4 ms·

But there is no function call overhead right? Replacing goto OUT_FULL; with return q1; would seem to be faster right? No need for a goto, just invoke return. W

But there is no function call overhead right? Replacing goto OUT_FULL; with return q1; would seem to be faster right? No need for a goto, just invoke return. What am I missing?

valarauca1 10y ago
There is some. A few register need to be assigned, and everything else gets pushed/copied onto the stack. Then you jump to a location and copy everything back off the stack. This doesn't take many cycles, but it does take some. While a GOTO is just a jump.
- xxs 10y ago
  the compilers are free to optimize. Creating a stack frame for each function is trivially avoided by inlining (esp. trivial for non-polymorph call sites)
  - valarauca1 10y ago
    It depends. In GCC/MSVC will only (attempt to) inline what you mark as inline. Then MSVC has a keyword which forces inlining. Unless you set a flag which tells the compiler to inline what ever it wants. But that being said Microsoft has a non-POSIX x64 ABI designed to allow better in-lining. How inlining works starts to dive pretty deep into the compiler rabbit hole.
    - lorenzhs 10y ago
      Are you sure? This does not match my experience. Rather, compilers inline stuff if they think it's a good idea, and treat the "inline" keyword more like a hint.
    - honkhonkpants 10y ago
      This is not true. Gcc will inline everything it damn well wants to. It will definitely inline any function that is smaller than a function call, like small C++ accessors, and any other small function available at compile time like trivial constructors and the like. The "inline" keyword mainly serves to avoid ODR violations, not to inform the optimizer.
    - xxs 10y ago
      Exported functions are entirely different than vast majority of the code, the exported ones do need call conventions, so they can be linked. But it's entirely not true that ONLY functions marked as 'inline' would be inlined. You can also direct GCC to try to integrate all “simple enough” functions into their callers with the option -finline-functions (includeded in -O3).[0]. O2 includes inline small functions [1]: [0]: https://gcc.gnu.org/onlinedocs/gcc/Inline.html https://gcc.gnu.org/onlinedocs/gcc/Inline.html [1]: https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html
    - astrange 10y ago
      Functions don't need to follow the ABI unless they're externally visible. If you use -fvisibility=hidden GCC will do whatever it wants for each function as it sees fit. Interprocedural register allocation can work better than inlining too, because it keeps the code size smaller, and direct calls have almost no speed penalty.

But there is no function call overhead right? Replacing goto OUT_FULL; with return q1; would seem to be faster right? No need for a goto, just invoke return. What am I missing?

valarauca1 10y ago

There is some. A few register need to be assigned, and everything else gets pushed/copied onto the stack. Then you jump to a location and copy everything back off the stack. This doesn't take many cycles, but it does take some. While a GOTO is just a jump.

xxs 10y ago
the compilers are free to optimize. Creating a stack frame for each function is trivially avoided by inlining (esp. trivial for non-polymorph call sites)
- valarauca1 10y ago
  It depends. In GCC/MSVC will only (attempt to) inline what you mark as inline. Then MSVC has a keyword which forces inlining. Unless you set a flag which tells the compiler to inline what ever it wants. But that being said Microsoft has a non-POSIX x64 ABI designed to allow better in-lining. How inlining works starts to dive pretty deep into the compiler rabbit hole.
  - lorenzhs 10y ago
    Are you sure? This does not match my experience. Rather, compilers inline stuff if they think it's a good idea, and treat the "inline" keyword more like a hint.
  - honkhonkpants 10y ago
    This is not true. Gcc will inline everything it damn well wants to. It will definitely inline any function that is smaller than a function call, like small C++ accessors, and any other small function available at compile time like trivial constructors and the like. The "inline" keyword mainly serves to avoid ODR violations, not to inform the optimizer.
  - xxs 10y ago
    Exported functions are entirely different than vast majority of the code, the exported ones do need call conventions, so they can be linked. But it's entirely not true that ONLY functions marked as 'inline' would be inlined. You can also direct GCC to try to integrate all “simple enough” functions into their callers with the option -finline-functions (includeded in -O3).[0]. O2 includes inline small functions [1]: [0]: https://gcc.gnu.org/onlinedocs/gcc/Inline.html https://gcc.gnu.org/onlinedocs/gcc/Inline.html [1]: https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html
  - astrange 10y ago
    Functions don't need to follow the ABI unless they're externally visible. If you use -fvisibility=hidden GCC will do whatever it wants for each function as it sees fit. Interprocedural register allocation can work better than inlining too, because it keeps the code size smaller, and direct calls have almost no speed penalty.