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Fix incorrect pseudocode for #[repr(C)] struct alignment #766

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merged 4 commits into from
Feb 26, 2020
Merged

Fix incorrect pseudocode for #[repr(C)] struct alignment #766

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5426423
Fix incorrect pseudocode for #[repr(C)] struct alignment
kinseytamsin Feb 22, 2020
bc5c540
Explicitly describe padding algorithm in pseudocode
kinseytamsin Feb 24, 2020
13e415a
Add warning for #[repr(C)] struct alignment pseudocode
kinseytamsin Feb 24, 2020
5d5b233
Add doc comment for padding_needed_for fn in pseudocode
kinseytamsin Feb 24, 2020
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8 changes: 6 additions & 2 deletions src/type-layout.md
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Original file line number Diff line number Diff line change
Expand Up @@ -205,14 +205,18 @@ for field in struct.fields_in_declaration_order() {
// Increase the current offset so that it's a multiple of the alignment
// of this field. For the first field, this will always be zero.
// The skipped bytes are called padding bytes.
current_offset += field.alignment % current_offset;
//
// padding_needed_for() is equivalent to
// std::alloc::Layout::padding_needed_for(), but takes an integer rather
// than a Layout as the first argument.
current_offset += padding_needed_for(current_offset, field.alignment);

struct[field].offset = current_offset;

current_offset += field.size;
}

struct.size = current_offset + current_offset % struct.alignment;
struct.size = current_offset + padding_needed_for(current_offset, struct.alignment);
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@rkanati you didn't touch this one in your PR... do you remember why? Was that an oversight, or was the original code actually correct for some reason?

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As far as I can tell, that was just an oversight.

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I think so, too. It would just have been good to get explicit confirmation.

```

> Note: This algorithm can produce zero-sized structs. This differs from
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