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111 lines
2.2 KiB
111 lines
2.2 KiB
/*
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* A fast checksum routine using movem
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* Copyright (c) 1998-2001, 2003 Axis Communications AB
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*
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* csum_partial(const unsigned char * buff, int len, unsigned int sum)
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*/
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.globl csum_partial
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csum_partial:
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;; r10 - src
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;; r11 - length
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;; r12 - checksum
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;; check for breakeven length between movem and normal word looping versions
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;; we also do _NOT_ want to compute a checksum over more than the
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;; actual length when length < 40
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cmpu.w 80,$r11
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blo _word_loop
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nop
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;; need to save the registers we use below in the movem loop
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;; this overhead is why we have a check above for breakeven length
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;; only r0 - r8 have to be saved, the other ones are clobber-able
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;; according to the ABI
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subq 9*4,$sp
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subq 10*4,$r11 ; update length for the first loop
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movem $r8,[$sp]
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;; do a movem checksum
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_mloop: movem [$r10+],$r9 ; read 10 longwords
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;; perform dword checksumming on the 10 longwords
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add.d $r0,$r12
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addc $r1,$r12
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addc $r2,$r12
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addc $r3,$r12
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addc $r4,$r12
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addc $r5,$r12
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addc $r6,$r12
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addc $r7,$r12
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addc $r8,$r12
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addc $r9,$r12
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;; fold the carry into the checksum, to avoid having to loop the carry
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;; back into the top
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addc 0,$r12
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addc 0,$r12 ; do it again, since we might have generated a carry
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subq 10*4,$r11
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bge _mloop
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nop
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addq 10*4,$r11 ; compensate for last loop underflowing length
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movem [$sp+],$r8 ; restore regs
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_word_loop:
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;; only fold if there is anything to fold.
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cmpq 0,$r12
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beq _no_fold
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;; fold 32-bit checksum into a 16-bit checksum, to avoid carries below.
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;; r9 and r13 can be used as temporaries.
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moveq -1,$r9 ; put 0xffff in r9, faster than move.d 0xffff,r9
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lsrq 16,$r9
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move.d $r12,$r13
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lsrq 16,$r13 ; r13 = checksum >> 16
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and.d $r9,$r12 ; checksum = checksum & 0xffff
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add.d $r13,$r12 ; checksum += r13
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move.d $r12,$r13 ; do the same again, maybe we got a carry last add
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lsrq 16,$r13
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and.d $r9,$r12
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add.d $r13,$r12
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_no_fold:
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cmpq 2,$r11
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blt _no_words
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nop
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;; checksum the rest of the words
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subq 2,$r11
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_wloop: subq 2,$r11
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bge _wloop
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addu.w [$r10+],$r12
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addq 2,$r11
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_no_words:
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;; see if we have one odd byte more
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cmpq 1,$r11
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beq _do_byte
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nop
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ret
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move.d $r12,$r10
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_do_byte:
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;; copy and checksum the last byte
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addu.b [$r10],$r12
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ret
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move.d $r12,$r10
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