<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/76684>76684</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            Clang considers the non-default variant of a multiversioned function unused
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            clang
      </td>
    </tr>

    <tr>
      <th>Assignees</th>
      <td>
      </td>
    </tr>

    <tr>
      <th>Reporter</th>
      <td>
          nabijaczleweli
      </td>
    </tr>
</table>

<pre>
    repro.cpp:
```cpp
#include <cstdint>
#include <cstdio>
#include <cstring>
#include <sys/mman.h>
#include <unistd.h>
#if __i386__ || __x86_64__
#include <nmmintrin.h>
#else
#error
#endif

namespace {
        __attribute__((target("default"))) std::uint32_t crc32c(std::uint32_t cur, const void * data, std::size_t data_len) {
                static const constexpr std::uint32_t crc_lut[] = {
                 0x00000000, 0xF26B8303, 0xE13B70F7, 0x1350F3F4, 0xC79A971F, 0x35F1141C, 0x26A1E7E8, 0xD4CA64EB, 0x8AD958CF, 0x78B2DBCC, 0x6BE22838, 0x9989AB3B,
                    0x4D43CFD0, 0xBF284CD3, 0xAC78BF27, 0x5E133C24, 0x105EC76F, 0xE235446C, 0xF165B798, 0x030E349B, 0xD7C45070, 0x25AFD373, 0x36FF2087, 0xC494A384,
                    0x9A879FA0, 0x68EC1CA3, 0x7BBCEF57, 0x89D76C54, 0x5D1D08BF, 0xAF768BBC, 0xBC267848, 0x4E4DFB4B, 0x20BD8EDE, 0xD2D60DDD, 0xC186FE29, 0x33ED7D2A,
                    0xE72719C1, 0x154C9AC2, 0x061C6936, 0xF477EA35, 0xAA64D611, 0x580F5512, 0x4B5FA6E6, 0xB93425E5, 0x6DFE410E, 0x9F95C20D, 0x8CC531F9, 0x7EAEB2FA,
                    0x30E349B1, 0xC288CAB2, 0xD1D83946, 0x23B3BA45, 0xF779DEAE, 0x05125DAD, 0x1642AE59, 0xE4292D5A, 0xBA3A117E, 0x4851927D, 0x5B016189, 0xA96AE28A,
                 0x7DA08661, 0x8FCB0562, 0x9C9BF696, 0x6EF07595, 0x417B1DBC, 0xB3109EBF, 0xA0406D4B, 0x522BEE48, 0x86E18AA3, 0x748A09A0, 0x67DAFA54, 0x95B17957,
                    0xCBA24573, 0x39C9C670, 0x2A993584, 0xD8F2B687, 0x0C38D26C, 0xFE53516F, 0xED03A29B, 0x1F682198, 0x5125DAD3, 0xA34E59D0, 0xB01EAA24, 0x42752927,
                    0x96BF4DCC, 0x64D4CECF, 0x77843D3B, 0x85EFBE38, 0xDBFC821C, 0x2997011F, 0x3AC7F2EB, 0xC8AC71E8, 0x1C661503, 0xEE0D9600, 0xFD5D65F4, 0x0F36E6F7,
