guiand marked an inline comment as done.
guiand added inline comments.
================
Comment at: clang/lib/CodeGen/CGCall.cpp:2082
+ const Type *RetTyPtr = RetTy.getTypePtr();
+ if (!RetTy->isVoidType() && !RetTyPtr->isRecordType() &&
+ RetAI.getKind() != ABIArgInfo::Indirect) {
----------------
guiand wrote:
> guiand wrote:
> > rsmith wrote:
> > > Other types that we should think about from a padding perspective:
> > >
> > > * `nullptr_t` (which has the same size and alignment as `void*` but is
> > > all padding)
> > > * 80-bit `long double` and `_Complex long double` (I don't know whether
> > > we guarantee to initialize the 6 padding bytes)
> > > * member pointers might contain padding under some ABI rules -- under
> > > the MS ABI, you get a struct containing N pointers followed by M ints,
> > > which could have 4 bytes of tail padding on 64-bit targets
> > > * vector types with tail padding (eg, a vector of 3 `char`s is sometimes
> > > passed as an `i32` with one byte `undef`)
> > > * matrix types (presumably the same concerns as for vector types apply
> > > here)
> > > * maybe Obj-C object types?
> > > * `_ExtInt` types (eg, returning an `_ExtInt(65)` initialized to 0
> > > produces an `{i64, i64}` containing 7 bytes of `undef`)
> > >
> > > It would be safer to list exactly those types for which we know this
> > > assumption is correct rather than assuming that it's correct by default
> > > -- I think more than half of the `Type` subclasses for concrete canonical
> > > types can have undef bits in their IR representation.
> > This is a good point, and I think there was some nuance that I had
> > previously included in determining this for records (since removed) that I
> > didn't think to include in CGCall.cpp.
> >
> > I think one particular check, `llvm::DataLayout::typeSizeEqualsStoreSize`,
> > handles a lot of these cases:
> > - `long double`
> > - oddly-sized vectors
> > - `_ExtInt` types
> >
> > I don't know if the `matrix` type has internal padding (like structs) but
> > if not it would cover that as well.
> > I believe that the struct with padding wouldn't be an issue as we're
> > excluding record types here.
> > And I'd have to take a closer look at `nullptr_t` to see how it behaves
> > here.
> >
> > ~~~
> >
> > But if I'd like to build an exhaustive list of types I think will work
> > correctly with a check like this:
> > - scalars
> > - floating point numbers
> > - pointers
> >
> > I think I'd need also need an inner `typeSizeEqualsStoreSize` check on the
> > base type of vectors/complex numbers.
> > nullptr_t (which has the same size and alignment as void* but is all
> > padding)
>
> From what I can gather looking at the IR of `test/CodeGenCXX/nullptr.cpp`, it
> looks like `nullptr_t` arguments and return values are represented as normal
> `i8*` types but given the special value `null`. From this it seems like we
> can mark them `noundef` without worry?
Actually I've been thinking more about these cases, and they should only really
be a problem if clang coerces them away from their native types to something
larger.
For example, if clang emits `i33` that's not a problem, as potential padding
belonging to `i33` can be always be determined by looking at the data layout.
But if it emits `{ i32, i32 }` or `i64` or something similar, it's introducing
new hidden/erased padding and so we can't mark it `noundef`.
Same thing goes for `long double`: if it's emitted as `x86_fp80` that's no
problem.
There's been some confusion wrt this in the LangRef patch as well. I think what
we really want is an attribute that indicates the presence of undef bits *not
inherent to the IR type*. So (for the sake of argument, we're not handling
structs right now) `struct { char; short }` emitted as `{ i8, i16 } noundef` is
fine, but emitting it as `i64` is a problem.
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D81678/new/
https://reviews.llvm.org/D81678
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