Re: [avr-gcc-list] Optimizing a 16-bit * 8-bit -> 24-bit multiplication

[Galen Seitz]

Shaun Jackman wrote:
> I would like to multiply a 16-bit number by an 8-bit number and
> produce a 24-bit result on the AVR. The AVR has a hardware 8-bit *
> 8-bit -> 16-bit multiplier.
>
> If I multiply a 16-bit number by a 16-bit number, it produces a 16-bit
> result, which isn't wide enough to hold the result.
>
> If I cast one of the operands to 32-bit and multiply a 32-bit number
> by a 16-bit number, GCC generates a call to __mulsi3, which is the
> routine to multiply a 32-bit number by a 32-bit number and produce a
> 32-bit result and requires ten 8-bit * 8-bit multiplications.
>
> A 16-bit * 8-bit -> 24-bit multiplication only requires two 8-bit *
> 8-bit multiplications. A 16-bit * 16-bit -> 32-bit multiplication
> requires four 8-bit * 8-bit multiplications.
>
> I could write a mul24_16_8 (16-bit * 8-bit -> 24-bit) function using
> unions and 8-bit * 8-bit -> 16-bit multiplications, but before I go
> down that path, is there any way to coerce GCC into generating the
> code I desire?

Not exactly what you want, but this might help you get started.

galen


extern inline int16_t
mult_s16_u8s16(uint8_t a, int16_t b)
{
    int16_t product;
    asm (
        "mul %A2, %1"   "\n\t"
        "movw %0, r0"   "\n\t"
        "mulsu %B2, %1" "\n\t"
        "add %B0, r0"   "\n\t"
        "clr r1"        "\n\t"
        : "=w" (product)
        : "a" (a), "a" (b)
        );
    return product;
}