{"id":7074,"date":"2023-02-03T15:14:51","date_gmt":"2023-02-03T07:14:51","guid":{"rendered":"https:\/\/yanjingang.com\/blog\/?p=7074"},"modified":"2023-02-03T15:17:48","modified_gmt":"2023-02-03T07:17:48","slug":"c%e7%bc%96%e8%af%91%e4%bc%98%e5%8c%96-likely","status":"publish","type":"post","link":"https:\/\/yanjingang.com\/blog\/?p=7074","title":{"rendered":"C++\u7f16\u8bd1\u4f18\u5316\u4e4b\u2014likely\/unlikely"},"content":{"rendered":"

\u4e00\u3001\u6982\u8ff0<\/h1>\n

\u5728\u4e00\u4e9b\u660e\u786e\u7684\u573a\u666f\u4e0b\uff0c\u7a0b\u5e8f\u5458\u6bd4CPU\u548c\u7f16\u8bd1\u5668\u66f4\u4e86\u89e3\u54ea\u4e2a\u5206\u652f\u6761\u4ef6\u66f4\u6709\u53ef\u80fd\u88ab\u6ee1\u8db3\u3002\u6211\u4eec\u662f\u5426\u53ef\u5c06\u8fd9\u4e00\u5148\u9a8c\u77e5\u8bc6\u544a\u77e5\u7f16\u8bd1\u5668\u548cCPU, \u63d0\u9ad8\u5206\u652f\u9884\u6d4b\u7684\u51c6\u786e\u7387\uff0c\u4ece\u800c\u51cf\u5c11CPU\u6d41\u6c34\u7ebf\u5206\u652f\u9884\u6d4b\u9519\u8bef\u5e26\u6765\u7684\u6027\u80fd\u635f\u5931\u5462\uff1f\u7b54\u6848\u662f\u53ef\u4ee5\uff01\u5b83\u4fbf\u662flikely\u548cunlikely\u3002\u5728Linux\u5185\u6838\u4ee3\u7801\u4e2d\uff0c\u8fd9\u4e24\u4e2a\u5b8f\u7684\u5e94\u7528\u6bd4\u6bd4\u7686\u662f\u3002\u4e0b\u9762\u662f\u4ed6\u4eec\u7684\u5b9a\u4e49\uff1a<\/p>\n

#define likely(x) __builtin_expect(!!(x), 1) \r\n#define unlikely(x) __builtin_expect(!!(x), 0)<\/code><\/pre>\n
likely\uff0c\u7528\u4e8e\u4fee\u9970if\/else if\u5206\u652f\uff0c\u8868\u793a\u8be5\u5206\u652f\u7684\u6761\u4ef6\u66f4\u6709\u53ef\u80fd\u88ab\u6ee1\u8db3\u3002\u800cunlikely\u4e0e\u4e4b\u76f8\u53cd
\n\u4ee5\u4e0b\u4e3a\u793a\u4f8b\u3002unlikely\u4fee\u9970argc > 0\u5206\u652f\uff0c\u8868\u793a\u8be5\u5206\u652f\u4e0d\u592a\u53ef\u80fd\u88ab\u6ee1\u8db3\u3002<\/p>\n
#include <cstdio>\r\n\r\n#define likely(x)\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 __builtin_expect(!!(x), 1)\r\n#define unlikely(x)\u00a0\u00a0\u00a0\u00a0 __builtin_expect(!!(x), 0)\r\n\r\nint main(int argc, char *argv[])\r\n{\r\n\u00a0\u00a0\u00a0\u00a0if (unlikely<\/span>(argc > 0)) {\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0puts (\"Positive\\n\");\r\n\u00a0\u00a0\u00a0\u00a0} else\r\n\u00a0\u00a0\u00a0\u00a0{\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0puts (\"Zero or Negative\\n\");\r\n\u00a0\u00a0\u00a0\u00a0}\r\n\u00a0\u00a0\u00a0\u00a0return 0;\r\n}<\/code><\/pre>\n
\u4e8c\u3001\u539f\u7406<\/h1>\n
\u63a5\u4e0b\u6765\uff0c\u6211\u4eec\u4ece\u6c47\u7f16\u6307\u4ee4\u5206\u6790likely\/unlikely\u5230\u5e95\u662f\u5982\u4f55\u8d77\u4f5c\u7528\u7684?\u9996\u5148\u6211\u4eec\u5c06\u4e0a\u8ff0\u4ee3\u7801\u4e2d\u7684unlikely\u53bb\u6389\uff0c\u7136\u540e\u53cd\u6c47\u7f16\uff0c\u4f5c\u4e3a\u5bf9\u7167\u7ec4\u3002<\/p>\n
\u6c47\u7f16\u5982\u4e0b\uff0c\u6211\u4eec\u770b\u5230\uff0cif\u5206\u652f\u4e2d\u7684\u6307\u4ee4\u88ab\u7f16\u8bd1\u5668\u653e\u7f6e\u4e8e\u5206\u652f\u8df3\u8f6c\u6307\u4ee4jle\u76f8\u90bb\u7684\u4f4d\u7f6e\uff0c\u5373CPU\u6d41\u6c34\u7ebf\u5728\u9047\u5230jle\u6307\u4ee4\u6240\u4ee3\u8868\u7684\u7684’\u5c94\u8def\u53e3’\u65f6\uff0c\u66f4\u503e\u5411\u4e8e\u8d70if\u5206\u652f<\/p>\n
