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From: "cassio.neri at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/107564] New: Fail to recognize overflow check
 for addition of __uint128_t operands
Date: Tue, 08 Nov 2022 00:00:40 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D107564

            Bug ID: 107564
           Summary: Fail to recognize overflow check for addition of
                    __uint128_t operands
           Product: gcc
           Version: 12.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: cassio.neri at gmail dot com
  Target Milestone: ---

Consider:

char f128(__uint128_t m, __uint128_t n) {
    #if !defined(USE_BUILTIN_ADD_OVERFLOW)
        m +=3D n;
        return m < n;
    #else
        __uint128_t r;
        return __builtin_add_overflow(m, n, &r);
    #endif
}

When USE_BUILTIN_ADD_OVERFLOW is undefined, GCC fails to recognise this is =
an
overflow check and with -O3 generates this:

        mov     r8, rdi
        mov     rax, rsi
        mov     rdi, rax
        mov     rsi, r8
        mov     rax, rdx
        mov     rdx, rcx
        add     rsi, rax
        adc     rdi, rcx
        cmp     rsi, rax
        mov     rcx, rdi
        sbb     rcx, rdx
        setc    al
        ret

When USE_BUILTIN_ADD_OVERFLOW is defined, it generates better code but still
suboptimal:

        mov     r8, rdi
        mov     rax, rsi
        mov     rsi, r8
        mov     rdi, rax
        add     rsi, rdx
        adc     rdi, rcx
        setc    al
        ret

For other unsigned integer types GCC generates the same optimal code for bo=
th
methods. For instance for uint64_t:

        add     rdi, rsi
        setc    al
        ret

https://godbolt.org/z/bj4M5no4j=