[LLVMdev] The definition of getTypeSize

[Duncan Sands]

Hi Evan, thanks for replying.

> > Now that I'm working on codegen support for arbitrary precision
> > integers (think i36 or i129), I've hit the problem of what
> > getTypeSize and friends should return for such integers (the
> > current implementations seem to me to be wrong).  However it's
> > not clear to me how getTypeSize and friends are defined.
> >
> > There seem to be several possible meanings for the size of a type
> > (only talking about primitive types here):
> >
> > (1) The minimum number of bits needed to hold all values of the type.
> > (2) The minimum number of bits read by a load (maybe a better way of
> > saying this: if you load a value and store it somewhere else, how many
> > bits are correctly copied?)
> > (3) The maximum number of bits that may be overwritten by a store.
> > (4) The amount of memory allocated by an alloca or malloc for a
> > variable of this type.
> > (5) The spacing between successive variables of this type in an
> > array or struct.
> >
> > For example, take i36.  For this type (1) is 36; (2) is also 36
> > (a load typically expands to two 32 bit loads, but any bits beyond 36
> > are discarded); (3) is 64 given my current implementation (a store  
> > writes
> > two 32 bit values; bits beyond bit 36 hold some rubbish which  
> > overwrites
> > whatever was originally at that memory location); (4) needs to be at
> > least 64; (5) will also be 64.
> 
> Why is (5) 64? Can it be 40?

This is due to a technical difficulty on big-endian machines.  In fact
only 40 bits will be written, but with the simplest implementation this
will be the first 5 bytes of 8 on a little-endian machine and the last
5 bytes of 8 on a big-endian machine.  Thus with this implementation in
general you need to allocate at least 8 bytes for a variable of this type.
In order to always only write to the first 5 bytes I would need to generalize
TRUNCSTORE to write to a range of bits starting from a possibly non-zero
bit-offset.  Even then I'm not sure it can always be done on all targets
(see below).  This doesn't seem worth the trouble.

> Should (4) be the same as (5) since alloca / malloc are allocating an  
> array of the specific type?

Yes, I think so.  Currently alloca allocates a multiple of getTypeSize
(see visitAlloca in SelectionDAGISel).  This seems to be a bug - it
needs to use getABITypeSize.  This also means that all the (many) places
that use getTypeSize as the amount of memory allocated by an alloca need
to be changed...

> > In general (1) and (2) will be the same.  (4) needs to be at least
> > as big as (3).  (5) needs to be at least as big as (4).
> 
> Do you really need all these "size"? What about just "size in bits",  
> "storage size in bits", and "abi size"? The first is the exact size  
> of the type (i.e. 36); the second is the size rounded up to some  
> nature boundary for load / store (i.e. 64); the last one is the  size  
> including alignment padding when it's part of a larger object (i.e.  
> 40?).

Given that (1) = (2) = "size in bits", (3) = "storage size in bits",
and (4) = (5) = "abi size", we are in agreement here.  But I wanted
to be sure that everyone agrees about these notions.  I disagree that
for i36 the "storage size in bits" should be 64 while the "abi size"
should be 40, though I understand where you are coming from.  Since the
"abi size" you describe is clearly (5), and you just suggested that
(4) and (5) should be the same, you seem to be saying that an alloca
should allocate 40 bits.  However at the same time you suggest that
stores ("storage size in bits") should be 64 bits, bigger than the size
that would be alloc'd!  Well, probably I misunderstood you.  This
ease of misunderstanding is one reason why I want to better document
the meaning of the existing size methods.

> > Another example is 80-bit floating point types.  Here (1), (2)
> > and (3) are presumably 80 bits.  On my machine (5) is 96 bits.
> > I'm not sure what (4) is, presumably 80 or 96.
> >
> > Which (if any) of these should getTypeSize, getABITypeSize,  
> > getTypeSizeInBits
> > and getABITypeSizeInBits correspond to?
> 
> TypeSize == "real size", ABITypeSize == "abi size". You will need  
> another pair for the storage size?

