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fix dynamic size/align computation logic for packed types with dyn trait tail

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cc rust-lang/rust#118538
This commit is contained in:
bjorn3 2023-12-18 16:13:54 +00:00
parent c4567c1841
commit 1ab05b6cbe
2 changed files with 37 additions and 21 deletions
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2
scripts/test_rustc_tests.sh
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@ -140,7 +140,7 @@ rm -r tests/run-make/extern-fn-explicit-align # argument alignment not yet suppo
rm tests/ui/codegen/subtyping-enforces-type-equality.rs # assert_assignable bug with Coroutine's
rm -r tests/ui/packed # rust-lang/rust#118537
rm tests/ui/packed/issue-118537-field-offset-ice.rs # rust-lang/rust#118540
# bugs in the test suite
# ======================

56
src/unsize.rs
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@ -252,7 +252,9 @@ pub(crate) fn size_and_align_of<'tcx>(
assert!(!layout.ty.is_simd());
let i = layout.fields.count() - 1;
let sized_size = layout.fields.offset(i).bytes();
let unsized_offset_unadjusted = layout.fields.offset(i).bytes();
let unsized_offset_unadjusted =
fx.bcx.ins().iconst(fx.pointer_type, unsized_offset_unadjusted as i64);
let sized_align = layout.align.abi.bytes();
let sized_align = fx.bcx.ins().iconst(fx.pointer_type, sized_align as i64);
@ -261,27 +263,41 @@ pub(crate) fn size_and_align_of<'tcx>(
let field_layout = layout.field(fx, i);
let (unsized_size, mut unsized_align) = size_and_align_of(fx, field_layout, 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 = fx.bcx.ins().iadd_imm(unsized_size, sized_size as i64);
// Packed types ignore the alignment of their fields.
if let ty::Adt(def, _) = layout.ty.kind() {
if def.repr().packed() {
unsized_align = sized_align;
// For packed types, we need to cap the alignment.
if let ty::Adt(def, _) = ty.kind() {
if let Some(packed) = def.repr().pack {
if packed.bytes() == 1 {
// We know this will be capped to 1.
unsized_align = fx.bcx.ins().iconst(fx.pointer_type, 1);
} else {
// We have to dynamically compute `min(unsized_align, packed)`.
let packed = fx.bcx.ins().iconst(fx.pointer_type, packed.bytes() as i64);
let cmp =
fx.bcx.ins().icmp(IntCC::UnsignedGreaterThan, unsized_align, packed);
unsized_align = fx.bcx.ins().select(cmp, unsized_align, packed);
}
}
}
// Choose max of two known alignments (combined value must
// be aligned according to more restrictive of the two).
let cmp = fx.bcx.ins().icmp(IntCC::UnsignedGreaterThan, sized_align, unsized_align);
let align = fx.bcx.ins().select(cmp, sized_align, unsized_align);
let full_align = fx.bcx.ins().select(cmp, sized_align, unsized_align);
// # 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 = fx.bcx.ins().iadd(unsized_offset_unadjusted, unsized_size);
// Issue #27023: must add any necessary padding to `size`
// (to make it a multiple of `align`) before returning it.
@ -293,12 +309,12 @@ pub(crate) fn size_and_align_of<'tcx>(
// emulated via the semi-standard fast bit trick:
//
// `(size + (align-1)) & -align`
let addend = fx.bcx.ins().iadd_imm(align, -1);
let add = fx.bcx.ins().iadd(size, addend);
let neg = fx.bcx.ins().ineg(align);
let size = fx.bcx.ins().band(add, neg);
let addend = fx.bcx.ins().iadd_imm(full_align, -1);
let add = fx.bcx.ins().iadd(full_size, addend);
let neg = fx.bcx.ins().ineg(full_align);
let full_size = fx.bcx.ins().band(add, neg);
(size, align)
(full_size, full_align)
}
_ => bug!("size_and_align_of_dst: {ty} not supported"),
}