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? Cheers, Shaun
