Fix test case from #2246 #2248

youknowone · 2022-07-26T21:38:05Z

close #2246
fix align codegen of #2240
fix align codegen of #2159
fix align codegen of #1538

tests/headers/packed-align-conflict.h

youknowone · 2022-07-27T19:57:27Z

mm.. test passes only because test doesn't include case about:

typedef unsigned char uint8_t;
typedef unsigned short uint16_t;

struct A {
  uint8_t a;
  uint16_t b;
  uint8_t c;
} __attribute__((aligned(2)));

struct B {
  uint8_t a;
  uint16_t b;
  uint8_t c;
} __attribute__((aligned(2), packed));


#include <assert.h>
#include <stdio.h>
int main() {
  printf("%lu %lu\n", sizeof(struct A), sizeof(struct B));
  assert(sizeof(struct A) == sizeof(struct B));
}

youknowone · 2022-07-27T20:05:17Z

is this the answer?

#[repr(C, align(2))]
#[derive(Debug, Default, Copy, Clone)]
pub struct B(B__packed);

#[repr(C, packed)]
#[derive(Debug, Default, Copy, Clone)]
pub struct B__packed {
    pub a: u8,
    pub b: u16,
    pub c: u8,
}

#[test]
fn bindgen_test_layout_B() {
    const UNINIT: ::std::mem::MaybeUninit<B> =
        ::std::mem::MaybeUninit::uninit();
    let ptr = UNINIT.as_ptr();
    assert_eq!(
        ::std::mem::size_of::<B>(),
        4usize,
        concat!("Size of: ", stringify!(B))
    );
    assert_eq!(
        ::std::mem::align_of::<B>(),
        2usize,
        concat!("Alignment of ", stringify!(B))
    );
    assert_eq!(
        unsafe { ::std::ptr::addr_of!((*ptr).0.a) as usize - ptr as usize },
        0usize,
        concat!("Offset of field: ", stringify!(B), "::", stringify!(a))
    );
    assert_eq!(
        unsafe { ::std::ptr::addr_of!((*ptr).0.b) as usize - ptr as usize },
        1usize,
        concat!("Offset of field: ", stringify!(B), "::", stringify!(b))
    );
    assert_eq!(
        unsafe { ::std::ptr::addr_of!((*ptr).0.c) as usize - ptr as usize },
        3usize,
        concat!("Offset of field: ", stringify!(B), "::", stringify!(c))
    );
}

youknowone

Still a few problems, but I think I made enough proof of concept how it works.

Now, packed struct also getting align attribute because I couldn't remove those case from requires_explicit_align
Also bitfield got problem. I need your advice for this case.

Except for those 2 cases, everything seems working.
The design probably is not very good. I feel like bindgen may already have a tool to handle those wrapper types.

youknowone · 2022-07-27T21:54:28Z

src/codegen/struct_layout.rs

@@ -365,7 +365,7 @@ impl<'a> StructLayoutTracker<'a> {
            return true;
        }

-        if self.max_field_align >= layout.align {
+        if !self.is_packed && self.max_field_align >= layout.align {


I want to check if it has explicit align in C code to avoid packed-only also get align attribute in rust side.

I'm confused about this change, can you elaborate? What case does this make a difference on?

Because packed or aligned cases are already being handled well, I need to check packed and aligned case here:
https://github.com/rust-lang/rust-bindgen/pull/2248/files#diff-01720a19d4518b9c16feea9cefbee210a853a1c99d1e9b19aec23e72877e52f0R2019-R2023

Without change, any packed type will not be marked as explicit_align. So the new handling condition doesn't hit.
The new problem introduced by this is only packed struct is also marked as aligned. Does bindgen have a feature to dinstinguish it?
Because the estimated size for clang is different, it looks like to be distinguishable.

emilio · 2022-07-28T17:28:55Z

src/codegen/mod.rs

                "packed".to_string()
            } else {
                format!("packed({})", n)
            };
            attributes.push(attributes::repr_list(&["C", &packed_repr]));
        } else {
            attributes.push(attributes::repr("C"));
-        }
-
-        if ctx.options().rust_features().repr_align {


Why removing the feature-check for repr(align)?

accidently happened during multiple revision 😂

emilio · 2022-07-28T17:29:28Z

src/codegen/mod.rs

+            .unwrap();
+            result.push(quote! {
+                #attributes
+                #[repr(C, align(#align))]


Probably better if it'd be #[repr(transparent)], that'd match the ABI exactly, though I don't think it would matter in practice.

