[libcxx-commits] [PATCH] D78763: Add optimization to basic_string::assign for compile-time known constant values.
Martijn Vels via Phabricator via libcxx-commits
libcxx-commits at lists.llvm.org
Thu Jun 18 12:34:34 PDT 2020
mvels marked 2 inline comments as done.
mvels added inline comments.
Comment at: libcxx/include/string:2306
_LIBCPP_ASSERT(__n == 0 || __s != nullptr, "string::assign received nullptr");
- return (__builtin_constant_p(__n) && __n <= 128 / sizeof(value_type))
- ? __assign_small_length(__s, __n)
+ return (_LIBCPP_BUILTIN_CONSTANT_P(__n) && __n < __min_cap)
+ ? __assign_short(__s, __n)
> It used to be `__n <= ...`, now it's `__n < ...` -- what am I missing?
The initial version was aimed at optimizing 2 things:
- short optimization, i.e., [n] chars < min_cap guaranteed to fit inside any SSO or heap allocated string
- small memcpy optimization, i.., n <= 128 which when inlined can be vectorized efficiently (i.e., a rough max memcpy inline)
For this change we should focus only on the former. The latter is better picked up as an FDO / LLVM compiler inlining opportunity.
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