rustc_codegen_ssa/size_of_val.rs
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//! Computing the size and alignment of a value.
use rustc_abi::WrappingRange;
use rustc_hir::LangItem;
use rustc_middle::bug;
use rustc_middle::ty::print::{with_no_trimmed_paths, with_no_visible_paths};
use rustc_middle::ty::{self, Ty};
use tracing::{debug, trace};
use crate::common::IntPredicate;
use crate::traits::*;
use crate::{common, meth};
pub fn size_and_align_of_dst<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
bx: &mut Bx,
t: Ty<'tcx>,
info: Option<Bx::Value>,
) -> (Bx::Value, Bx::Value) {
let layout = bx.layout_of(t);
trace!("size_and_align_of_dst(ty={}, info={:?}): layout: {:?}", t, info, layout);
if layout.is_sized() {
let size = bx.const_usize(layout.size.bytes());
let align = bx.const_usize(layout.align.abi.bytes());
return (size, align);
}
match t.kind() {
ty::Dynamic(..) => {
// Load size/align from vtable.
let vtable = info.unwrap();
let size = meth::VirtualIndex::from_index(ty::COMMON_VTABLE_ENTRIES_SIZE)
.get_usize(bx, vtable, t);
let align = meth::VirtualIndex::from_index(ty::COMMON_VTABLE_ENTRIES_ALIGN)
.get_usize(bx, vtable, t);
// Size is always <= isize::MAX.
let size_bound = bx.data_layout().ptr_sized_integer().signed_max() as u128;
bx.range_metadata(size, WrappingRange { start: 0, end: size_bound });
// Alignment is always nonzero.
bx.range_metadata(align, WrappingRange { start: 1, end: !0 });
(size, align)
}
ty::Slice(_) | ty::Str => {
let unit = layout.field(bx, 0);
// The info in this case is the length of the str, so the size is that
// times the unit size.
(
// All slice sizes must fit into `isize`, so this multiplication cannot (signed)
// wrap.
// NOTE: ideally, we want the effects of both `unchecked_smul` and `unchecked_umul`
// (resulting in `mul nsw nuw` in LLVM IR), since we know that the multiplication
// cannot signed wrap, and that both operands are non-negative. But at the time of
// writing, the `LLVM-C` binding can't do this, and it doesn't seem to enable any
// further optimizations.
bx.unchecked_smul(info.unwrap(), bx.const_usize(unit.size.bytes())),
bx.const_usize(unit.align.abi.bytes()),
)
}
ty::Foreign(_) => {
// `extern` type. We cannot compute the size, so panic.
let msg_str = with_no_visible_paths!({
with_no_trimmed_paths!({
format!("attempted to compute the size or alignment of extern type `{t}`")
})
});
let msg = bx.const_str(&msg_str);
// Obtain the panic entry point.
let (fn_abi, llfn, _instance) =
common::build_langcall(bx, None, LangItem::PanicNounwind);
// Generate the call. Cannot use `do_call` since we don't have a MIR terminator so we
// can't create a `TerminationCodegenHelper`. (But we are in good company, this code is
// duplicated plenty of times.)
let fn_ty = bx.fn_decl_backend_type(fn_abi);
bx.call(
fn_ty,
/* fn_attrs */ None,
Some(fn_abi),
llfn,
&[msg.0, msg.1],
None,
None,
);
// This function does not return so we can now return whatever we want.
let size = bx.const_usize(layout.size.bytes());
let align = bx.const_usize(layout.align.abi.bytes());
(size, align)
}
ty::Adt(..) | ty::Tuple(..) => {
// First get the size of all statically known fields.
// Don't use size_of because it also rounds up to alignment, which we
// want to avoid, as the unsized field's alignment could be smaller.
assert!(!t.is_simd());
debug!("DST {} layout: {:?}", t, layout);
let i = layout.fields.count() - 1;
let unsized_offset_unadjusted = layout.fields.offset(i).bytes();
let sized_align = layout.align.abi.bytes();
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
// the last field).
let field_ty = layout.field(bx, i).ty;
let (unsized_size, mut unsized_align) = size_and_align_of_dst(bx, field_ty, info);
// # First compute the dynamic alignment
// 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 full_align = match (
bx.const_to_opt_u128(sized_align, false),
bx.const_to_opt_u128(unsized_align, false),
) {
(Some(sized_align), Some(unsized_align)) => {
// If both alignments are constant, (the sized_align should always be), then
// pick the correct alignment statically.
bx.const_usize(std::cmp::max(sized_align, unsized_align) as u64)
}
_ => {
let cmp = bx.icmp(IntPredicate::IntUGT, sized_align, unsized_align);
bx.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 = 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.
//
// Namely, the returned size should be, in C notation:
//
// `size + ((size & (align-1)) ? align : 0)`
//
// emulated via the semi-standard fast bit trick:
//
// `(size + (align-1)) & -align`
let one = bx.const_usize(1);
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);
(full_size, full_align)
}
_ => bug!("size_and_align_of_dst: {t} not supported"),
}
}