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brevzin opened this issue Apr 21, 2025 · 1 comment · Fixed by #4429
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Don't use __builtin_strlen #4423

brevzin opened this issue Apr 21, 2025 · 1 comment · Fixed by #4429
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@brevzin
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brevzin commented Apr 21, 2025

When upgrading {fmt}, b4d1d7f8e broke some of our usage. We had a place where we were building up a format string using a type like this:

template <size_t N>
struct fixed_string {
    char data[N] = {}; 

    constexpr fixed_string(char const (&m)[N]) {
        for (size_t i = 0; i != N; ++i) {
            data[i] = m[i];
        }
    }   
};

And trying to do something like:

static constexpr auto f = fixed_string("x={}");
fmt::print(f.data, 42);

This works on fmt 10.2.1 and 11.0, fails on trunk — after that commit, with:

/opt/compiler-explorer/libs/fmt/trunk/include/fmt/base.h:543:31: error: '__builtin_strlen(((const char*)(& f.fixed_string<5>::data)))' is not a constant expression
  543 |       size_ = __builtin_strlen(detail::narrow(s));
      |               ~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~

gcc says that __builtin_strlen isn't usable like this. Which... if they say so. But seems unlikely this will change, so I think {fmt} should also revert to previous usage.
@vitaut
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vitaut commented Apr 21, 2025

That's unfortunate but I agree that we shouldn't break this reasonable usage. A PR to revert and add a test case would be welcome.

@brevzin brevzin mentioned this issue Apr 26, 2025
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Avoiding __builtin_strlen brevzin/fmt
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@vitaut @brevzin