Question

9.7.2 Byteops.exe

Byteops 是一个与 intops 类似的程序，差别在于这两个变量是字节并非整型数。反汇编的程序见图 9-18 所示，反编译的 C 版本在图 9-19，最初的 C 程序在图 9-20。该程序有下列调用图：

main printf

从程序的反汇编可知，局部变量被放在栈的偏移-1 和-2 上。和 intops.a2 程序相比，这个程序多出 22.41%的指令，因为有些机器指令比如 IDIV 取字寄存器作为操作数而不是用字节寄存器；因此，字节寄存器或被填补或被符号扩展以形成一个字寄存器。两个程序中最后的高级指令数目是相同的，因此在这个程序里面中间指令数目的缩减率比较大。它达到 82.76%，如图 9-21 所示。

	main	PROC NEAR
000	0002FA	55	PUSH	bp
001	0002FB	8BEC	MOV	bp,sp
002	0002FD	83EC02	SUB	sp,2
003	000300	C646FEFF	MOV	byte ptr [bp-2],0FFh
004	000304	C646FF8F	MOV	byte ptr [bp-1],8Fh
005	000308	8A46FE	MOV	al,[bp-2]
006	00030B	0246FF	ADD	al,[bp-1]
007	00030E	8846FF	MOV	[bp-1],al
008	000311	8A46FE	MOV	al,[bp-2]
009	000314	2A46FF	SUB	al,[bp-1]
010	000317	8846FE	MOV	[bp-2],al
011	00031A	8A46FE	MOV	al,[bp-2]
012	00031D	B400	MOV	ah,0
013	00031F	8A56FF	MOV	dl,[bp-1]
014	000322	B600	MOV	dh,0
015	000324	F7E2	MUL	dx
016	000326	8846FE	MOV	[bp-2],al
017	000329	8A46FF	MOV	al,[bp-1]
018	00032C	B400	MOV	ah,0
019	00032E	8A56FE	MOV	dl,[bp-2]
020	000331	B600	MOV	dh,0
021	000333	8BDA	MOV	bx,dx
022	000335	99	CWD
023			MOV	tmp,dx:ax	;Synthetic inst
024	000336	F7FB	IDIV	bx
025			MOD	bx	;Synthetic inst
026	000338	8846FF	MOV	[bp-1],al
027	00033B	8A46FF	MOV	al,[bp-1]
028	00033E	B400	MOV	ah,0
029	000340	8A56FE	MOV	dl,[bp-2]
030	000343	B600	MOV	dh,0
031	000345	8BDA	MOV	bx,dx
032	000347	99	CWD
033			MOV	tmp,dx:ax	;Synthetic inst
034	000348	F7FB	IDIV	bx
035			MOD	bx	;Synthetic inst
036	00034A	8856FF	MOV	[bp-1],dl
037	00034D	8A46FE	MOV	al,[bp-2]
038	000350	B105	MOV	cl,5
039	000352	D2E0	SHL	al,cl
040	000354	8846FE	MOV	[bp-2],al
041	000357	8A46FF	MOV	al,[bp-1]
042	00035A	8A4EFE	MOV	cl,[bp-2]
043	00035D	D2E8	SHR	al,cl
044	00035F	8846FF	MOV	[bp-1],al
045	000362	8A46FF	MOV	al,[bp-1]
046	000365	B400	MOV	ah,0
047	000367	50	PUSH	ax
048	000368	8A46FE	MOV	al,[bp-2]
049	00036B	B400	MOV	ah,0
050	00036D	50	PUSH	ax
051	00036E	B89401	MOV	ax,194h
052	000371	50	PUSH	ax
053	000372	E8AB06	CALL	near ptr printf
054	000375	83C406	ADD	sp,6
055	000378	8BE5	MOV	sp,bp
056	00037A	5D	POP	bp
057	00037B	C3	RET
	main	ENDP

图 9-18: Byteops.a2

/* * Input file : byteops.exe * File type : EXE */ #include "dcc.h" void main () /* Takes no parameters. * High-level language prologue code. */ { int loc1; int loc2; loc1 = 255; loc2 = 143; loc2 = (loc1 + loc2); loc1 = (loc1 - loc2); loc1 = (loc1 * loc2); loc2 = (loc2 / loc1); loc2 = (loc2 % loc1); loc1 = (loc1 << 5); loc2 = (loc2 >> loc1); printf ("a = %d, b = %d\n", loc1, loc2); }

图 9-19: Byteops.b

#define TYPE unsigned char main() { TYPE a, b; a = 255; b = 143; b = a + b; a = a - b; a = a * b; b = b / a; b = b % a; a = a << 5; b = b >> a; printf ("a = %d, b = %d\n", a, b); }

图 9-20: Byteops.c

子程序	低级	高级	% 缩减率
main	58	10	82.76
合计	58	10	82.76

图 9-21: Byteops 统计