Division by zero is undefined behavior

[iacore]

The official language website claims that V has "no undefined behavior". However, division by zero is undefined with C as backend. I think this makes division by zero undefined as well.

Also, overflow/underflow is undefined as well.

OS: linux, "Arch Linux"
CC version: cc (GCC) 11.1.0
V full version: V 0.2.2 69f31d8

What did you do?
Run the following program with v run test.v

fn main() {
    a := 0
    println(1 / a)
}

What did you expect to see?
Make x / 0 a defined value, e.g. 0, and allow or block after it.
Or, like in Pony, add operators like /? that propagates the error.

What did you see instead?
Runtime error in debug mode.

SIGILL in release mode.

[yuyi98]

It's also runtime error in Golang.

package main

import "fmt"

func main() {
	a := 0
	fmt.Println(1 / a)
}

D:/Test/go/ttt/ttt.exe  [D:/Test/go/ttt]
panic: runtime error: integer divide by zero

goroutine 1 [running]:
main.main()
	D:/Test/go/ttt/main.go:7 +0x11

[crthpl]

Also, overflow/underflow is undefined as well.

overflow/underflow is defined: For unsigned numbers, it will wrap around (u8(0)-1 == 255 and u8(255)+1 == 0, and signed numbers will behave according to two's complement arithmetic (i8(-128)-1 == 127 and i8(127)+1 == -128)

[iacore]

Also, overflow/underflow is undefined as well.

overflow/underflow is defined: For unsigned numbers, it will wrap around (u8(0)-1 == 255 and u8(255)+1 == 0, and signed numbers will behave according to two's complement arithmetic (i8(-128)-1 == 127 and i8(127)+1 == -128)

I don't see how, since signed overflow/underflow is undefined in C and the following code

mut a := byte(244)
mut b := i8(244)
a += 100
b += 100
println(a)
println(b)

compiles to

VV_LOCAL_SYMBOL void main__main(void) {
        byte a = ((byte)(244));
        i8 b = ((i8)(244));
        a += 100;
        b += 100;
        println(byte_str(a));
        println(i8_str(b));
}

[danieldaeschle]

@locriacyber Same behaviour as in Go. What do you mean with defined behaviour? What is your expected output?

[iacore]

@locriacyber Same behaviour as in Go. What do you mean with defined behaviour? What is your expected output?

I expect those special cases of arithmetic to either defined and documented in V (like, always throw an error, or be 0), or the website stop stating V has "No undefined behavior".

[danieldaeschle]

TODO: add test case for that

[justinas]

@danieldaeschle @yuyi98 any idea why this was closed? Is it "completed", or "won't fix" as the label suggests?

It is still UB today:

$ v version
V 0.4.6 4a7c70c
$ cat division.v
fn main() {
    i := 0
    println(1/i)
}
$ v -autofree -gc none -o division.c division.v
$ cc -fsanitize=undefined division.c -o division
$ ./division
division.c:13104:31: runtime error: division by zero
0

[danieldaeschle]

Autofree is still not working really afaik

[justinas]

This is not about autofree, sorry about the confusion. I added -autofree -gc none so I can compile the generated C code without needing to include flags to link Boehm.

I don't think the memory management model changes what C code is generated for division by V?

[iacore]

It is still UB today:

It's always runtime error now.