45 ms·

You still have the overhead of a function call. If you just use / % operators then you'll get a call inserted to the libgcc or compiler-rt routine if you don't

You still have the overhead of a function call. If you just use / % operators then you'll get a call inserted to the libgcc or compiler-rt routine if you don't have the M extension, and those routines are div or mod only. Using stdlib for integer division seems like an odd choice. If stdlib div() were promoted to a builtin one day (it currently is not in GCC afaict), and its implementation were inlined, then the compiler would recognise the common case of one side of the struct being dead, and you'd still end up with a single div/rem instruction.

cpgxiii 1y ago
Interesting, this is a case where GCC and Clang are "dumb" and MSVC does a better job. For code #include <cstdlib> #include <cstdint> #include <utility> std::pair<int64_t, int64_t> LibDivWithRemainder(int64_t numerator, int64_t denominator) { const auto res = std::div(numerator, denominator); return std::make_pair(res.quot, res.rem); } std::pair<int64_t, int64_t> ManDivWithRemainder(int64_t numerator, int64_t denominator) { const int64_t quot = numerator / denominator; const int64_t rem = numerator % denominator; return std::make_pair(quot, rem); } GCC (x86-64 trunk @ -O2) produces "LibDivWithRemainder(long, long)": sub rsp, 8 call "ldiv" add rsp, 8 ret "ManDivWithRemainder(long, long)": mov rax, rdi cqo idiv rsi ret Clang (x86-64 @ -O2) produces LibDivWithRemainder(long, long): jmp ldiv@PLT ManDivWithRemainder(long, long): mov rax, rdi mov rcx, rdi or rcx, rsi shr rcx, 32 je .LBB1_1 cqo idiv rsi ret .LBB1_1: xor edx, edx div esi ret while MSVC (x64 @ /O2) produces mov rax, rdx cdq idiv r8 mov QWORD PTR [rcx], rax mov rax, rcx mov QWORD PTR [rcx+8], rdx ret 0 for both

cpgxiii 1y ago

Interesting, this is a case where GCC and Clang are "dumb" and MSVC does a better job. For code #include <cstdlib> #include <cstdint> #include <utility> std::pair<int64_t, int64_t> LibDivWithRemainder(int64_t numerator, int64_t denominator) { const auto res = std::div(numerator, denominator); return std::make_pair(res.quot, res.rem); } std::pair<int64_t, int64_t> ManDivWithRemainder(int64_t numerator, int64_t denominator) { const int64_t quot = numerator / denominator; const int64_t rem = numerator % denominator; return std::make_pair(quot, rem); } GCC (x86-64 trunk @ -O2) produces "LibDivWithRemainder(long, long)": sub rsp, 8 call "ldiv" add rsp, 8 ret "ManDivWithRemainder(long, long)": mov rax, rdi cqo idiv rsi ret Clang (x86-64 @ -O2) produces LibDivWithRemainder(long, long): jmp ldiv@PLT ManDivWithRemainder(long, long): mov rax, rdi mov rcx, rdi or rcx, rsi shr rcx, 32 je .LBB1_1 cqo idiv rsi ret .LBB1_1: xor edx, edx div esi ret while MSVC (x64 @ /O2) produces mov rax, rdx cdq idiv r8 mov QWORD PTR [rcx], rax mov rax, rcx mov QWORD PTR [rcx+8], rdx ret 0 for both