https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112440
Bug ID: 112440
Summary: Compiler does not grok basic_string::resize and
basic_string::reserve if _CharT is char
Product: gcc
Version: 13.2.1
Status: UNCONFIRMED
Keywords: missed-optimization
Severity: normal
Priority: P3
Component: libstdc++
Assignee: unassigned at gcc dot gnu.org
Reporter: antoshkka at gmail dot com
Target Milestone: ---
Consider the example:
#include <string>
void test1(std::size_t summ) {
std::string result;
result.resize(summ);
if (result.size() > summ) {
__builtin_abort();
}
}
The resulting assembly contains `call abort` and code to check the string size:
https://godbolt.org/z/zcj3Pc3G8
Looks like this is due to char* aliasing with string internals, switching to
std::u8string removes the `call abort` related assembly:
https://godbolt.org/z/a6bKaqqn5
I've failed to come up with a generic solution, but looks like adding
__builtin_unreachable() to the end of basic_string::resize and
basic_string::reserve helps: https://godbolt.org/z/vWcjqGK94
P.S.: such hints help to shorten the assembly for reserve+append*n cases
https://godbolt.org/z/nsEGsWdP3 , https://godbolt.org/z/qMf4b7dd8 ,
https://godbolt.org/z/1r6dd6d5M which are quire common
