ref: a98f911a178d3cc044d612e5ad96ee487bd887c8
dir: /sys/src/libmp/amd64/mpvecdigmulsub.s/
/* * mpvecdigmulsub(mpdigit *b, int n, mpdigit m, mpdigit *p) * * p -= b*m * * each step look like: * hi,lo = m*b[i] * lo += oldhi + carry * hi += carry * p[i] += lo * oldhi = hi * * the registers are: * hi = DX - constrained by hardware * lo = AX - constrained by hardware * b = SI - can't be BP * p = DI - can't be BP * i = BP * n = CX - constrained by LOOP instr * m = BX * oldhi = R8 * */ TEXT mpvecdigmulsub(SB),$0 /* MOVL b+0(FP),SI */ MOVQ RARG,SI MOVL n+8(FP),CX MOVL m+16(FP),BX MOVQ p+24(FP),DI XORL BP,BP MOVL BP,R8 _mulsubloop: MOVL (SI)(BP*4),AX /* lo = b[i] */ MULL BX /* hi, lo = b[i] * m */ ADDL R8,AX /* lo += oldhi */ JCC _mulsubnocarry1 INCL DX /* hi += carry */ _mulsubnocarry1: SUBL AX,(DI)(BP*4) JCC _mulsubnocarry2 INCL DX /* hi += carry */ _mulsubnocarry2: MOVL DX,R8 INCL BP LOOP _mulsubloop SUBL R8,(DI)(BP*4) JCC _mulsubnocarry3 MOVQ $-1,AX RET _mulsubnocarry3: MOVQ $1,AX RET