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From: "antoshkka at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug libstdc++/112440] New: Compiler does not grok
 basic_string::resize and basic_string::reserve if _CharT is char
Date: Wed, 08 Nov 2023 11:40:20 +0000
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X-Bugzilla-Type: new
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: libstdc++
X-Bugzilla-Version: 13.2.1
X-Bugzilla-Keywords: missed-optimization
X-Bugzilla-Severity: normal
X-Bugzilla-Who: antoshkka at gmail dot com
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X-Bugzilla-Priority: P3
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D112440

            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 s=
ize:
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=