[Bug target/82858] __builtin_add_overflow() generates suboptimal code with unsigned types on x86

glisse at gcc dot gnu.org gcc-bugzilla@gcc.gnu.org
Mon Nov 6 12:33:00 GMT 2017


https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82858

--- Comment #4 from Marc Glisse <glisse at gcc dot gnu.org> ---
    unsigned c;
    unsigned d = __builtin_add_overflow(a, b, &c)?-1:0;
    return c|d;

gives the expected asm. Ideally phiopt would recognize a saturing add pattern,
but we have nothing to model it in gimple. We could turn it into the branchless
BIT_IOR form though.

(the problem isn't with __builtin_add_overflow but with what comes afterwards)


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