.LC0:\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0.string \"Positive\\n\"\r\n.LC1:\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0.string \"Zero or Negative\\n\"\r\nmain:\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0sub\u00a0\u00a0\u00a0\u00a0 rsp, 8\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0test\u00a0\u00a0\u00a0 edi, edi\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0jle<\/span>\u00a0\u00a0\u00a0\u00a0 .L2\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 ; \u5982\u679cargc <= 0, \u8df3\u8f6c\u5230L2<\/span>\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0mov\u00a0\u00a0\u00a0\u00a0 edi, OFFSET FLAT:.LC0<\/span>\u00a0\u00a0 ; \u5982\u679cargc > 0, \u4ece\u8fd9\u91cc\u6267\u884c<\/span>\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0call\u00a0\u00a0\u00a0 puts<\/span>\r\n.L3:\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0xor\u00a0\u00a0\u00a0\u00a0 eax, eax\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0add\u00a0\u00a0\u00a0\u00a0 rsp, 8\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0ret\r\n.L2:\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0mov\u00a0\u00a0\u00a0\u00a0 edi, OFFSET FLAT:.LC1<\/span>\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0call\u00a0\u00a0\u00a0 puts<\/span>\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0jmp\u00a0\u00a0\u00a0\u00a0 .L3<\/code><\/pre>\n
\u63a5\u7740\u6211\u4eec\u5728if\u5206\u652f\u4e2d\u52a0\u4e0aunlikely, \u53cd\u6c47\u7f16\u5982\u4e0b\u3002\u8fd9\u91cc\u7684\u60c5\u51b5\u6b63\u597d\u4e0e\u5bf9\u7167\u7ec4\u76f8\u53cd\uff0cif\u5206\u652f\u4e0b\u7684\u6307\u4ee4\u88ab\u7f16\u8bd1\u5668\u653e\u7f6e\u4e8e\u8fdc\u79bb\u8df3\u8f6c\u6307\u4ee4jg\u7684\u4f4d\u7f6e\u3002\u8fd9\u610f\u5473\u7740CPU\u6b64\u65f6\u66f4\u503e\u5411\u4e8e\u8d70else\u5206\u652f\u3002<\/p>\n
.LC0:\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0.string \"Positive\\n\"\r\n.LC1:\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0.string \"Zero or Negative\\n\"\r\nmain:\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0sub\u00a0\u00a0\u00a0\u00a0 rsp, 8\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0test\u00a0\u00a0\u00a0 edi, edi\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0jg<\/span>\u00a0\u00a0\u00a0\u00a0\u00a0 .L6\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 ; \u5982\u679cargc > 0, \u8df3\u8f6c\u5230L\uff16<\/span>\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0mov\u00a0\u00a0\u00a0\u00a0 edi, OFFSET FLAT:.LC1\u00a0\u00a0 ;<\/span> \u5982\u679cargc <= 0, \u4ece\u8fd9\u91cc\u6267\u884c<\/span>\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0call\u00a0\u00a0\u00a0 puts<\/span>\r\n.L3:\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0xor\u00a0\u00a0\u00a0\u00a0 eax, eax\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0add\u00a0\u00a0\u00a0\u00a0 rsp, 8\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0ret\r\n.L6:\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0mov\u00a0\u00a0\u00a0\u00a0 edi, OFFSET FLAT:.LC0<\/span>\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0call\u00a0\u00a0\u00a0 puts<\/span>\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0jmp\u00a0\u00a0\u00a0\u00a0 .L3<\/code><\/pre>\n