                    0x61C69362, 0x93AD1061, 0x80FDE395, 0x72966096, 0xA65C047D, 0x5437877E, 0x4767748A, 0xB50CF789, 0xEB1FCBAD, 0x197448AE, 0x0A24BB5A, 0xF84F3859,
                    0x2C855CB2, 0xDEEEDFB1, 0xCDBE2C45, 0x3FD5AF46, 0x7198540D, 0x83F3D70E, 0x90A324FA, 0x62C8A7F9, 0xB602C312, 0x44694011, 0x5739B3E5, 0xA55230E6,
                 0xFB410CC2, 0x092A8FC1, 0x1A7A7C35, 0xE811FF36, 0x3CDB9BDD, 0xCEB018DE, 0xDDE0EB2A, 0x2F8B6829, 0x82F63B78, 0x709DB87B, 0x63CD4B8F, 0x91A6C88C,
                    0x456CAC67, 0xB7072F64, 0xA457DC90, 0x563C5F93, 0x082F63B7, 0xFA44E0B4, 0xE9141340, 0x1B7F9043, 0xCFB5F4A8, 0x3DDE77AB, 0x2E8E845F, 0xDCE5075C,
                    0x92A8FC17, 0x60C37F14, 0x73938CE0, 0x81F80FE3, 0x55326B08, 0xA759E80B, 0xB4091BFF, 0x466298FC, 0x1871A4D8, 0xEA1A27DB, 0xF94AD42F, 0x0B21572C,
                    0xDFEB33C7, 0x2D80B0C4, 0x3ED04330, 0xCCBBC033, 0xA24BB5A6, 0x502036A5, 0x4370C551, 0xB11B4652, 0x65D122B9, 0x97BAA1BA, 0x84EA524E, 0x7681D14D,
                    0x2892ED69, 0xDAF96E6A, 0xC9A99D9E, 0x3BC21E9D, 0xEF087A76, 0x1D63F975, 0x0E330A81, 0xFC588982, 0xB21572C9, 0x407EF1CA, 0x532E023E, 0xA145813D,
                 0x758FE5D6, 0x87E466D5, 0x94B49521, 0x66DF1622, 0x38CC2A06, 0xCAA7A905, 0xD9F75AF1, 0x2B9CD9F2, 0xFF56BD19, 0x0D3D3E1A, 0x1E6DCDEE, 0xEC064EED,
                    0xC38D26C4, 0x31E6A5C7, 0x22B65633, 0xD0DDD530, 0x0417B1DB, 0xF67C32D8, 0xE52CC12C, 0x1747422F, 0x49547E0B, 0xBB3FFD08, 0xA86F0EFC, 0x5A048DFF,
                    0x8ECEE914, 0x7CA56A17, 0x6FF599E3, 0x9D9E1AE0, 0xD3D3E1AB, 0x21B862A8, 0x32E8915C, 0xC083125F, 0x144976B4, 0xE622F5B7, 0xF5720643, 0x07198540,
                    0x590AB964, 0xAB613A67, 0xB831C993, 0x4A5A4A90, 0x9E902E7B, 0x6CFBAD78, 0x7FAB5E8C, 0x8DC0DD8F, 0xE330A81A, 0x115B2B19, 0x020BD8ED, 0xF0605BEE,
                    0x24AA3F05, 0xD6C1BC06, 0xC5914FF2, 0x37FACCF1, 0x69E9F0D5, 0x9B8273D6, 0x88D28022, 0x7AB90321, 0xAE7367CA, 0x5C18E4C9, 0x4F48173D, 0xBD23943E,
                 0xF36E6F75, 0x0105EC76, 0x12551F82, 0xE03E9C81, 0x34F4F86A, 0xC69F7B69, 0xD5CF889D, 0x27A40B9E, 0x79B737BA, 0x8BDCB4B9, 0x988C474D, 0x6AE7C44E,