It doesn't work. we need align here but transparent seems doesn't accept other things.

error[E0692]: transparent struct cannot have other repr hints --> tests/packed-align-conflict.rs:16:8 | 16 | #[repr(transparent, align(2))] |

emilio · 2022-07-28T17:32:04Z

src/codegen/struct_layout.rs

@@ -365,7 +365,7 @@ impl<'a> StructLayoutTracker<'a> {
            return true;
        }

-        if self.max_field_align >= layout.align {
+        if !self.is_packed && self.max_field_align >= layout.align {


I'm confused about this change, can you elaborate? What case does this make a difference on?

emilio · 2022-07-28T17:34:40Z

tests/expectations/tests/issue-537-repr-packed-n.rs

    pub i: ::std::os::raw::c_int,
 }
+#[derive(Debug, Default, Copy, Clone)]
+#[repr(C, align(2))]
+pub struct AlignedToTwo(AlignedToTwo__packed);


This should be public, how do folks access the fields otherwise? More to the point, we should try to only add this wrapper when absolutely needed (I think packed && max_field_align < explicit_align, right?), and also we should probably impl Deref and DerefMut on the struct to get more convenient access / avoid most code breakage as a result of this change.

About Deref, Is there another wrapper mechanism to do it? Or this is the first place to add those Deref stuff?
~~The condition is logically right but I have no idea how to do yet.~~

Counter example about the condition.

// sizeof(B) == 6 struct B { uint8_t a; uint16_t b; uint8_t c; } __attribute__((aligned(2), packed));

This is packed but not max_field_align < explicit_align (2 < 2).

The key idea we need to complete this task is distinguishing it from

// sizeof(B) == 4 struct B { uint8_t a; uint16_t b; uint8_t c; } __attribute__(packed));

…on hints. Fixes rust-lang#2240

youknowone · 2022-07-28T21:14:14Z

I added the last commit to check the CI result. I don't believe that's the correct way. Please suppose I am not understanding the remaining problems and how bindgen handle packed and aligned.

youknowone · 2022-08-01T11:25:09Z

@emilio do you have advices?

youknowone · 2022-08-18T14:51:53Z

@emilio can I get advice for remaining issues?

emilio · 2022-08-18T16:58:54Z

Sorry for the lag, it's on my list of things to do. I plan to dig a bit into how C++ compilers treat repr+aligned hints because I'm not super-convinced there's a way to emulate it in Rust, but if you've done that research and can point to it it'd be extremely helpful.

youknowone · 2022-08-18T17:57:03Z

I think emulating in rust is done here. I forgot the datails now but If you need explanation, I can review it again.

If bindgen cannot distinguish explicitly aligned and implicitly aligned struct, could you guide me to how to get clang-calcuated size and bindgen-calculated size for the struct? Then I will try to detect which alignment used by them.

bors-servo · 2022-10-05T01:59:34Z

☔ The latest upstream changes (presumably #2284) made this pull request unmergeable. Please resolve the merge conflicts.

CGMossa · 2023-04-10T15:10:10Z

Can I kindly ask that this gets looked at please?

pvdrz · 2023-04-10T15:44:09Z

I've seen several issues and PRs discussing problems with packed and align representations. I'll gather some info and have a proper discussion around it with other contributors.

emilio requested changes Jul 27, 2022

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tests/headers/packed-align-conflict.h Show resolved Hide resolved

youknowone marked this pull request as draft July 27, 2022 19:03

youknowone force-pushed the packed-align-conflict branch 3 times, most recently from 065d903 to 7cc3a34 Compare July 27, 2022 19:43

youknowone marked this pull request as ready for review July 27, 2022 19:48

youknowone requested a review from emilio July 27, 2022 19:50

youknowone marked this pull request as draft July 27, 2022 20:07

youknowone force-pushed the packed-align-conflict branch 2 times, most recently from cc0ce21 to ec4094f Compare July 27, 2022 21:41

youknowone commented Jul 27, 2022

View reviewed changes

emilio requested changes Jul 28, 2022

View reviewed changes

codegen: Don't generate conflicting packed() and align() representati…

61f307d

…on hints. Fixes rust-lang#2240

youknowone force-pushed the packed-align-conflict branch 4 times, most recently from 2f67f2a to 03837ca Compare July 28, 2022 20:57

youknowone added 2 commits July 29, 2022 06:10

poc packed wrapper

3785a8e

XXX: I am not understanding how things going. don't merge this commit

8f31017

youknowone force-pushed the packed-align-conflict branch from aa0db92 to 8f31017 Compare July 28, 2022 21:11

pvdrz deleted the branch rust-lang:master November 2, 2023 17:50

pvdrz closed this Nov 2, 2023

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Fix test case from #2246 #2248

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Fix test case from #2246 #2248

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