  int64_t a, b, c, r;

  r = (a * b) / c; /* multiplication step could overflow so use 128bits */

▲ cmovq 2 hours ago | parent [-]

Last time I checked LLVM had surprisingly bad codegen for this using int128. On x86 you only need two instructions:

    __asm (
        "mulq %[multiplier]\n"
        "divq %[divisor]\n"
        : "=a"(result)
        : "a"(num), [multiplier]"r"(multiplier), [divisor]"r"(divisor)
        : "rdx"
    );

The intermediate 128bit number is in rdx:rax.

	▲	bonzini an hour ago \| parent [-]
		That only works if you are sure to have a 64-bit result. If you can have divisor < multiplier and need to detect overflow, it's more complicated.