                    0xBE2DA0A5, 0x4C4623A6, 0x5F16D052, 0xAD7D5351};

                auto bytes = reinterpret_cast<const std::uint8_t *>(data), end = bytes + data_len;
                for(; bytes != end; ++bytes)
                        cur = (cur >> 8) ^ crc_lut[(cur & 0xFF) ^ *bytes];
                return cur;
        }

#if __i386__ || __x86_64__
        __attribute__((target("sse4.2"))) std::uint32_t crc32c(std::uint32_t cur_r, const void * data, std::size_t data_len) {
#if __x86_64__  // this isn't a simple std::size_t+#if _LP64 because x32 is amd64 ILP32
                using sse42_t = std::uint64_t;
#define _mm_crc32_big _mm_crc32_u64
#else
                using sse42_t = std::uint32_t;
#define _mm_crc32_big _mm_crc32_u32
#endif

                sse42_t cur = cur_r;

                auto bulk_iter = reinterpret_cast<const sse42_t *>(data);
                for(auto bulk = data_len / sizeof(sse42_t); bulk; --bulk, ++bulk_iter)
                        cur = _mm_crc32_big(cur, *bulk_iter);

                auto single_iter = reinterpret_cast<const std::uint8_t *>(bulk_iter);
                for(auto single = data_len % sizeof(sse42_t); single; --single, ++single_iter)
                        cur = _mm_crc32_u8(cur, *single_iter);

                return cur;
        }
#endif
}

int main() {
        auto m = mmap(nullptr, 1024ull * 1024 * 1024 , PROT_READ, MAP_PRIVATE | MAP_ANONYMOUS | MAP_POPULATE, -1, 0);

        int ret{};
        for(int i = 0; i < 1000; ++i)
                ret ^= crc32c(~0, m, 1024ull * 1024 * 1024 );
        std::printf("%08x\n", ret);
}
```

```
$ c++  -O3 -g -Wall -Wextra -fno-exceptions -fno-rtti -std=c++20 -D_FILE_OFFSET_BITS=64 -DTEXTDOMAIN_DIRNAME='"/usr/local/share/locale"'   repro.cpp    -o repro
repro.cpp:16:50: warning: unused function 'crc32c' [-Wunused-function]
   16 |         __attribute__((target("sse4.2"))) std::uint32_t crc32c(std::uint32_t cur_r, const void * data, std::size_t data_len) {
      | ^~~~~~
1 warning generated.
```
with
```
$ c++ --version
