ref: d540dcf6834b4bec8a2d21f2fe95eccf49f97b03
dir: /libmp/Plan9-amd64/mpvecsub.s/
/* * mpvecsub(mpdigit *a, int alen, mpdigit *b, int blen, mpdigit *diff) * * diff[0:alen-1] = a[0:alen-1] - b[0:blen-1] * * prereq: alen >= blen, diff has room for alen digits */ TEXT mpvecsub(SB),$0 /* MOVQ a+0(FP),SI */ MOVQ RARG, SI MOVQ b+16(FP),BX MOVL alen+8(FP),DX MOVL blen+24(FP),CX MOVQ diff+32(FP),DI SUBL CX,DX XORL BP,BP /* this also sets carry to 0 */ /* skip subraction if b is zero */ TESTL CX,CX JZ _sub1 /* diff[0:blen-1],borrow = a[0:blen-1] - b[0:blen-1] */ _subloop1: MOVL (SI)(BP*4),AX SBBL (BX)(BP*4),AX MOVL AX,(DI)(BP*4) INCL BP LOOP _subloop1 _sub1: INCL DX MOVL DX,CX LOOP _subloop2 RET /* diff[blen:alen-1] = a[blen:alen-1] - 0 */ _subloop2: MOVL (SI)(BP*4),AX SBBL $0,AX MOVL AX,(DI)(BP*4) INCL BP LOOP _subloop2 RET