3 ms·

There are explicit instructions on x64 and aarch64 for sign and zero extensions. There are also common patterns like 'xor r64, r64' to clear out a reg (and the

There are explicit instructions on x64 and aarch64 for sign and zero extensions. There are also common patterns like 'xor r64, r64' to clear out a reg (and the zero-register arm variant). Why are there no higher level language abstractions for these types of patterns? Or maybe there are and I'm just unaware. I would really like to see single operand operators, similar to 'i++'. '!!i' to do 'i~=i', '<<i' to do 'i=i<<1'. The 'rep' instructions are also nice: https://www.felixcloutier.com/x86/rep:repe:repz:repne:repnz https://www.felixcloutier.com/x86/rep:repe:repz:repne:repnz Imagine doing something like this in C or Rust: 'int i=10;rep i printf("called %u times", i);' where rep would store the value of i in rcx, sets it to zero and stores in rax and jmp's to whatever function or codeblock you specified (could be inline code, or lambda expression), rcx (i) times, passing 'i''s current value optionally to the target code block. It would essentially be a shorthand form of 'for(int i=0;i<10;i++){printf("called %u times",i);}' except it's easier to use for simpler constructs like 'rep 8 <<i;' (just an example, you can just do 'i = i << 8;') if you combine it with my earlier proposed left shift operator.

toast0 2y ago
> There are explicit instructions on x64 and aarch64 for sign and zero extensions. There are also common patterns like 'xor r64, r64' to clear out a reg (and the zero-register arm variant). Why are there no higher level language abstractions for these types of patterns? Or maybe there are and I'm just unaware. The high level abstraction for xor r64, r64 is foo = 0. High level abstraction for sign/zero extension is casting to a larger type.
- notepad0x90 2y ago
  I guess I was just being lazy and wanting something like '~foo'. If you won't want to change the type and are fine with losing bits, it's slightly more verbose, but I admit to being lazy again with that as well :(.
adrian_b 2y ago
If you define in C/C++ a structure with bit fields, and you declare the bit fields as being either signed or unsigned, assigning the bit fields to a bigger integer, which is appropriately signed or unsigned, will perform an optimal sign extension or zero extension, as necessary, without having to write any expression. When you need sign extension or zero extension for conversion between standard integer types, you have to write only the type casting operator. Having to use any of the inefficient tricks presented in the parent article is necessary only when you do not declare the correct types for your variables.
- notepad0x90 2y ago
  I was think more about convenience than necessity.

toast0 2y ago

> There are explicit instructions on x64 and aarch64 for sign and zero extensions. There are also common patterns like 'xor r64, r64' to clear out a reg (and the zero-register arm variant). Why are there no higher level language abstractions for these types of patterns? Or maybe there are and I'm just unaware. The high level abstraction for xor r64, r64 is foo = 0. High level abstraction for sign/zero extension is casting to a larger type.

notepad0x90 2y ago
I guess I was just being lazy and wanting something like '~foo'. If you won't want to change the type and are fine with losing bits, it's slightly more verbose, but I admit to being lazy again with that as well :(.

adrian_b 2y ago

If you define in C/C++ a structure with bit fields, and you declare the bit fields as being either signed or unsigned, assigning the bit fields to a bigger integer, which is appropriately signed or unsigned, will perform an optimal sign extension or zero extension, as necessary, without having to write any expression. When you need sign extension or zero extension for conversion between standard integer types, you have to write only the type casting operator. Having to use any of the inefficient tricks presented in the parent article is necessary only when you do not declare the correct types for your variables.

notepad0x90 2y ago
I was think more about convenience than necessity.