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From: "antoshkka at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/109931] Knowledge on literal not used in
 optimization
Date: Mon, 22 May 2023 12:57:40 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D109931

--- Comment #3 from Antony Polukhin <antoshkka at gmail dot com> ---
> But that's because nothing in the function asserts this?  Without fully
> specializing and unrolling on the constant "hello" argument at least.

Yes, I was hoping for that unrolling to happen

Probably a more simplified case:


constexpr bool EqualICase(const char* lowercase, const char* y) noexcept {
    for (;;) {
        const auto lowercase_c =3D *lowercase;
        if (!lowercase_c) return true;
        if (lowercase_c !=3D *y) {
            return false;
        }
        ++lowercase;
        ++y;
    }
}
bool test2(const char* y) {
    return EqualICase("he", y);
}

With range info for loads from read-only constants I'd expect this to become
just a

test2(char const*):
        cmp     BYTE PTR [rdi], 104
        jne     .L3
        cmp     BYTE PTR [rdi+1], 101
        sete    al
        ret
.L3:
        xor     eax, eax
        ret


rather than a fair loop with checks for \0

Godbolt: https://godbolt.org/z/z6rTYEzWx=