5 ms·

> I try to use ++x instead of x++, even though 99% of the time it makes no difference. I think 100% of time is closer to truth, unless you're using ancient or

> I try to use ++x instead of x++, even though 99% of the time it makes no difference. I think 100% of time is closer to truth, unless you're using ancient or embedded 8/16-bit compilers.

Animats 9y ago
That hasn't really been a thing since the PDP-11, which had an auto-increment option on the register used for pointer offsets. That's why that feature is in C. It really mattered in 1978.
- vram22 9y ago
  Interesting, so the C creators used a feature in their language, that was hardware-specific. I thought (from having read K&R) that one goal of C was to be hardware-independent. Maybe this was an exception, or maybe that auto-increment was common to many computer designs at that time.
  - mark-r 9y ago
    Wikipedia states that its inclusion in B (the ancestor to C) was independent of the PDP-11 instruction set, which hadn't been invented yet [1]. [1] https://en.wikipedia.org/wiki/Increment_and_decrement_operators#History https://en.wikipedia.org/wiki/Increment_and_decrement_operat...
    - vram22 9y ago
      Cool. Wow, B. A blast from the past, sort of. I had read a good book about BCPL (the ancestor to B) many years ago. IIRC, it was by Martin Richards, inventor of BCPL. Pretty interesting book and language. BCPL and B were both typeless languages, or languages with just one type, the machine word (16 or 32 bits, don't remember). Still I found that many algorithms and programs were expressed rather compactly in BCPL - or so it seemed to me at the time. Was quite junior then, and without exposure to more advanced programming languages - only knew BASIC and Pascal, probably; even C, I only learned a bit later. https://en.wikipedia.org/wiki/B_(programming_language) https://en.wikipedia.org/wiki/B_(programming_language) https://en.wikipedia.org/wiki/BCPL https://en.wikipedia.org/wiki/BCPL https://en.wikipedia.org/wiki/Martin_Richards_(computer_scientist) https://en.wikipedia.org/wiki/Martin_Richards_(computer_scie... Also just saw some other interesting stuff from the BCPL article above: [ BCPL is the language in which the original hello world program was written.[3] The first MUD was also written in BCPL (MUD1). Several operating systems were written partially or wholly in BCPL (for example, TRIPOS and the earliest versions of AmigaDOS). BCPL was also the initial language used in the seminal Xerox PARC Alto project, the first modern personal computer; among other projects, the Bravo document preparation system was written in BCPL. ]
jcelerier 9y ago
naye. Even for the following trivial function: #include <vector> #include <string> std::size_t foo(const std::vector<std::string>& vec) { std::size_t n = 0; for(auto it = vec.begin(), end = vec.end(); it != end; ++it) { n += vec.size(); } return n; } MSVC 2017 generates different assembly for ++it vs it++, at the highest optimization level.
- vardump 9y ago
  That's very surprising, perhaps a compiler bug? Does it perform any differently? Just tried gcc 7.1, both post and pre-increment compile identically.
- moefh 9y ago
  That's sad. Both gcc and clang at -O3 can optimize it++ to ++it (see [1]). Hopefully MSVC will improve soon. [1] https://godbolt.org/g/pxzVdP https://godbolt.org/g/pxzVdP
- Arnavion 9y ago
  Looks identical to me. https://godbolt.org/g/mELzSE https://godbolt.org/g/mELzSE https://godbolt.org/g/nGcdqT https://godbolt.org/g/nGcdqT
  - apta 9y ago
    For the it++ case: std::_Vector_const_iterator<std::_Vector_val<std::_Simple_types<std::basic_string<char,std::char_traits<char>,std::allocator<char> > > > >::operator++, COMDAT PROC mov rax, QWORD PTR [rcx] mov QWORD PTR [rdx], rax add rax, 32 ; 00000020H mov QWORD PTR [rcx], rax mov rax, rdx ret 0 For the ++it case: std::_Vector_const_iterator<std::_Vector_val<std::_Simple_types<std::basic_string<char,std::char_traits<char>,std::allocator<char> > > > >::operator++, COMDAT PROC add QWORD PTR [rcx], 32 ; 00000020H mov rax, rcx ret
    - 9y ago

> I try to use ++x instead of x++, even though 99% of the time it makes no difference. I think 100% of time is closer to truth, unless you're using ancient or embedded 8/16-bit compilers.