Debian clang version 18.0.0 (++20231231112334+c7c912cff945-1~exp1~20231231112352.433)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /bin
```

GCC 12.2.0-14 correctly doesn't warn about this.
</pre>
<img width="1px" height="1px" alt="" src="http://email.email.llvm.org/o/eJzUWU1v47iW_TXKhnBAXn4vsuDnoIDurkJ3vemZlSHbSqI3thxIcnX6Lfq3DyRdqpKUM1WDXr0gSEyRl_fw8pwjia6HoX3omuaukr6S8aa-jI_n_q6rd-0_6_2_js0fzbG92Z0Pf971zVN_vt0_PVXcVTRW1FWKLr_TxeUK8LbbHy-HhlQ87Ifx0HZjxdO7vef3O_u2e3ind_hzqCCfTnV3-_jOkEvXDuPhTfc92W5bbtR2SyodKh3Idvts1FaJ7fbaJN3p1HYTkNfzNMeh-dro-3P_tdUd2ntszX-7-tQMT_W-IZX2pcNut_U49u3uMjbbbQWmAjPW_UMzzp_h0NzXl-NYAVRgl18yjIep8Nxd2m7ksB3Jvt9z2FdgrnRd-goC2Z-7YSRfzu2BVODIoR7r6fI6fmj_1WzH-fr22HRTmpcoK2qHsR7bPU40_22en_rrYLbHy7jQiFQ8vp2J0GeKPxMG-pxBecMpX1qJca9p1kuLcUkzz2JpBW2d1SwvLS4zY4KFpQXKsaSTWVpRBKdE8kvLuGilCRinjYfoA8YpnwAMxzhrjXWeT3GvMBNC6LOIgoccEbbPYESICNsFbXwGhC0T4zwAwmZUpqAVpk_ApRAK02empNcW01NOExcWYUcdhKQa84F0OXKN-bjKGajBfEFY4bgRV2FbZ7TNDqdRJgUWHE6jvQ8pS5zG2KhVkAhbRhap8QjbZa2M9wjbB1DaCIQtkojZC4QN1EeTYsJFQFQ0xohAmVE5TUSeF8FT1BHcVdhJg2Y2MKyhFMG6AFgnxYKyXGENhdbJcYlAnRJRMYyThmYpGcYJL7NTCeO85QJkwjgVcxKMImybrQxAEbYJQXKWEbZOLnnI12HjDmL6AMYE5zF9ZNFwKzA9cM-9E5g-a21jcpieSgYyOkzPlACXJKZPAixE6XARjjvGNMYJI5kFjXHSU6aYwThnlUtgvoFNn3V01CiFmE0OnkqFmG2wPiuLmFXKVEuLmAXTnsWVE5xRm1a-UEFVLJyQAD6lwhejEjNu5aAwjtqVnzq67AoHrfRM25mf35Y6eAdCroqwwQa1qsVZy6XBaaLJ4FVRCw3cRFgVmCSXbFVnpNxBUSDLygAr6sQ9KYrnIkm7ugFlybmieAFagoXrsK3yWcTVf0QUIa3epI3gkRffkin7VLwp-hwMrH5nraZs9UIXdIbid8G4oFnxQhaUYnL110SjVav3RhmVLP5KM1dJ5euwUXKFFtxFRlfK0BwTL7TQYJWihTJOyUDFSknBtdErXbXS0_ZjDSUNWRe6Js9y8KsErBbCrPJwILwvEshGZG5meXwLG4KRMqwKTCnFvKoz-gShKJDnKF0u6tTMGilWA-CZR72aA3UcRMb0CoJxupiDVxQCXw1HKCvoakaaW8-L4TgpgdPZjN7oMXvBaFjNzoIzeTVCp50OxeySYSznYoQ8RG_96rXJU2ZWH46JJg-IGbLxyhQfNpAV9xr5oqmN3mjkkuIhCm-QZ5Y5FYwJ1--PUgUXFKrMa6ohK6SWE1LHYJF2UvEgs0VKUsyPu-mESNRjXLJMMC4wjnmdLRUYF7KXWTiEzWNMWrtyE0omGSERdgxJUi2vw8byYnpFA9eZYXrNLTchYXrDsqE5YXopOShPMb3T0iZDMb0X1DKfMb1QCqzJqFxmNHMiYlxyzIGOGJetcFEAxlEPTGq4Djvm5DkPCBuioZ4GhM1TpIJzhB2C94Hy4luLdJAykgLlyhVD55oGKZFqnjEvlEQaKhkZgEfKWO2dYx7pZERyEgRSTSvDIhPxuh6NhRQVThNdtiopnCZYZ220OA33AViyyOaUqdFOI2wWFc9WI2yaOKfOIOwcpDHWIGwsIeYTVKfMAuaTHBIFjvkcE9Iw_g1s-qylyUlGzG10EkpFzG2FF1YC5lYqZqYAc3MTAjiKccE57SzFuGizli5jHHgbos0Yl7NUPjLETCOPPDHEzJKKISbEnAJVIqXrpcbbXOEES8rJlS_glVSFE3F6QpOFLxTv6whG6cBhpauEEBgUKmuhBRS6CiuFTqsEPM85rvIwKtNUJCAdFSbO8vgWtkkhTbJHMgUnlVvVmbO0tihw4gpzRZ1YpmIAzBsFqzlAMpZJTB-o4QyKOTAhrFar4SiALFczkhqoKoZD8aZwFba01Hm7-p1XjLvVCw1nwRa_E0464YoX2mQppNVuQ_YurlacnZfJIGwTA42xWDFyvtCCSQ9-pQw-g-MiqKLSz5S5okfhHM8rK1VgPqyMlZaJXFjJdXYhFMYqm2ymqwq8Ac1XhZgIhhYVaOct5UUhLmmu9KrAwEwSqzqzMExzhO0jcCv4N7DpMz6mFPXj-xUWAqRkuag_UZ5sKM7ARRbZrG6jbNZ-dSIZsjHFbUA7QX1xIm295no1Ox-DF6sRGhOEFhinXNJBiOul9gmio6vXBqGArz6cmYq0eK2LOk5PpZWOFfcvjxGW-erLeCa7P8dmmN-w-6btxqZ_6ptxu6-HseJheVF_9XputuP09l9NFTXLEcC8hqY7zLPgfOC_ngPwV2_u9-e-AlNxvw5lU1zTHaZrFfgK_NwzTfwirqJ2f-mXswAwy8dU8UTMfNAg08tDAxwAavbBMqACt8ws4xtQfTNe-m4-6HjZoeOrqv3Ymc93z2OGoRG38LeOY7Z__0AGF1PAE1JNDpzJ-NgOpB26CvRIajK0p6dj83bOeZvmCX76pATZNfv6MjTkmQNpB1KfDkqQDz994vCyyJeh7R7ItPppFdNGvlqbEtvxa_WBH5r7tmvI9nTazsXY7tqHF62LEteO0H4k01TF_0-mdRlXTuTwYAtTFYYuW_S-6C7H_9m2Y9N_R3hlAW_1dk1R68TznGXPp10l056d7ycuLRMuU8yDp_-bzfwJQlFfQfe-Al-ValHbEu9eBb9bgGl_js2PlOBd73kn0ZuCLInelkS-V5Jl-FIU_LyW5QXmHynMxbyqy5voa5X5ngm9Zt9rb2q7kZzqtpsd5vW561yH04zudKqfKjDd5Xh8GmdkjIK4HI-zf0yfX34I5NOvHz9vf03L6_PP7tP2068f_tN9TpP3zW33y8df_vvnj__4bb3y6eOnf_zkPs_3vM1yx7y63glw34wT0Fe3J9y8qbudMdNpN6aPE1hKv94k2je70Dfj5PKz_Ip5_jU_Hp2-t9JXBFop99S33Xi_eHYFkprnSoauWp5IJuwv49btKN9ivFrvm4sgyH5ZBCGbj5xsHsjm9_p4JJvfm-exr8nmvjtvmud98zS2525Y2v04tmQzw4sYDpRs4jZ_-CltP-b8W_q89R8-_1bxqATZxM_pvz7Hjz-7D79s44dff3E_T8WpQM9LyJehryAfz_v6WEEeHuu-Ke1mHqEJIevXNNPDx-a8tJdVvPwGh6mKO0kr7sgfdd_N37U4cukuQ3Mg95duPy2DVKDLzmhSSb_5fRmxKSOmG_M8NyGEqZlT5eff4Z66IJ1QVzL9Nf0s11kpCnlouqavx-Zwe5UYf7Tj43cZs9l8afphqtbcFZtdW3dkf6y7B4I9hJlbekvJXJuFJ8AZcMYYcC4q8Hu9twz29_dWyA37q3l-Yn-9HCThVkzvdaiwz0upuSPL88Lmab85tt3lefPQXXDIY9_UB3I6H5rjNPDpPLTPS9eHbhjr47E5xLafuirIu7b7P_TyHyEQBrdwSzdMkP2575v9ePyTHM4NPphMBSX17nwZ5weW25vDHT9Ybuub5o5pKhgoacXN4x3YXc12nO11fV_Xu7o27H5Pd4paqi1j4qa9AwqCMsqYAivFrTpIIYy0TcPZTjaiErQ51e3x9nj8cro99w837TBcmjutlBE3x3rXHIf5C0-AeRMmEsp4099N4ze7y8NQCXpsh3H4OsPYjsfmLsx7NjGuPTT9QMbHhnTnboNf2JEvdd_W3UjO96Qmp8txbHF_X4pqkdDNpT_ePY7j0zDRdH6Se2jHx8vudn8-TbI-fin_Nk_9-Z_NfqwgzwsZKsjzWv43AAD__8R7ntA">