Caesar cipher in x86 ASM with MMX

caesar.s

	# MMX caeasar chiffre implementation
	# for i686 with MMX
	# signature:
	# caesar(out, in, len, key)
	# key is between 0 and 25

	.section .text
	.globl caesar
	.type caesar,@function
	.align 16
caesar:
	push %ebp
	mov %esp,%ebp
	push %ebx
	push %edi
	push %esi

	mov 8(%ebp),%edi		# edi: destination
	mov 12(%ebp),%esi		# esi: source
	mov 16(%ebp),%edx		# edx: length
	and $~7,%edx			# only process full qwords
	jz 1f

	xor %ecx,%ecx			# ecx: index
	movd 20(%ebp),%mm5
	punpcklbw %mm5,%mm5
	punpcklwd %mm5,%mm5
	punpckldq %mm5,%mm5		# mm5: key bytes

	movq Amask,%mm7			# for later use

	movq %mm5,%mm4
	psubb twentysix,%mm4		# mm4: key - 26

	movq %mm7,%mm6
	psubb %mm4,%mm6			# mm6: 'A' - 1 - (key - 26)

	.align 16
0:	movq (%esi,%ecx),%mm0
	movq %mm0,%mm1
	pand ucmask,%mm1		# mm1: xmm0 in upper case

	movq %mm1,%mm3
	movq %mm1,%mm2

	pcmpgtb %mm7,%mm2		# mm2: 0xff where 'A' <= buf[i]
	pcmpgtb Zmask,%mm3		# mm3: 0xff where 'Z' < buf[i]
	pcmpgtb %mm6,%mm1		# mm1: 0xff where 'A' + key <= buf[i]

	pandn %mm1,%mm3
	pand %mm4,%mm3			# mm3: key-26 where 'A' + (26 - key) <= buf[i] <= 'Z'

	pandn %mm2,%mm1
	pand %mm5,%mm1			# mm1: key where 'A' <= buf[i] < 'A' + (26 - key)

	por %mm3,%mm1			# mm1: 0/key/key-26
	paddb %mm1,%mm0
	movq %mm0,(%edi,%ecx)

	add $8,%ecx
	cmp %edx,%ecx
	jb 0b
	emms
	add %ecx,%edi
	add %ecx,%esi

1:	mov 16(%ebp),%edx		# length
	mov 20(%ebp),%ecx		# key
	and $7,%edx			# loop tail
	jz 1f				# all bytes processed alread?

	# process remaining bytes
	add %edx,%edi			# end of output buffer
	add %edx,%esi			# end of input buffer
	neg %edx			# index counts up to 0

	lea -26(%ecx),%ebx		# bl: key - 26
	mov $'A'-1,%ah
	sub %bl,%ah			# ah: threshold ('A' - 1 - (key - 26))

	.align 16
0:	mov (%esi,%edx),%al
	and $~0x20,%al			# al: uppercase c
	cmp $'Z'+1,%al			# cf: uc <= 'Z'
	sbb %ch,%ch			# ch: 0xff if uc <= 'Z'
	cmp %al,%ah			# cf: thresh < uc
	sbb %bh,%bh			# bh: 0xff if thresh < uc
	and %bh,%ch
	and %al,%ch			# ch: key - 26 & isleZ & isgttthres
	not %bh				# bh: ~isgtthresh
	cmp $'A',%al
	cmc				# cf: 'A' <= uc
	sbb %al,%al			# al: 0xff if 'A' <= uc
	and %bh,%al
	and %cl,%al			# al: key & isgeA & ~isgtthresh
	or %ch,%al
	add (%esi,%edx),%al
	mov %al,(%edi,%edx)
	inc %edx
	jnz 0b

1:	pop %esi
	pop %edi
	pop %ebx
	leave
	ret
	.size caesar,.-caesar

	.section .rodata.cst8,"aM",@progbits,8
	.align 8
	.type ucmask,@object
ucmask:	.fill 8, 1, ~0x20
	.size ucmask, 8
	.type Amask,@object
Amask:	.fill 8, 1, 'A' - 1
	.size Amask, 8
	.type Zmask,@object
Zmask:	.fill 8, 1, 'Z'
	.size Zmask, 8
	.type twentysix,@object
twentysix:
	.fill 8, 1, 26
	.size twentysix, 8

main.c

#define _POSIX_C_SOURCE 200809L
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

extern void caesar(const char *out, char *in, size_t len, int key);

extern int
main(int argc, char *argv[])
{
	char *buf = NULL;
	size_t len = 0;
	ssize_t count;
	int key = 13;

	if (argc > 1)
		key = atoi(argv[1]) % 26;

	while (count = getline(&buf, &len, stdin), count != EOF) {
		caesar(buf, buf, strlen(buf), key);
		fputs(buf, stdout);
	}

	return (EXIT_SUCCESS);
}