[llvm-dev] Semantics for non-byte-sized stores? (or whenever "store size in bits" is different than "size in bits")

[Björn Pettersson A via llvm-dev]

Thanks Eli!

I was looking for the information in the LangRef, but I guess I was stupid enough not to read the semantics for load/store (I thought that I would find the information in the earlier parts before the description of operations).

Afaict we can end up with different kinds of problems in DeadStoreElimination today.
Worst case scenario includes miscompiles.
So, I wrote a PR here:
  https://bugs.llvm.org/show_bug.cgi?id=41949



One part in the LangRef baffled me:

"When loading a value of a type like ``i20`` with a size that is not an integral number
of bytes, the result is undefined if the value was not originally
written using a store of the same type."

I think that when using unions it is common that there could be different types
used in the store and the load. Example first using two separate i16 stores to
store an { i16, i16 } structure, followed by an i32 load that loads all the 32 bits
in one single operation.

  %a = alloca i32
  %b = bitcast i32* %a to { i16, i16 }*
  %b0 = getelementptr inbounds { i16, i16 }, { i16, i16 }* %b, i32 0, i32 0
  %b1 = getelementptr inbounds { i16, i16 }, { i16, i16 }* %b, i32 0, i32 1
  store i16 10, i16* %b0
  store i16 20, i16* %b1
  ...
  %x = load i32, i32* %a

Is the result of the load really undefined for such IR?
(Or maybe this is something that would need to be formalized as well?)

/Björn

> -----Original Message-----
> From: Eli Friedman <efriedma at quicinc.com>
> Sent: den 17 maj 2019 21:09
> To: Björn Pettersson A <bjorn.a.pettersson at ericsson.com>; llvm-dev <llvm-
> dev at lists.llvm.org>
> Subject: RE: [llvm-dev] Semantics for non-byte-sized stores? (or whenever
> "store size in bits" is different than "size in bits")
> 
> Comments inline
> 
> > -----Original Message-----
> > From: llvm-dev <llvm-dev-bounces at lists.llvm.org> On Behalf Of Björn
> > Pettersson A via llvm-dev
> > Sent: Friday, May 17, 2019 7:34 AM
> > To: llvm-dev at lists.llvm.org
> > Subject: [EXT] [llvm-dev] Semantics for non-byte-sized stores? (or
> whenever
> > "store size in bits" is different than "size in bits")
> >
> > Assume that I have LLVM IR for a union between two types that are mapped
> to
> > i32 and i19.
> > And then I have two overlapping stores to this union such as:
> >
> >   %u = alloca %union
> >   %u32 = bitcast %union* %u to i32*
> >   %u19 = bitcast %union* %u to i19*
> >   store i32 -1, i32* %u32
> >   store i19  0, i19* %u19
> >   %result = load i32, i32* %u32
> >
> > How many bits are guaranteed to be zero in %result?
> 
> According to LangRef, 19.  In practice, 24.
> 
> > How many bits are guaranteed to be one in %result?
> 
> 8.
> 
> > To be more specific:
> >
> >  a) Is the i19 store defined as only writing 19 bits (even if store size
> is larger, so it
> > will become a load-modify-write after legalization)?
> 
> Loads and stores only operate on a whole number of bytes, determined by
> getTypeStoreSize.  So 24 bits are stored, and no bits are loaded.
> 
> >  b) Is the i19 store defined as touching 24 bits, so we get 5 bits that
> are
> > undefined (or always zero-extended/sign-extended)?
> 
> LangRef says the 5 bits are determined by the target: an i19 load is only
> defined if the value was constructed using an i19 store.  But in practice,
> SelectionDAG legalization always zero-extends stores, and loads assume the
> value is zero-extended.
> 
> >   %u = alloca %union
> >   %u32 = bitcast %union* %u to i32*
> >   %u19 = bitcast %union* %u to i19*
> >   %u16 = bitcast %union* %u to i16*
> >   store i19  0, i19* %u19
> >   store i16 -1, i16* %u16
> >   %result = load i32, i32* %u32
> 
> The first store stores three bytes, the second store stores two bytes, the
> load loads four bytes.  So there's one byte of data from the first store,
> and two bytes from the second.
> 
> > Yet another question is if the i19 value is guaranteed to be placed in
> the least
> > significant end of the 3 bytes that are tainted by the store?
> 
> IIRC, this isn't formally specified in LangRef, but in practice an i19 is
> laid out as if it were zero-extended to i24.  I'd suggest bailing out of
> any transform that would care, though.
> 
> I've been considering formalizing the current SelectionDAG behavior in
> LangRef, but I haven't written a patch.
> 
> -Eli