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Workaround lack of static constexpr in constexpr functions

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This commit is contained in:
Victor Zverovich 2021-06-07 07:16:58 -07:00
parent 2039dce75f
commit 1de80f5b22
1 changed files with 13 additions and 7 deletions
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20
include/fmt/format.h
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@ -935,15 +935,14 @@ FMT_CONSTEXPR auto count_digits(UInt n) -> int {
template <> auto count_digits<4>(detail::fallback_uintptr n) -> int; template <> auto count_digits<4>(detail::fallback_uintptr n) -> int;
#ifdef FMT_BUILTIN_CLZ #ifdef FMT_BUILTIN_CLZ
// Optional version of count_digits for better performance on 32-bit platforms.
FMT_CONSTEXPR20 inline auto count_digits(uint32_t n) -> int { // It is a separate function rather than a part of count_digits to workaround
if (is_constant_evaluated()) { // the lack of static constexpr in constexpr functions.
return count_digits_fallback(n); FMT_INLINE uint64_t count_digits_inc(int n) {
}
// An optimization by Kendall Willets from https://bit.ly/3uOIQrB. // An optimization by Kendall Willets from https://bit.ly/3uOIQrB.
// This increments the upper 32 bits (log10(T) - 1) when >= T is added. // This increments the upper 32 bits (log10(T) - 1) when >= T is added.
# define FMT_INC(T) (((sizeof(# T) - 1ull) << 32) - T) # define FMT_INC(T) (((sizeof(# T) - 1ull) << 32) - T)
FMT_STATIC_CONSTEXPR uint64_t table[] = { static constexpr uint64_t table[] = {
FMT_INC(0), FMT_INC(0), FMT_INC(0), // 8 FMT_INC(0), FMT_INC(0), FMT_INC(0), // 8
FMT_INC(10), FMT_INC(10), FMT_INC(10), // 64 FMT_INC(10), FMT_INC(10), FMT_INC(10), // 64
FMT_INC(100), FMT_INC(100), FMT_INC(100), // 512 FMT_INC(100), FMT_INC(100), FMT_INC(100), // 512
@ -956,7 +955,14 @@ FMT_CONSTEXPR20 inline auto count_digits(uint32_t n) -> int {
FMT_INC(1000000000), FMT_INC(1000000000), FMT_INC(1000000000), // 1024M FMT_INC(1000000000), FMT_INC(1000000000), FMT_INC(1000000000), // 1024M
FMT_INC(1000000000), FMT_INC(1000000000) // 4B FMT_INC(1000000000), FMT_INC(1000000000) // 4B
}; };
return static_cast<int>((n + table[FMT_BUILTIN_CLZ(n | 1) ^ 31]) >> 32); return table[n];
}
// Optional version of count_digits for better performance on 32-bit platforms.
FMT_CONSTEXPR20 inline auto count_digits(uint32_t n) -> int {
if (is_constant_evaluated()) return count_digits_fallback(n);
auto inc = count_digits_inc(FMT_BUILTIN_CLZ(n | 1) ^ 31);
return static_cast<int>((n + inc) >> 32);
} }
#endif #endif