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fix computing the dynamic alignment of packed structs with dyn trait tails

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This commit is contained in:
Ralf Jung 2023-12-02 13:40:24 +01:00
parent 7176b8babd
commit 7e4c4271f4
4 changed files with 98 additions and 47 deletions
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60
compiler/rustc_codegen_ssa/src/size_of_val.rs
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@ -84,10 +84,13 @@ pub fn size_and_align_of_dst<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
debug!("DST {} layout: {:?}", t, layout);
let i = layout.fields.count() - 1;
let sized_size = layout.fields.offset(i).bytes();
let unsized_offset_unadjusted = layout.fields.offset(i).bytes();
let sized_align = layout.align.abi.bytes();
debug!("DST {} statically sized prefix size: {} align: {}", t, sized_size, sized_align);
let sized_size = bx.const_usize(sized_size);
debug!(
"DST {} offset of dyn field: {}, statically sized align: {}",
t, unsized_offset_unadjusted, sized_align
);
let unsized_offset_unadjusted = bx.const_usize(unsized_offset_unadjusted);
let sized_align = bx.const_usize(sized_align);
// Recurse to get the size of the dynamically sized field (must be
@ -95,26 +98,26 @@ pub fn size_and_align_of_dst<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
let field_ty = layout.field(bx, i).ty;
let (unsized_size, mut unsized_align) = size_and_align_of_dst(bx, field_ty, info);
// FIXME (#26403, #27023): We should be adding padding
// to `sized_size` (to accommodate the `unsized_align`
// required of the unsized field that follows) before
// summing it with `sized_size`. (Note that since #26403
// is unfixed, we do not yet add the necessary padding
// here. But this is where the add would go.)
// # First compute the dynamic alignment
// Return the sum of sizes and max of aligns.
let size = bx.add(sized_size, unsized_size);
// Packed types ignore the alignment of their fields.
if let ty::Adt(def, _) = t.kind() {
if def.repr().packed() {
unsized_align = sized_align;
// For packed types, we need to cap the alignment.
if let ty::Adt(def, _) = t.kind()
&& let Some(packed) = def.repr().pack
{
if packed.bytes() == 1 {
// We know this will be capped to 1.
unsized_align = bx.const_usize(1);
} else {
// We have to dynamically compute `min(unsized_align, packed)`.
let packed = bx.const_usize(packed.bytes());
let cmp = bx.icmp(IntPredicate::IntULT, unsized_align, packed);
unsized_align = bx.select(cmp, unsized_align, packed);
}
}
// Choose max of two known alignments (combined value must
// be aligned according to more restrictive of the two).
let align = match (
let full_align = match (
bx.const_to_opt_u128(sized_align, false),
bx.const_to_opt_u128(unsized_align, false),
) {
@ -129,6 +132,19 @@ pub fn size_and_align_of_dst<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
}
};
// # Then compute the dynamic size
// The full formula for the size would be:
// let unsized_offset_adjusted = unsized_offset_unadjusted.align_to(unsized_align);
// let full_size = (unsized_offset_adjusted + unsized_size).align_to(full_align);
// However, `unsized_size` is a multiple of `unsized_align`.
// Therefore, we can equivalently do the `align_to(unsized_align)` *after* adding `unsized_size`:
// let full_size = (unsized_offset_unadjusted + unsized_size).align_to(unsized_align).align_to(full_align);
// Furthermore, `align >= unsized_align`, and therefore we only need to do:
// let full_size = (unsized_offset_unadjusted + unsized_size).align_to(full_align);
let full_size = bx.add(unsized_offset_unadjusted, unsized_size);
// Issue #27023: must add any necessary padding to `size`
// (to make it a multiple of `align`) before returning it.
//
@ -140,12 +156,12 @@ pub fn size_and_align_of_dst<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
//
// `(size + (align-1)) & -align`
let one = bx.const_usize(1);
let addend = bx.sub(align, one);
let add = bx.add(size, addend);
let neg = bx.neg(align);
let size = bx.and(add, neg);
let addend = bx.sub(full_align, one);
let add = bx.add(full_size, addend);
let neg = bx.neg(full_align);
let full_size = bx.and(add, neg);
(size, align)
(full_size, full_align)
}
_ => bug!("size_and_align_of_dst: {t} not supported"),
}