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Auto merge of #73453 - erikdesjardins:tuplayout, r=eddyb

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Ignore ZST offsets when deciding whether to use Scalar/ScalarPair layout

This is important because Scalar/ScalarPair layout previously would not be used if any ZST had nonzero offset.
For example, before this change, only `((), u128)` would be laid out like `u128`, not `(u128, ())`.

Fixes #63244
This commit is contained in:
bors 2020-09-25 14:42:20 +00:00
commit 5b9e886403
7 changed files with 230 additions and 76 deletions
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36
compiler/rustc_codegen_ssa/src/mir/place.rs
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@ -93,15 +93,33 @@ impl<'a, 'tcx, V: CodegenObject> PlaceRef<'tcx, V> {
let effective_field_align = self.align.restrict_for_offset(offset);
let mut simple = || {
// Unions and newtypes only use an offset of 0.
let llval = if offset.bytes() == 0 {
self.llval
} else if let Abi::ScalarPair(ref a, ref b) = self.layout.abi {
// Offsets have to match either first or second field.
assert_eq!(offset, a.value.size(bx.cx()).align_to(b.value.align(bx.cx()).abi));
bx.struct_gep(self.llval, 1)
} else {
bx.struct_gep(self.llval, bx.cx().backend_field_index(self.layout, ix))
let llval = match self.layout.abi {
_ if offset.bytes() == 0 => {
// Unions and newtypes only use an offset of 0.
// Also handles the first field of Scalar, ScalarPair, and Vector layouts.
self.llval
}
Abi::ScalarPair(ref a, ref b)
if offset == a.value.size(bx.cx()).align_to(b.value.align(bx.cx()).abi) =>
{
// Offset matches second field.
bx.struct_gep(self.llval, 1)
}
Abi::Scalar(_) | Abi::ScalarPair(..) | Abi::Vector { .. } if field.is_zst() => {
// ZST fields are not included in Scalar, ScalarPair, and Vector layouts, so manually offset the pointer.
let byte_ptr = bx.pointercast(self.llval, bx.cx().type_i8p());
bx.gep(byte_ptr, &[bx.const_usize(offset.bytes())])
}
Abi::Scalar(_) | Abi::ScalarPair(..) => {
// All fields of Scalar and ScalarPair layouts must have been handled by this point.
// Vector layouts have additional fields for each element of the vector, so don't panic in that case.
bug!(
"offset of non-ZST field `{:?}` does not match layout `{:#?}`",
field,
self.layout
);
}
_ => bx.struct_gep(self.llval, bx.cx().backend_field_index(self.layout, ix)),
};
PlaceRef {
// HACK(eddyb): have to bitcast pointers until LLVM removes pointee types.