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libstdc++: Micro-optimize __from_chars_pow2_base 

In the first iteration of __from_chars_pow2_base's main loop, we need
to remember the value of the leading significant digit for sake of the
overflow check at the end (for base > 2).

This patch manually unrolls this first iteration so as to not encumber
the entire loop with logic that only the first iteration needs.  This
seems to significantly improve performance:

Base  Before  After (seconds, lower is better)
   2    9.36   9.37
   8    3.66   2.93
  16    2.93   1.91
  32    2.39   2.24

libstdc++-v3/ChangeLog:

	* include/std/charconv (__from_chars_pow2_base): Manually
	unroll the first iteration of the main loop and simplify
	accordingly.
This commit is contained in:
Patrick Palka 2022-04-18 17:22:55 -04:00
parent 021b51814d
commit d210653f39
1 changed files with 20 additions and 8 deletions
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28
libstdc++-v3/include/std/charconv
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@ -469,25 +469,37 @@ namespace __detail
while (__i < __len && __first[__i] == '0')
++__i;
const ptrdiff_t __leading_zeroes = __i;
if (__i >= __len) [[__unlikely__]]
{
__first += __i;
return true;
}
// Remember the leading significant digit value if necessary.
unsigned char __leading_c = 0;
if (__base != 2)
{
__leading_c = __from_chars_alnum_to_val<_DecOnly>(__first[__i]);
// __glibcxx_assert(__leading_c != 0);
if (__leading_c >= __base) [[__unlikely__]]
{
__first += __i;
return true;
}
__val = __leading_c;
++__i;
}
for (; __i < __len; ++__i)
{
const unsigned char __c = __from_chars_alnum_to_val<_DecOnly>(__first[__i]);
if (__c >= __base)
break;
__val = (__val << __log2_base) | __c;
if (__i == __leading_zeroes)
{
// At the first iteration, remember the leading significant digit.
// __glibcxx_assert(__leading_c == 0 && __c != 0);
__leading_c = __c;
}
}
__first += __i;
auto __significant_bits = (__i - __leading_zeroes) * __log2_base;
if (__base != 2 && __leading_c != 0)
if (__base != 2)
// Compensate for a leading significant digit that didn't use all
// of its available bits.
__significant_bits -= __log2_base - __bit_width(__leading_c);