ref: 052abe1cc8dd6403f4983b1b587f14d3eed1fe8c
dir: /sys/src/libmp/mips/mpvecdigmulsub.s/
/* * mpvecdigmulsub(mpdigit *b, int n, mpdigit m, mpdigit *p) * * p -= b*m * * each step looks like: * hi,lo = m*b[i] * lo += oldhi + carry * hi += carry * p[i] += lo * oldhi = hi * * the registers are: * b = R1 * n = R4 * m = R5 * p = R6 * i = R7 * hi = R8 - constrained by hardware * lo = R9 - constrained by hardware * oldhi = R10 * tmp = R11 * */ TEXT mpvecdigmulsub(SB),$0 MOVW n+4(FP),R4 MOVW m+8(FP),R5 MOVW p+12(FP),R6 MOVW R0, R10 /* oldhi = 0 */ _mulsubloop: MOVW 0(R1), R9 /* lo = b[i] */ ADDU $4, R1 MOVW 0(R6), R11 /* tmp = p[i] */ MULU R9, R5 MOVW HI, R8 /* hi = (b[i] * m)>>32 */ MOVW LO, R9 /* lo = b[i] * m */ ADDU R10, R9 /* lo += oldhi */ SGTU R10, R9, R2 ADDU R2, R8 /* hi += carry */ SUBU R9, R11, R3 /* tmp -= lo */ SGTU R3, R11, R2 ADDU R2, R8 /* hi += carry */ MOVW R3, 0(R6) /* p[i] = tmp */ ADDU $4, R6 MOVW R8, R10 /* oldhi = hi */ SUBU $1, R4 BNE R4, _mulsubloop MOVW 0(R6), R11 /* tmp = p[i] */ SUBU R10, R11, R3 /* tmp -= oldhi */ MOVW R3, 0(R6) /* p[i] = tmp */ SGTU R3, R11, R1 BNE R1, _mulsub2 MOVW $1, R1 /* return +1 for positive result */ RET _mulsub2: MOVW $-1, R1 /* return -1 for negative result */ RET