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fix alignment

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This commit is contained in:
nora 2022-04-20 10:59:40 +02:00
parent c750e0758f
commit ea45532926
1 changed files with 12 additions and 8 deletions
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20
src/main.rs
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@ -8,19 +8,22 @@ use std::mem::MaybeUninit;
compile_error!("not supported"); compile_error!("not supported");
// a pointer sized buffer used to store a single pointer // a pointer sized buffer used to store a single pointer
type Buf = MaybeUninit<[usize; 1]>; type Buf = MaybeUninit<[u8; 1]>;
// a double pointer sized buffer used to store two pointers and rip out the center // a double pointer sized buffer used to store two pointers and rip out the center
type DBuf = MaybeUninit<[usize; 2]>; type DBuf = MaybeUninit<[u8; 2]>;
/// just a pointer, doesn't matter which one /// just a pointer, doesn't matter which one
type Ptr = *const u8; type Ptr = *const u8;
#[repr(align(8))]
struct Align8<T>(T);
/// combines two provenances into a single pointer sized value /// combines two provenances into a single pointer sized value
unsafe fn combine(prov_a: Ptr, prov_b: Ptr) -> Buf { unsafe fn combine(prov_a: Ptr, prov_b: Ptr) -> Buf {
// a buffer where we write in both pointers and then read out of from center, 1/2 of each provenance // a buffer where we write in both pointers and then read out of from center, 1/2 of each provenance
let mut double_buf = DBuf::zeroed(); let mut double_buf = Align8(DBuf::zeroed());
let ptr = double_buf.as_mut_ptr(); let ptr = double_buf.0.as_mut_ptr();
// write the a pointer to the first slot // write the a pointer to the first slot
ptr.cast::<Ptr>().write(prov_a); ptr.cast::<Ptr>().write(prov_a);
@ -34,11 +37,12 @@ unsafe fn combine(prov_a: Ptr, prov_b: Ptr) -> Buf {
/// extracts the two provenances from [`combine`] /// extracts the two provenances from [`combine`]
unsafe fn extract(buf: Buf) -> (Ptr, Ptr) { unsafe fn extract(buf: Buf) -> (Ptr, Ptr) {
let mut double_buf = DBuf::zeroed(); let mut double_buf = Align8(DBuf::zeroed());
// write the the pointer sized value into the center of the double buffer // write the the pointer sized value into the center of the double buffer
// splitting the provenances between the first and second slow // splitting the provenances between the first and second slow
double_buf double_buf
.0
.as_mut_ptr() .as_mut_ptr()
.cast::<u8>() .cast::<u8>()
.add(4) .add(4)
@ -46,12 +50,12 @@ unsafe fn extract(buf: Buf) -> (Ptr, Ptr) {
.write(buf); .write(buf);
// a copy of the first half of the dbuf, where the second half of it contains a provenance // a copy of the first half of the dbuf, where the second half of it contains a provenance
let mut a_buf: Buf = double_buf.as_ptr().cast::<Buf>().read(); let mut a_buf: Buf = double_buf.0.as_ptr().cast::<Buf>().read();
// a copy of the second half of the dbuf, where the first half of it contains b provenance // a copy of the second half of the dbuf, where the first half of it contains b provenance
let mut b_buf: Buf = double_buf.as_ptr().cast::<Buf>().add(1).read(); let mut b_buf: Buf = double_buf.0.as_ptr().cast::<Buf>().add(1).read();
// the pointer to the dbuf // the pointer to the dbuf
let ptr = double_buf.as_ptr(); let ptr = double_buf.0.as_ptr();
// copy the 4 a provenance bytes from the dbuf into the empty space in the a_buf // copy the 4 a provenance bytes from the dbuf into the empty space in the a_buf
// this way a_buf now contains 8 a provenance bytes // this way a_buf now contains 8 a provenance bytes