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* [gcc r12-8185] libstdc++: Micro-optimize __from_chars_pow2_base
@ 2022-04-18 21:23 Patrick Palka
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From: Patrick Palka @ 2022-04-18 21:23 UTC (permalink / raw)
To: gcc-cvs, libstdc++-cvs
https://gcc.gnu.org/g:d210653f3907ac1829fd275d067b2855ea53da24
commit r12-8185-gd210653f3907ac1829fd275d067b2855ea53da24
Author: Patrick Palka <ppalka@redhat.com>
Date: Mon Apr 18 17:22:55 2022 -0400
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.
Diff:
---
libstdc++-v3/include/std/charconv | 28 ++++++++++++++++++++--------
1 file changed, 20 insertions(+), 8 deletions(-)
diff --git a/libstdc++-v3/include/std/charconv b/libstdc++-v3/include/std/charconv
index dda4ec87779..f1ace406017 100644
--- a/libstdc++-v3/include/std/charconv
+++ b/libstdc++-v3/include/std/charconv
@@ -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);
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