12 ms·

Interesting, so that would mean that `int` would be faster than `unsigned int` in tight loops because the compiler would not have to check for overflow and do t

Interesting, so that would mean that `int` would be faster than `unsigned int` in tight loops because the compiler would not have to check for overflow and do the wrapping (if the upper bound is unknown so that it cannot be optimized out). I think that in Rust the unsigned types do not wrap by default, so that the compiler does not have to introduce these checks at runtime. If one wants wrapping, they have to directly use the wrap types instead which would be more instruction-heavy. I wonder if there is a way in C to reproduce this mechanism in order to not generate any check instructions as well for unsigned types.

junon 5y ago
> I wonder if there is a way in C to reproduce this mechanism in order to not generate any check instructions as well for unsigned types. Correct me if I'm wrong (I probably am, don't trust me) but simply using `unsigned` as the type gives you the most optimal signed unsigned integer type for the system.
- dumael 5y ago
  'signed' and 'unsigned' on their own--act as short-hand for 'signed int' and 'unsigned int' in C and C++. Note that the size of an 'int' is dependant on the "data model"[1]. As for whether it's the most optimal is far too context dependant. A data model that defaults 'int's to 32 bits on today's (and yesterday's) architectures is fine in many cases as the range of that type is acceptable for most usages without excessive wastage. Certain data models do specify that 'int' is 64 bits which can break some programmer's assumptions, and also lead to space wastage as a struct member or a stack slot has to have 64 bits allocated for it on paper. Data models are part of the ABI your program uses, so it's not necessarily optimal for any given system. [1] Wikipedia has a table summarizing some of the differences: https://en.wikipedia.org/wiki/64-bit_computing#64-bit_data_models https://en.wikipedia.org/wiki/64-bit_computing#64-bit_data_m...
celegans25 5y ago
I am not sure if this generalizes to non-64 bit platforms, but in my experience using size_t gives you unsigned integer indexing without requiring overflow checks (as it's normally a 64 bit quantity).
- deaddabe 5y ago
  This would make sense, since this type is almost always used for iterations and sizes. Anyways I have played with Compiler Explorer and could not spot the "overflow checks" inside some dumb code: https://godbolt.org/z/4znWG8GoP https://godbolt.org/z/4znWG8GoP Maybe this is because x86_64 instruction set is already wrapping up unsigned integers, and maybe the majority of instruction sets are also.

junon 5y ago

> I wonder if there is a way in C to reproduce this mechanism in order to not generate any check instructions as well for unsigned types. Correct me if I'm wrong (I probably am, don't trust me) but simply using `unsigned` as the type gives you the most optimal signed unsigned integer type for the system.

dumael 5y ago
'signed' and 'unsigned' on their own--act as short-hand for 'signed int' and 'unsigned int' in C and C++. Note that the size of an 'int' is dependant on the "data model"[1]. As for whether it's the most optimal is far too context dependant. A data model that defaults 'int's to 32 bits on today's (and yesterday's) architectures is fine in many cases as the range of that type is acceptable for most usages without excessive wastage. Certain data models do specify that 'int' is 64 bits which can break some programmer's assumptions, and also lead to space wastage as a struct member or a stack slot has to have 64 bits allocated for it on paper. Data models are part of the ABI your program uses, so it's not necessarily optimal for any given system. [1] Wikipedia has a table summarizing some of the differences: https://en.wikipedia.org/wiki/64-bit_computing#64-bit_data_models https://en.wikipedia.org/wiki/64-bit_computing#64-bit_data_m...

celegans25 5y ago

I am not sure if this generalizes to non-64 bit platforms, but in my experience using size_t gives you unsigned integer indexing without requiring overflow checks (as it's normally a 64 bit quantity).

deaddabe 5y ago
This would make sense, since this type is almost always used for iterations and sizes. Anyways I have played with Compiler Explorer and could not spot the "overflow checks" inside some dumb code: https://godbolt.org/z/4znWG8GoP https://godbolt.org/z/4znWG8GoP Maybe this is because x86_64 instruction set is already wrapping up unsigned integers, and maybe the majority of instruction sets are also.