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Rollup merge of #58615 - RalfJung:ref-to-place-alignment, r=oli-obk

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miri: explain why we use static alignment in ref-to-place conversion

@eddyb @oli-obk do you think this makes sense? Or should we use the run-time alignment (`align_of_val`)? I am a bit worried about custom DSTs, but that affects way more areas of Miri so I'd ignore them for now.

r? @oli-obk
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Mazdak Farrokhzad 2019-02-22 14:58:12 +01:00 committed by GitHub
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1 changed files with 8 additions and 2 deletions
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10
src/librustc_mir/interpret/place.rs
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@ -326,6 +326,10 @@ where
let mplace = MemPlace {
ptr: val.to_scalar_ptr()?,
// We could use the run-time alignment here. For now, we do not, because
// the point of tracking the alignment here is to make sure that the *static*
// alignment information emitted with the loads is correct. The run-time
// alignment can only be more restrictive.
align: layout.align.abi,
meta: val.to_meta()?,
};
@ -385,9 +389,11 @@ where
// above). In that case, all fields are equal.
let field_layout = base.layout.field(self, usize::try_from(field).unwrap_or(0))?;
// Offset may need adjustment for unsized fields
// Offset may need adjustment for unsized fields.
let (meta, offset) = if field_layout.is_unsized() {
// re-use parent metadata to determine dynamic field layout
// Re-use parent metadata to determine dynamic field layout.
// With custom DSTS, this *will* execute user-defined code, but the same
// happens at run-time so that's okay.
let align = match self.size_and_align_of(base.meta, field_layout)? {
Some((_, align)) => align,
None if offset == Size::ZERO =>