{"id":1181,"date":"2013-03-28T04:27:57","date_gmt":"2013-03-27T18:27:57","guid":{"rendered":"http:\/\/brnz.org\/hbr\/?p=1181"},"modified":"2015-01-25T08:09:11","modified_gmt":"2015-01-24T22:09:11","slug":"can-c11s-final-qualifier-affect-codegen","status":"publish","type":"post","link":"https:\/\/brnz.org\/hbr\/?p=1181","title":{"rendered":"Can C++11’s final qualifier affect codegen?"},"content":{"rendered":"

To satisfy my curiosity, I compiled this<\/span><\/p>\n

struct A { void f(); };\r\nstruct B { virtual void f(); };\r\nstruct C { virtual void f() final; };\r\nvoid a(A& a) { a.f(); }\r\nvoid b(B& b) { b.f(); }\r\nvoid c(C& c) { c.f(); }<\/pre>\n
like this<\/span><\/p>\n
g++ call.cpp -std=c++11 -O3 -S -o- -masm=intel # gcc-4.7.2 i686-pc-cygwin<\/pre>\n
which produced [output that included] this<\/p>\n
__Z1aR1A:\r\n    jmp __ZN1A1fEv\r\n\r\n__Z1bR1B:\r\n    mov eax, DWORD PTR [esp+4]\r\n    mov edx, DWORD PTR [eax]\r\n    mov eax, DWORD PTR [edx]\r\n    jmp eax\r\n\r\n__Z1cR1C:\r\n    jmp __ZN1C1fEv<\/pre>\n
What does it mean? Because C::f()<\/strong>\u00a0is marked final<\/strong>, calls to it can be devirtualized by the compiler, so there is no extra indirection when calling it in the above case.<\/p>\n
(Trying the same code with VS2012’s cl.exe, the call to\u00a0C::f()<\/strong> does not appear to be devirtualized)<\/p>\n
struct C<\/strong> does still appear to have a virtual function table pointer: (sizeof(C) == sizeof(B) == 4)<\/strong>, whereas (sizeof(A) == 1)<\/strong>.\u00a0It seems to me that the vtable pointer could be omitted in this case…<\/del><\/p>\n
Update (2015-01-24):<\/strong> As Tom points out below, anyone with a B*<\/strong> or B&<\/strong> that is actually pointing to an object of type C<\/strong> will still require the vtable pointer to find the correct implementation of f()<\/strong>.<\/p>\n","protected":false},"excerpt":{"rendered":"
To satisfy my curiosity, I compiled this struct A { void f(); }; struct B { virtual void f(); }; struct C { virtual void f() final; }; void a(A& a) { a.f(); } void b(B& b) { b.f(); } void c(C& c) { c.f(); } like this g++ call.cpp -std=c++11 -O3 -S -o- -masm=intel … Continue reading “Can C++11’s final qualifier affect codegen?”<\/span><\/a><\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":[],"categories":[5,26],"tags":[],"_links":{"self":[{"href":"https:\/\/brnz.org\/hbr\/index.php?rest_route=\/wp\/v2\/posts\/1181"}],"collection":[{"href":"https:\/\/brnz.org\/hbr\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/brnz.org\/hbr\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/brnz.org\/hbr\/index.php?rest_route=\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/brnz.org\/hbr\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=1181"}],"version-history":[{"count":10,"href":"https:\/\/brnz.org\/hbr\/index.php?rest_route=\/wp\/v2\/posts\/1181\/revisions"}],"predecessor-version":[{"id":1425,"href":"https:\/\/brnz.org\/hbr\/index.php?rest_route=\/wp\/v2\/posts\/1181\/revisions\/1425"}],"wp:attachment":[{"href":"https:\/\/brnz.org\/hbr\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=1181"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/brnz.org\/hbr\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=1181"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/brnz.org\/hbr\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=1181"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}