{"id":89971,"date":"2020-05-13T22:22:14","date_gmt":"2020-05-13T14:22:14","guid":{"rendered":"http:\/\/4563.org\/?p=89971"},"modified":"2020-05-13T22:22:14","modified_gmt":"2020-05-13T14:22:14","slug":"a-%e5%92%8c-0-a-%e6%9c%89%e4%bb%80%e4%b9%88%e5%8c%ba%e5%88%ab%ef%bc%9f","status":"publish","type":"post","link":"http:\/\/4563.org\/?p=89971","title":{"rendered":"-a \u548c 0-a \u6709\u4ec0\u4e48\u533a\u522b\uff1f"},"content":{"rendered":"<div>\n<div>\n<div>\n<h1>                  -a \u548c 0-a \u6709\u4ec0\u4e48\u533a\u522b\uff1f               <\/h1>\n<p> <\/p>\n<div>\n<div> <span>\u8cc7\u6df1\u5927\u4f6c : nyanyh <\/span>  <span><i><\/i> 13<\/span> <\/div>\n<div> <\/div>\n<\/p><\/div>\n<\/p><\/div>\n<\/p><\/div>\n<div isfirst=\"1\"> <\/p>\n<p>\u8ba4\u8bc6\u7684\u4e00\u4f4d\u6280\u672f\u4eba\u5458\u544a\u8bc9\u6211\u7528 0-a \u66ff\u6362-a\uff0c\u53ef\u4ee5\u63d0\u5347\u6027\u80fd\uff0c\u6211\u5c06\u4fe1\u5c06\u7591\uff0c\u5199\u4e86\u5c0f\u4ee3\u7801\u6d4b\u8bd5\u4e00\u4e0b<\/p>\n<pre><code>int main() {     int a = 100;      auto a1 = -a;     auto a2 = 0-a;      return 0; } <\/code><\/pre>\n<p>\u7136\u540e\u770b\u770b LLVM \u7684\u4e2d\u95f4\u7801<\/p>\n<pre><code>%6:gr32 = SUB32rm %0:gr32(tied-def 0), %stack.1, 1, $noreg, 0, $noreg, implicit-def $eflags, debug-location !19 :: (load 4 from %ir.2); .\/test.cpp:5:15   MOV32mr %stack.2, 1, $noreg, 0, $noreg, killed %6:gr32, debug-location !17 :: (store 4 into %ir.3); .\/test.cpp:5:10    %3:gr32 = SUB32rm %0:gr32(tied-def 0), %stack.1, 1, $noreg, 0, $noreg, implicit-def $eflags, debug-location !23 :: (load 4 from %ir.2); .\/test.cpp:6:16   MOV32mr %stack.3, 1, $noreg, 0, $noreg, killed %3:gr32, debug-location !21 :: (store 4 into %ir.4); .\/test.cpp:6:10 <\/code><\/pre>\n<p>\u662f\u5b8c\u5168\u4e00\u6837\u7684<\/p>\n<p>\u518d\u8bd5\u8bd5\u5982\u679c a \u662f double \u5462\uff1f<\/p>\n<pre><code>int main() {     double a = 100;      auto a1 = -a;     auto a2 = 0-a;      return 0; } <\/code><\/pre>\n<p>\u51fa\u73b0\u4e86\u660e\u663e\u4e0d\u540c\uff1a<\/p>\n<pre><code>  %11:fr64 = MOVSDrm_alt %stack.1, 1, $noreg, 0, $noreg, debug-location !19 :: (load 8 from %ir.2); .\/test.cpp:5:16   %7:gr64 = MOVSDto64rr killed %11:fr64, debug-location !20; .\/test.cpp:5:15   %8:gr64 = MOV64ri -9223372036854775808, debug-location !20; .\/test.cpp:5:15   %9:gr64 = XOR64rr killed %7:gr64(tied-def 0), %8:gr64, implicit-def $eflags, debug-location !20; .\/test.cpp:5:15   %10:fr64 = MOV64toSDrr killed %9:gr64, debug-location !20; .\/test.cpp:5:15   MOVSDmr %stack.2, 1, $noreg, 0, $noreg, killed %10:fr64, debug-location !18 :: (store 8 into %ir.3); .\/test.cpp:5:10    %2:fr64 = FsFLD0SD debug-location !24; .\/test.cpp:6:16   %4:fr64 = SUBSDrm %2:fr64(tied-def 0), %stack.1, 1, $noreg, 0, $noreg, debug-location !24 :: (load 8 from %ir.2); .\/test.cpp:6:16   MOVSDmr %stack.3, 1, $noreg, 0, $noreg, killed %4:fr64, debug-location !22 :: (store 8 into %ir.4); .\/test.cpp:6:10   $eax = COPY %0:gr32, debug-location !25; .\/test.cpp:8:5 <\/code><\/pre>\n<p>\u4e0a\u9762\u7684\u4ee3\u7801\u90fd\u662f\u7528<code>-O0<\/code>\u6d4b\u8bd5\u7684\uff0c\u6253\u5f00\u4f18\u5316\u7684\u8bdd\u5c31\u6d4b\u8bd5\u4e0d\u51fa\u4e86\uff0c\u4e24\u884c\u4ee3\u7801\u76f4\u63a5\u88ab\u4f18\u5316\u6389\u4e86<\/p>\n<p>\u6240\u4ee5\u5728\u67d0\u4e9b\u60c5\u51b5\u4e0b\u786e\u5b9e 0-a \u6bd4-a \u5c11\u51e0\u6761\u6307\u4ee4\uff0c\u6240\u4ee5\u4f1a\u5feb\u4e00\u4e9b\uff1f<\/p>\n<\/p><\/div>\n<div> <b>\u5927\u4f6c\u6709\u8a71\u8aaa<\/b> (<span>5<\/span>)        <\/div>\n<div> <\/div>\n<\/p><\/div>\n<\/p><\/div>\n<ul>\n<li data-pid=\"1329739\" data-uid=\"2\">\n<div>\n<div>\n<div> <span>\u8cc7\u6df1\u5927\u4f6c : xcstream <\/span>  <\/div>\n<div> <i title=\"\u5f15\u7528\"><\/i>  <span>          <\/span> <\/div>\n<\/p><\/div>\n<div>                                                             \u4e00\u4e2a\u662f\u51cf\u6cd5 \u4e00\u4e2a\u76f4\u63a5\u53d6\u53cd                                                            <\/div>\n<\/p><\/div>\n<\/li>\n<li data-pid=\"1329740\" data-uid=\"2\">\n<div>\n<div>\n<div> <span>\u8cc7\u6df1\u5927\u4f6c : msg7086 <\/span>  <\/div>\n<div> <i title=\"\u5f15\u7528\"><\/i>  <span>          <\/span> <\/div>\n<\/p><\/div>\n<div>                                                             \u6d4b\u4f18\u5316\u540e\u4ee3\u7801\u7684\u8bdd\u53ef\u4ee5\u5728\u540e\u9762\u63a5\u4e00\u4e2a printf \u6253\u5370\u51fa\u6765\u3002<br \/>\u5927\u90e8\u5206\u60c5\u51b5\u5e94\u8be5\u90fd\u662f\u80fd\u4f18\u5316\u6389\u7684\uff0c\u6d6e\u70b9\u6570\u56e0\u4e3a\u6d89\u53ca\u5230 rounding error \u6240\u4ee5\u5c11\u6570\u64cd\u4f5c\u53ef\u80fd\u4e0d\u4f1a\u7ed9\u4f60\u4f18\u5316\u3002<br \/>\uff08\u6bd4\u5982\u6d6e\u70b9\u4e0b a+b+c+d \u548c(a+b)+(c+d)\u7684\u7ed3\u679c\u662f\u4e0d\u540c\u7684\u3002\u800c\u4e14\u7406\u8bba\u4e0a\u540e\u8005\u6bd4\u524d\u8005\u5feb\u3002\uff09                                                            <\/div>\n<\/p><\/div>\n<\/li>\n<li data-pid=\"1329741\" data-uid=\"2\">\n<div>\n<div>\n<div> <span>\u8cc7\u6df1\u5927\u4f6c : msg7086 <\/span>  <\/div>\n<div> <i title=\"\u5f15\u7528\"><\/i>  <span>          <\/span> <\/div>\n<\/p><\/div>\n<div>                                                             https:\/\/godbolt.org\/z\/RYVwp7<\/p>\n<p>\u610f\u6599\u4e4b\u4e2d\uff0c\u6d6e\u70b9\u6570\u51cf\u6cd5\u8c03\u7528\u7684\u662f sub\uff0c\u800c\u6d6e\u70b9\u6570\u53d6\u53cd\u5219\u662f\u548c -0.0 \u53d6\u5f02\u6216\u3002                                                            <\/p><\/div>\n<\/p><\/div>\n<\/li>\n<li data-pid=\"1329742\" data-uid=\"2\">\n<div>\n<div>\n<div> <span>\u8cc7\u6df1\u5927\u4f6c : galileo1214 <\/span>  <\/div>\n<div> <i title=\"\u5f15\u7528\"><\/i>  <span>          <\/span> <\/div>\n<\/p><\/div>\n<div>                                                             \u5b66\u5230\u4e86                                                            <\/div>\n<\/p><\/div>\n<\/li>\n<li data-pid=\"1329743\" data-uid=\"2\">\n<div>\n<div>\n<div> <span>\u8cc7\u6df1\u5927\u4f6c : fgwmlhdkkkw <\/span>  <\/div>\n<div> <i title=\"\u5f15\u7528\"><\/i>  <span>          <\/span> <\/div>\n<\/p><\/div>\n<div>                                                             @msg7086 \u8fd8\u6709\u8fd9\u6837\u7684\u7f51\u7ad9\uff01                                                            <\/div>\n<\/p><\/div>\n<\/li>\n<li>\n","protected":false},"excerpt":{"rendered":"<p>-a \u548c 0-a \u6709\u4ec0\u4e48\u533a\u522b\uff1f \u8cc7&hellip;<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":[],"categories":[],"tags":[],"_links":{"self":[{"href":"http:\/\/4563.org\/index.php?rest_route=\/wp\/v2\/posts\/89971"}],"collection":[{"href":"http:\/\/4563.org\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"http:\/\/4563.org\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"http:\/\/4563.org\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"http:\/\/4563.org\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=89971"}],"version-history":[{"count":0,"href":"http:\/\/4563.org\/index.php?rest_route=\/wp\/v2\/posts\/89971\/revisions"}],"wp:attachment":[{"href":"http:\/\/4563.org\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=89971"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"http:\/\/4563.org\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=89971"},{"taxonomy":"post_tag","embeddable":true,"href":"http:\/\/4563.org\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=89971"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}