原创 GCC-AVR编译 算术运算中 字符型将被扩展为int类型运算

http://www.ouravr.com/bbs/bbs_content.jsp?bbs_sn=838295&bbs_page_no=1&bbs_id=1000

编译器使用：WinAVR-20070525

        uint l=0;
        uchar i=0;
        uchar j=0;
        uchar tmp[6];

tmp[4]=0xA0;
tmp[5]=0;
j=0x40;
问题是，下面第一种编译出来if条件竟然不成立，我明明把变量已经定义为无符号类型。
将j-tmp[4]的结果强制转换后，才能成立。
第一种情况下编译结果
        if(((j-tmp[4])==tmp[4])&&(tmp[5]==0))   //
+00000507:   811D        LDD     R17,Y+5          Load indirect with displacement
+00000508:   2F21        MOV     R18,R17          Copy register
+00000509:   2733        CLR     R19              Clear Register
+0000050A:   2F80        MOV     R24,R16          Copy register
+0000050B:   2799        CLR     R25              Clear Register
+0000050C:   1B82        SUB     R24,R18          Subtract without carry
+0000050D:   0B93        SBC     R25,R19          Subtract with carry
+0000050E:   1728        CP      R18,R24          Compare
+0000050F:   0739        CPC     R19,R25          Compare with carry
+00000510:   F419        BRNE    PC+0x04          Branch if not equal
+00000511:   818E        LDD     R24,Y+6          Load indirect with displacement
+00000512:   2388        TST     R24              Test for Zero or Minus
+00000513:   F081        BREQ    PC+0x11          Branch if equal
第二种编译结果

if(((uchar)(j-tmp[4])==tmp[4])&&(tmp[5]==0))   //
+00000507:   811D        LDD     R17,Y+5          Load indirect with displacement
+00000508:   2EF0        MOV     R15,R16          Copy register
+00000509:   1AF1        SUB     R15,R17          Subtract without carry
+0000050A:   151F        CP      R17,R15          Compare
+0000050B:   F419        BRNE    PC+0x04          Branch if not equal
+0000050C:   818E        LDD     R24,Y+6          Load indirect with displacement
+0000050D:   2388        TST     R24              Test for Zero or Minus
+0000050E:   F079        BREQ    PC+0x10          Branch if equal

在第一种运算中j-tmp[4]被扩展为整型运算，通过反汇编看到tmp[4]-->R19,R18;     j-->R25,R24中。

所以当j>tmp[4]的时候，可以得到预期的结果，当j<tmp[4]的时候就出问题了。如果将j-tmp[4]得值赋给第三个变量，例如k=j-tmp[4]，再判断k?=tmp[4]得时候,也可以得到预期得结果。

感谢yyccaa 给出了解释。http://www.ouravr.com/bbs/bbs_content.jsp?bbs_sn=795415

http://www.avrfreaks.net/index.php?name=PNphpBB2&file=viewtopic&t=53586

在ISO C99规范中相关得解释：P79,

1 The operand of the unary + or - operator shall have arithmetic type; of the ˜ operator,
integer type; of the ! operator, scalar type.
Semantics
2 The result of the unary + operator is the value of its (promoted) operand. The integer
promotions are performed on the operand, and the result has the promoted type.
3 The result of the unary - operator is the negative of its (promoted) operand. The integer
promotions are performed on the operand, and the result has the promoted type.