[strlen的GNU实现］有趣的代码

先搁这吧，晚上回去分析。

1. #include<string.h>
2. #include<stdlib.h>
4. #undefstrlen
6. /*
7. Returnthelengthofthenull-terminatedstringSTR.Scanfor
8. thenullterminatorquicklybytestingfourbytesatatime.
9. */
10. size_tstrlen(str)
11. constchar*str;
12. {
13. constchar*char_ptr;
14. constunsignedlongint*longword_ptr;
15. unsignedlongintlongword,magic_bits,himagic,lomagic;
17. /*
18. Handlethefirstfewcharactersbyreadingonecharacteratatime.
19. DothisuntilCHAR_PTRisalignedonalongwordboundary.
20. */
21. for(char_ptr=str;((unsignedlongint)char_ptr
22. &(sizeof(longword)-1))!=0;++char_ptr)
24. if(*char_ptr=='/0')
25. returnchar_ptr-str;
27. /*
28. Alltheseelucidatorycommentsreferto4-bytelongwords,
29. butthetheoryappliesequallywellto8-bytelongwords.
30. */
31. longword_ptr=(unsignedlongint*)char_ptr;
33. /*
34. Bits31,24,16,and8ofthisnumberarezero.Callthesebits
35. the"holes."Notethatthereisaholejusttotheleftof
36. eachbyte,withanextraattheend:
38. bits:01111110111111101111111011111111
39. bytes:AAAAAAAABBBBBBBBCCCCCCCCDDDDDDDD
41. The1-bitsmakesurethatcarriespropagatetothenext0-bit.
42. The0-bitsprovideholesforcarriestofallinto.
43. */
44. magic_bits=0x7efefeffL;
45. himagic=0x80808080L;
46. lomagic=0x01010101L;
47. if(sizeof(longword)>4)
48. {
49. /*64-bitversionofthemagic.*/
50. /*Dotheshiftintwostepstoavoidawarningiflonghas32bits.*/
51. magic_bits=((0x7efefefeL<<16)<<16)|0xfefefeffL;
52. himagic=((himagic<<16)<<16)|himagic;
53. lomagic=((lomagic<<16)<<16)|lomagic;
54. }
55. if(sizeof(longword)>8)
56. abort();
58. /*
59. Insteadofthetraditionalloopwhichtestseachcharacter,
60. wewilltestalongwordatatime.Thetrickypartistesting
61. if*anyofthefour*bytesinthelongwordinquestionarezero.
62. */
63. for(;;)
64. {
65. /*WetentativelyexittheloopifaddingMAGIC_BITSto
66. LONGWORDfailstochangeanyoftheholebitsofLONGWORD.
68. 1)Isthissafe?Willitcatchallthezerobytes?
69. Supposethereisabytewithallzeros.Anycarrybits
70. propagatingfromitsleftwillfallintotheholeatits
71. leastsignificantbitandstop.Sincetherewillbeno
72. carryfromitsmostsignificantbit,theLSBofthe
73. bytetotheleftwillbeunchanged,andthezerowillbe
74. detected.
76. 2)Isthisworthwhile?Willitignoreeverythingexcept
77. zerobytes?SupposeeverybyteofLONGWORDhasabitset
78. somewhere.Therewillbeacarryintobit8.Ifbit8
79. isset,thiswillcarryintobit16.Ifbit8isclear,
80. oneofbits9-15mustbeset,sotherewillbeacarry
81. intobit16.Similarly,therewillbeacarryintobit
82. 24.Ifoneofbits24-30isset,therewillbeacarry
83. intobit31,soalloftheholebitswillbechanged.
85. Theonemisfireoccurswhenbits24-30areclearandbit
86. 31isset;inthiscase,theholeatbit31isnot
87. changed.Ifwehadaccesstotheprocessorcarryflag,
88. wecouldclosethisloopholebyputtingthefourthhole
89. atbit32!
91. Soitignoreseverythingexcept128's,whenthey'realigned
92. properly.*/
94. longword=*longword_ptr++;
96. if(
97. #if0
98. /*AddMAGIC_BITStoLONGWORD.*/
99. (((longword+magic_bits)
101. /*Setthosebitsthatwereunchangedbytheaddition.*/
102. ^~longword)
104. /*Lookatonlytheholebits.Ifanyoftheholebits
105. areunchanged,mostlikelyoneofthebyteswasa
106. zero.*/
107. &~magic_bits)
108. #else
109. ((longword-lomagic)&himagic)
110. #endif
111. !=0)
112. {
113. /*
114. Whichofthebyteswasthezero?Ifnoneofthemwere,itwas
115. amisfire;continuethesearch.
116. */
118. constchar*cp=(constchar*)(longword_ptr-1);
120. if(cp[0]==0)
121. returncp-str;
122. if(cp[1]==0)
123. returncp-str+1;
124. if(cp[2]==0)
125. returncp-str+2;
126. if(cp[3]==0)
127. returncp-str+3;
128. if(sizeof(longword)>4)
129. {
130. if(cp[4]==0)
131. returncp-str+4;
132. if(cp[5]==0)
133. returncp-str+5;
134. if(cp[6]==0)
135. returncp-str+6;
136. if(cp[7]==0)
137. returncp-str+7;
138. }
139. }
140. }
141. }
142. libc_hidden_builtin_def(strlen)