Animats 9y ago

That hasn't really been a thing since the PDP-11, which had an auto-increment option on the register used for pointer offsets. That's why that feature is in C. It really mattered in 1978.

vram22 9y ago
Interesting, so the C creators used a feature in their language, that was hardware-specific. I thought (from having read K&R) that one goal of C was to be hardware-independent. Maybe this was an exception, or maybe that auto-increment was common to many computer designs at that time.
- mark-r 9y ago
  Wikipedia states that its inclusion in B (the ancestor to C) was independent of the PDP-11 instruction set, which hadn't been invented yet [1]. [1] https://en.wikipedia.org/wiki/Increment_and_decrement_operators#History https://en.wikipedia.org/wiki/Increment_and_decrement_operat...
  - vram22 9y ago
    Cool. Wow, B. A blast from the past, sort of. I had read a good book about BCPL (the ancestor to B) many years ago. IIRC, it was by Martin Richards, inventor of BCPL. Pretty interesting book and language. BCPL and B were both typeless languages, or languages with just one type, the machine word (16 or 32 bits, don't remember). Still I found that many algorithms and programs were expressed rather compactly in BCPL - or so it seemed to me at the time. Was quite junior then, and without exposure to more advanced programming languages - only knew BASIC and Pascal, probably; even C, I only learned a bit later. https://en.wikipedia.org/wiki/B_(programming_language) https://en.wikipedia.org/wiki/B_(programming_language) https://en.wikipedia.org/wiki/BCPL https://en.wikipedia.org/wiki/BCPL https://en.wikipedia.org/wiki/Martin_Richards_(computer_scientist) https://en.wikipedia.org/wiki/Martin_Richards_(computer_scie... Also just saw some other interesting stuff from the BCPL article above: [ BCPL is the language in which the original hello world program was written.[3] The first MUD was also written in BCPL (MUD1). Several operating systems were written partially or wholly in BCPL (for example, TRIPOS and the earliest versions of AmigaDOS). BCPL was also the initial language used in the seminal Xerox PARC Alto project, the first modern personal computer; among other projects, the Bravo document preparation system was written in BCPL. ]

jcelerier 9y ago

naye. Even for the following trivial function: #include <vector> #include <string> std::size_t foo(const std::vector<std::string>& vec) { std::size_t n = 0; for(auto it = vec.begin(), end = vec.end(); it != end; ++it) { n += vec.size(); } return n; } MSVC 2017 generates different assembly for ++it vs it++, at the highest optimization level.

vardump 9y ago
That's very surprising, perhaps a compiler bug? Does it perform any differently? Just tried gcc 7.1, both post and pre-increment compile identically.
moefh 9y ago
That's sad. Both gcc and clang at -O3 can optimize it++ to ++it (see [1]). Hopefully MSVC will improve soon. [1] https://godbolt.org/g/pxzVdP https://godbolt.org/g/pxzVdP
Arnavion 9y ago
Looks identical to me. https://godbolt.org/g/mELzSE https://godbolt.org/g/mELzSE https://godbolt.org/g/nGcdqT https://godbolt.org/g/nGcdqT
- apta 9y ago
  For the it++ case: std::_Vector_const_iterator<std::_Vector_val<std::_Simple_types<std::basic_string<char,std::char_traits<char>,std::allocator<char> > > > >::operator++, COMDAT PROC mov rax, QWORD PTR [rcx] mov QWORD PTR [rdx], rax add rax, 32 ; 00000020H mov QWORD PTR [rcx], rax mov rax, rdx ret 0 For the ++it case: std::_Vector_const_iterator<std::_Vector_val<std::_Simple_types<std::basic_string<char,std::char_traits<char>,std::allocator<char> > > > >::operator++, COMDAT PROC add QWORD PTR [rcx], 32 ; 00000020H mov rax, rcx ret
  - 9y ago