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From: "federico at kircheis dot it" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug c++/111388] New: std:.get_if variant, unnecessary branch when outside of if statement Date: Tue, 12 Sep 2023 11:13:29 +0000 [thread overview] Message-ID: <bug-111388-4@http.gcc.gnu.org/bugzilla/> (raw) https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111388 Bug ID: 111388 Summary: std:.get_if variant, unnecessary branch when outside of if statement Product: gcc Version: 13.2.1 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c++ Assignee: unassigned at gcc dot gnu.org Reporter: federico at kircheis dot it Target Milestone: --- Example of code (https://godbolt.org/z/M1bWf5sz3) ---- #include <variant> struct interface{ virtual ~interface()= default; virtual int foo() const = 0; }; struct a : interface{ int foo() const override; }; struct b : interface{ int foo() const override; }; class a_or_b{ std::variant<a,b> ab; public: a_or_b() = delete; a_or_b(a _) : ab(_){} a_or_b(b _) : ab(_){} interface* operator->() noexcept { if (interface* ptr = std::get_if<0>(&ab); ptr) { return ptr; } #if 0 else if (interface* ptr = std::get_if<1>(&ab); ptr) { return ptr; } #else return std::get_if<1>(&ab); #endif } }; int bar3(a_or_b& ab){ return ab->foo()+1; } ---- With `#if 1`, the generated code looks like ---- bar3(a_or_b&): sub rsp, 8 mov rax, QWORD PTR [rdi] call [QWORD PTR [rax+16]] add rsp, 8 add eax, 1 ret ---- while with `#if 0`, the assembly looks like ---- bar3(a_or_b&): cmp BYTE PTR [rdi+8], 0 jne .L2 sub rsp, 8 mov rax, QWORD PTR [rdi] call [QWORD PTR [rax+16]] add rsp, 8 add eax, 1 ret bar3(a_or_b&) [clone .cold]: .L2: mov rax, QWORD PTR ds:0 ud2 ---- If I'm not mistake, with `return std::get_if<1>(&ab);` the compiler verifies if the return of get_if is nullptr, and if it is, then sets the return value to nullptr, which is unnecessary. With `if 1`, the result is passed as is. AFAIK the generated assembly is functionally equivalant, but the "more safe"(1) version less optimal 1) more safe as in "there is no UB if the class changes and the variant could be empty or hold another type". NOTE: replacing "std::get_if<0>"/"std::get_if<1>" with "std::get_if<a>/std::get_if<b>" does not make a relevante difference, the generated code is the same for both "if 0" and "if 1". For what is worth, clang generates the same code for "if 0" and "if 1".
next reply other threads:[~2023-09-12 11:13 UTC|newest] Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top 2023-09-12 11:13 federico at kircheis dot it [this message] 2023-09-12 14:55 ` [Bug tree-optimization/111388] " pinskia at gcc dot gnu.org 2023-09-12 14:58 ` pinskia at gcc dot gnu.org
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