Re: [PATCH] Optimize to_chars

[Jonathan Wakely <jwakely@redhat.com>]

On 30/08/19 11:03 -0600, Martin Sebor wrote:
>On 8/30/19 8:27 AM, Antony Polukhin wrote:
>>Bunch of micro optimizations for std::to_chars:
>>* For base == 8 replacing the lookup in __digits table with arithmetic
>>computations leads to a same CPU cycles for a loop (exchanges two
>>movzx with 3 bit ops https://godbolt.org/z/RTui7m ). However this
>>saves 129 bytes of data and totally avoids a chance of cache misses on
>>__digits.
>>* For base == 16 replacing the lookup in __digits table with
>>arithmetic computations leads to a few additional instructions, but
>>totally avoids a chance of cache misses on __digits (- ~9 cache misses
>>for worst case) and saves 513 bytes of const data.
>>* Replacing __first[pos] and __first[pos - 1] with __first[1] and
>>__first[0] on final iterations saves ~2% of code size.
>>* Removing trailing '\0' from arrays of digits allows the linker to
>>merge the symbols (so that "0123456789abcdefghijklmnopqrstuvwxyz" and
>>"0123456789abcdef" could share the same address). This improves data
>>locality and reduces binary sizes.
>>* Using __detail::__to_chars_len_2 instead of a generic
>>__detail::__to_chars_len makes the operation O(1) instead of O(N). It
>>also makes the code two times shorter ( https://godbolt.org/z/Peq_PG)
>>.
>>
>>In sum: this significantly reduces the size of a binary (for about
>>4KBs only for base-8 conversion https://godbolt.org/z/WPKijS ), deals
>>with latency (CPU cache misses) without changing the iterations count
>>and without adding costly instructions into the loops.
>
>Would it make sense to move some of this code into GCC as
>a built-in so that it could also be used by GCC to expand
>some strtol and sprintf calls?

Makes sense, although we'd still need it in libstdc++ until Clang and
EDG implement the same built-in.