Implement 128bit integer conversion for limited API #1310

kngwyu · 2020-12-12T08:05:49Z

Fix #1309
They would be slow since they use eval... but could there be any alternative?

birkenfeld · 2020-12-12T08:09:42Z

Why use eval when you can call the method directly with call_method?

birkenfeld · 2020-12-12T08:12:17Z

This is the snippet I tested:

        let bytes = obj.call_method("to_bytes", (16, "little"), None)?;
        let res = u128::from_le_bytes(bytes.extract()?);

Or do you think this is slower due to conversion of the arguments from Rust? Hm, you need to construct a whole dictionary for the eval...

Of course, both are missing the PyNumber_Index call that the other conversion is doing.

birkenfeld · 2020-12-12T08:13:29Z

Alternative, as I said in #1309, is to use PyNumber_ APIs to shift and mask the number to get two values that fit into u64's.

kngwyu · 2020-12-12T08:21:39Z

Why use eval when you can call the method directly with call_method?

Thanks, I just failed to come up with it. We can call PyNumber_Index manually.

to use PyNumber_ APIs to shift and mask the number to get two values that fit into u64's.

I tried that first but I found it too complex, especially for i128, though it is faster...

birkenfeld · 2020-12-12T08:29:12Z

So in my tests (FromPyObject only) it was slightly faster to call with call_method, but then use your method of downcast instead of extract.

kngwyu · 2020-12-12T08:59:07Z

src/types/num.rs

@@ -183,11 +173,12 @@ mod int128_conversion {
            impl IntoPy<PyObject> for $rust_type {
                fn into_py(self, py: Python) -> PyObject {
                    unsafe {
-                        let bytes = self.to_ne_bytes();
+                        // Always use little endian
+                        let bytes = self.to_le_bytes();


Though it's not related to this PR, I noticed that to_le_bytes is added in Rust 1.32.
Using always little-endian simplified the code a lot.

kngwyu · 2020-12-12T08:59:32Z

src/types/num.rs

                        let obj = ffi::_PyLong_FromByteArray(
-                            bytes.as_ptr() as *const c_uchar,
-                            $byte_size,


I don't know why 16 is not hard-coded...

maybe bytes.len() is most correct? I would think that the compiler will still optimize this at compile time.

davidhewitt

Nice!

I think it's better to use std::mem::size_of{_val} instead of hard-coding 16, as it's easier to understand what that means for a future reader rather than just seeing a magic number 16.

src/types/num.rs

davidhewitt · 2020-12-12T09:04:49Z

src/types/num.rs

                        let obj = ffi::_PyLong_FromByteArray(
-                            bytes.as_ptr() as *const c_uchar,
-                            $byte_size,


maybe bytes.len() is most correct? I would think that the compiler will still optimize this at compile time.

davidhewitt · 2020-12-12T09:08:15Z

src/types/num.rs

@@ -202,83 +193,131 @@ mod int128_conversion {
                        if num.is_null() {
                            return Err(PyErr::fetch(ob.py()));
                        }
-                        let mut buffer = [0; $byte_size];
+                        let mut buffer = [0; 16];


Suggested change

let mut buffer = [0; 16];

let mut buffer = [0; std::mem::size_of::<$rust_type>()];

davidhewitt · 2020-12-12T09:09:33Z

src/types/num.rs

                        let ok = ffi::_PyLong_AsByteArray(
                            num as *mut ffi::PyLongObject,
                            buffer.as_mut_ptr(),
-                            $byte_size,
-                            IS_LITTLE_ENDIAN,
+                            16,


Suggested change

16,

buffer.len(),

davidhewitt · 2020-12-12T09:13:09Z

src/types/num.rs

+                    let num = unsafe { ffi::PyNumber_Index(ob.as_ptr()) };
+                    let num: &PyLong =
+                        unsafe { crate::FromPyPointer::from_owned_ptr_or_err(py, num)? };
+                    let args = (16, "little".to_object(py));


Suggested change

let args = (16, "little".to_object(py));

let args = (std::mem::size_of::<$rust_type>(), "little".to_object(py));

or could define a const SIZE: usize = std::mem::size_of::<$rust_type>() and use here and on line 258.

birkenfeld · 2020-12-12T10:02:48Z

src/types/num.rs

+                            ffi::PyNumber_Index(ob.as_ptr()),
+                        )
+                    }?;
+                    let kwargs = [("signed", $is_signed)].into_py_dict(py);


ah damn, it requires a dict here too then. Can you at least make this

let kwargs = if $is_signed { Some(...) } else { None };

?

CHANGELOG.md

Co-authored-by: David Hewitt <1939362+davidhewitt@users.noreply.github.com>

kngwyu · 2020-12-12T10:39:22Z

Thank you for your reviews!

davidhewitt

💯

kngwyu mentioned this pull request Dec 12, 2020

abi3 causes compile failures where u128s are involved #1309

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kngwyu force-pushed the abi3-128bit-integer branch from 09423d0 to 7f814f7 Compare December 12, 2020 08:57

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kngwyu force-pushed the abi3-128bit-integer branch from 7f814f7 to 0cafe3d Compare December 12, 2020 09:54

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Implement 128bit integer conversion for limited API

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kngwyu force-pushed the abi3-128bit-integer branch from 0cafe3d to 8c4cba2 Compare December 12, 2020 10:14

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Co-authored-by: David Hewitt <1939362+davidhewitt@users.noreply.github.com>

davidhewitt approved these changes Dec 12, 2020

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kngwyu merged commit 22359e9 into master Dec 12, 2020

programmerjake mentioned this pull request Dec 13, 2020

Use shifting and masking to convert between 128-bit Rust integers and Python integers #1314

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Implement 128bit integer conversion for limited API #1310

Implement 128bit integer conversion for limited API #1310

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	let mut buffer = [0; 16];
	let mut buffer = [0; std::mem::size_of::<$rust_type>()];

	let args = (16, "little".to_object(py));
	let args = (std::mem::size_of::<$rust_type>(), "little".to_object(py));

Implement 128bit integer conversion for limited API #1310

Implement 128bit integer conversion for limited API #1310

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