On Mon, Jul 11, 2011 at 2:02 PM, Paulo Marques <pmarq...@grupopie.com> wrote:
> Some time ago I toyed with trying to build a super-optimizer for the > avr. The project didn't went very far, but it produced a few small bits > of code that I think are worth sharing. > > Basically I was trying to produce functions to multiply an 8 bit value > by 8 bit constants with a 16 bit result in avr families without a mul > instruction. Your results looked similar to what I've seen out of the "star-chain" multiplication routine from Doctor Dobb's Journal #125, March 1987. [The old multiplication one is not on line, but the newer division one is, with broken formatting: http://drdobbs.com/high-performance-computing/184408499 ] Curious if you were doing the some thing similar? > multiply x129: > MOV r2, r0 > ADD r0, r0 > ADC r1, r2 > ROR r1 > ROR r0 [Output from the 'starchain.exe' I hacked together long ago:] /* The factors of 129 are: 3 43 */ Star-Chain Multiply of 129: /* Multiplier = 129 */ Rw = R1 = /* Your Multiplicand */ Rw <<= 7; Rw += R1; Star-Chain Division by 129: R1 = /* Your divisor */ Rw = R1; Rw <<= 7; Rw -= R1; Rw <<= 2; Rw += 511; Rw >>= 16; > multiply x254 > NEG r0 > SBC r1, r0 > SBC r0, r1 > SBC r1, r2 /* The factors of 254 are: 2 127 */ Star-Chain Multiply of 254: /* Multiplier = 254 */ Rw = R1 = /* Your Multiplicand */ Rw <<= 7; Rw -= R1; Rw <<= 1; Star-Chain Division by 254: R1 = /* Your divisor */ R1 >>= 1; Rw = R1; Rw <<= 7; Rw += R1; Rw <<= 2; Rw += 519; Rw >>= 16; > multiply x255 > NEG r0 > SBC r1, r0 /* The factors of 255 are: 3 5 17 */ Star-Chain Multiply of 255: /* Multiplier = 255 */ Rw = R1 = /* Your Multiplicand */ Rw <<= 8; Rw -= R1; Star-Chain Division by 255: R1 = /* Your divisor */ Rw = R1; Rw <<= 8; Rw += R1; Rw += 257; Rw >>= 16; _______________________________________________ AVR-GCC-list mailing list AVR-GCC-list@nongnu.org https://lists.nongnu.org/mailman/listinfo/avr-gcc-list
