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Pointless optimization 2 Comments

Filed under Programming by Gwynne Raskind

So I was looking at the macro used to calculate 16-bit parity in pure C without branching: 

#define parity(v)   ({ \
        uint16_t pv = (v); \
        pv ^= (uint16_t)(pv < < 8); \
        pv ^= (uint16_t)(pv << 4); \
        pv ^= (uint16_t)(pv << 2); \
        pv ^= (uint16_t)(pv << 1); \
        (uint16_t)(pv & 0x0001); \
    })

It uses GCC’s handy compound statement syntax, but otherwise it’s plain old C. Let’s look at the 64-bit ASM this compiles to at -Os: (more…)

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January 25, 2010 at 12:45 pm