\u56e0\u6b64\uff0c\u901a\u8fc7\u5bf9\u5206\u652f\u6761\u4ef6\u4f7f\u7528likely\u548cunlikely\uff0c\u6211\u4eec\u53ef\u7ed9\u7f16\u8bd1\u5668\u4e00\u79cd\u6697\u793a\uff0c\u5373\u8be5\u5206\u652f\u6761\u4ef6\u88ab\u6ee1\u8db3\u7684\u6982\u7387\u6bd4\u8f83\u5927\u6216\u6bd4\u8f83\u5c0f\u3002\u800c\u7f16\u8bd1\u5668\u5229\u7528\u8fd9\u4e00\u4fe1\u606f\u4f18\u5316\u5176\u673a\u5668\u6307\u4ee4\uff0c\u4ece\u800c\u6700\u5927\u9650\u5ea6\u51cf\u5c11CPU\u5206\u652f\u9884\u6d4b\u5931\u8d25\u5e26\u6765\u7684\u60e9\u7f5a\u3002<\/p>\n
\u4e09\u3001\u9002\u7528\u573a\u666f<\/h1>\n
CPU\u6709\u81ea\u5e26\u7684\u5206\u652f\u9884\u6d4b\u5668\uff0c\u5728\u5927\u591a\u6570\u573a\u666f\u4e0b\u6548\u679c\u4e0d\u9519\u3002\u56e0\u6b64\u5728\u5206\u652f\u53d1\u751f\u6982\u7387\u4e25\u91cd\u503e\u659c\u3001\u8ffd\u6c42\u6781\u81f4\u6027\u80fd\u7684\u573a\u666f\u4e0b\uff0c\u4f7f\u7528likely\/unlikely\u624d\u5177\u6709\u8f83\u5927\u610f\u4e49\u3002<\/p>\n
\u56db\u3001\u5176\u4ed6<\/h1>\n
C++20\u4e4b\u524d\u7684\uff0clikely\u548cunlikely\u53ea\u4e0d\u8fc7\u662f\u4e00\u5bf9\u81ea\u5b9a\u4e49\u7684\u5b8f\u3002\u800cC++20\u4e2d\u6b63\u5f0f\u5c06likely\u548cunlikely\u786e\u5b9a\u4e3a\u5c5e\u6027\u5173\u952e\u5b57\u3002\u4f7f\u7528\u793a\u4f8b\uff1a<\/p>\n
int test(int i) {\r\n\u00a0\u00a0\u00a0\u00a0switch(i) {\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0case 1: handle1();\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0break;\r\n\u00a0\u00a0\u00a0\u00a0[[likely]] case 2: handle2();\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0break;\r\n\u00a0\u00a0\u00a0\u00a0}\r\n}<\/code><\/pre>\n
 <\/p>\n
yan 23.2.3<\/p>\n","protected":false},"excerpt":{"rendered":"
\u4e00\u3001\u6982\u8ff0 \u5728\u4e00\u4e9b\u660e\u786e\u7684\u573a\u666f\u4e0b\uff0c\u7a0b\u5e8f\u5458\u6bd4CPU\u548c\u7f16\u8bd1\u5668\u66f4\u4e86\u89e3\u54ea\u4e2a\u5206\u652f\u6761\u4ef6\u66f4\u6709\u53ef\u80fd\u88ab\u6ee1\u8db3\u3002\u6211\u4eec\u662f\u5426\u53ef\u5c06\u8fd9\u4e00\u5148\u9a8c\u77e5\u8bc6 […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":[],"categories":[1027],"tags":[1255,1256],"_links":{"self":[{"href":"https:\/\/yanjingang.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/7074"}],"collection":[{"href":"https:\/\/yanjingang.com\/blog\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/yanjingang.com\/blog\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/yanjingang.com\/blog\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/yanjingang.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=7074"}],"version-history":[{"count":3,"href":"https:\/\/yanjingang.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/7074\/revisions"}],"predecessor-version":[{"id":7079,"href":"https:\/\/yanjingang.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/7074\/revisions\/7079"}],"wp:attachment":[{"href":"https:\/\/yanjingang.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=7074"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/yanjingang.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=7074"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/yanjingang.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=7074"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}