Implement the sign function

[neverchanje]

Purpose

The sign() function returns the sign of the specified argument.

Parameters

The sign() function takes one parameter:

dp or numeric: The argument to compute the sign for.

Return Value

The sign() function returns an integer that represents the sign of the specified argument, according to the following rules:

• If the argument is positive, 1 is returned.
• If the argument is negative, -1 is returned.
• If the argument is zero, 0 is returned.

Example

SELECT sign(5); -- Returns 1
SELECT sign(-5); -- Returns -1
SELECT sign(0); -- Returns 0

Regress test to enable

risingwave/src/tests/regress/data/sql/float8.sql

Lines 102 to 103 in ac104a7

	-- sign
	--@ select sign(f1) as sign_f1 from float8_tbl f;

risingwave/src/tests/regress/data/expected/float8.out

Lines 322 to 330 in ac104a7

	select sign(f1) as sign_f1 from float8_tbl f;
	sign_f1
	---------
	0
	1
	-1
	1
	1
	(5 rows)

[broccoliSpicy]

there are actually some bit hacks we could utilize for this function.
for example:
sign = (v > 0) - (v < 0);

bit_hack

do we care about performance that much here?
to implement these hacks we might need to introduce C to risingwave, seems too trivial a reason to mess up the current compilation/build
what about assembly, what if doing so limits risingwave's supporting architecture

[xxchan]

Hmmm, why can't this hack be done in Rust? 👀

[xxchan]

And actually, there's i32::signum https://doc.rust-lang.org/std/primitive.i32.html#method.signum

[broccoliSpicy]

ha, my bad, I thought casting from boolean to integer is disallowed in Rust

// Type your code here, or load an example.
pub fn sign(num: i32) -> i32 {
    ((num > 0) as i32) - ((num < 0) as i32)
}

// If you use `main()`, declare it as `pub` to see it in the output:
// pub fn main() { ... }

example::sign:
        mov     ecx, edi
        sar     ecx, 31
        xor     eax, eax
        test    edi, edi
        setne   al
        or      eax, ecx
        ret

rustc's optimization is doing quite a good job here(all 'cause of llvm?)
Rust is cool here

this result is produced by this tool rustc-nightly
-C opt-level=3 -C target-cpu=cannonlake

[xxchan]

Hmmm, is this bit hack better? 👀 You might find a chance to contribute to Rust std! 😄

[xxchan]

Oh, things become different in nightly... 🤔

[broccoliSpicy]

haha:smile:, we might be a few months late.

I am sure there will be plenty of other chances for us

this signum function's implementation is lit, I think we should use it

[broccoliSpicy]

reference:
Rust PR for signum optimizition

[neverchanje]

Hi @xxchan Can I assign you as the owner for this issue? Please feel free to uncheck the assignee if you don't have time.