to 99
// into ascii characters as described by Andrei Alexandrescu in
// https://www.facebook.com/notes/facebook-engineering/three-optimization-tips-for-c/10151361643253920/
const char digits[201] = "00010203040506070809"
"10111213141516171819"
"20212223242526272829"
"30313233343536373839"
"40414243444546474849"
"50515253545556575859"
"60616263646566676869"
"70717273747576777879"
"80818283848586878889"
"90919293949596979899";
ALWAYS_INLINE inline char * outTwoDigits(char * p, uint8_t value)
{
memcpy(p, &digits[value * 2], 2);
p += 2;
return p;
}
const uint64_t max_multiple_of_hundred_that_fits_in_64_bits = 1'00'00'00'00'00'00'00'00'00ull;
const int max_multiple_of_hundred_blocks = 9;
static_assert(max_multiple_of_hundred_that_fits_in_64_bits % 100 == 0);