[PATCH] D81678: Introduce noundef attribute at call sites for stricter poison analysis

[Gui Andrade via Phabricator via llvm-commits]

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