What is "real size"?  Do you mean getTypeSizeInBits rounded up to a
multiple of 8?  That was my first thought too, but I came to the conclusion
that it isn't useful.  What seems to me more useful is (3), the number
of bits that may be overwritten by a store.  You may well wonder why this
isn't the same thing.  Why isn't it 40 for an i36?  As I explained above this
because the simplest implementation causes 5 bytes to be overwritten, but
starting at a 3 byte offset from the pointer on a big-endian machine.
Maybe it is worth me working harder and ensuring that always only the first
5 bytes are overwritten.  Then getTypeSize could be 5 which would be what
everyone expects.  But how is this to work on a target that can't do byte
stores?  On such a machine it seems to me that getTypeSize would have to be
strictly bigger than getTypeSizeInBits rounded up to a multiple of 8...
How does this work for i8 on such machines?

> > It seems clear that getTypeSizeInBits corresponds to (1) and (2), as
> > shown by it returning 36 for i36.  This is like gcc's TYPE_PRECISION,
> > and is a useful concept - but I think the name should be changed,  
> > since
> > right now it implicitly suggests it returns 8*getTypeSize.  If no one
> > objects, I will rename it to getBitsUsedByType.
> 
> Isn't it the other way around? Type information should be specified  
> in bits, not in bytes. So getTypeSizeInBits returns the exact size in  
> bits. I don't see how the new name is any clearer. I actually prefer  
> the current name.

For me the problem is that "Size" is too overloaded.  I'm fine with keeping
the name - in that case I'll just add some more documentation.

> > Currently getTypeSize doesn't seem to correspond to any of these  
> > possibilities,
> > at least for APInt's: the current implementation returns the APInt  
> > bitwidth rounded
> > up to a multiple of the alignment.  That makes it sound like it's  
> > trying to be (5).
> > I think getTypeSize should be defined to be (3), the maximum number  
> > of bits that
> > may be overwritten by a store [except that it's in bytes].  This  
> > means changing the
> > implementation for APInts, but not for other types.
> 
> To me getTypeSize is getTypeSizeInBits divided by 8 and rounded up. I  
> think renaming it to getTypeSizeInBytes make sense.

Many parts of LLVM are using getTypeSize as the size of an alloca.  That
seems to be wrong - I guess they should all be using getABITypeSize.  In
that case is getTypeSize useful?  I don't see the point of having it be
getTypeSizeInBits rounded up to a multiple of 8.  In fact that seems dangerous.
What would it be used for?  The number of bits stored in a write?  But that
is (3) and as I pointed out it may need to be a bigger number.  The number
of bits read in a load?  But that might be less bits (36 for an i36) so you
may make wrong conclusions.  I would rather define it as (3), since that has
a pretty clear meaning, and seems useful.

> > Clearly getABITypeSize corresponds to (5) [except that it's in  
> > bytes].  This
> > corresponds to gcc's TYPE_SIZE.
> 
> Right.
> 
> >
> > Currently getABITypeSizeInBits returns 36 for i36, and otherwise  
> > 8*getABITypeSize.
> > It seems to me that this is clearly wrong for APInts, and that it  
> > should always
> > return 8*getABITypeSize (in which case it can be eliminated).  If  
> > no one objects,
> > I will delete it as redundant.
> 
> I'd suggest you not deleting anything for now. Let these evolve and  
> see what really makes sense after a while.

Sure.

Ciao,

Duncan.

> > Finally, looking through various users of getTypeSize, it seems  
> > that they assume
> > that getTypeSize returns (4), the amount of memory allocated for a  
> > variable of
> > this type.  That seems reasonable.  If everyone agrees, I will  
> > document that
> > for LLVM (3) and (4) coincide, and is what getTypeSize returns  
> > [except that it
> > returns the number of bytes, rather than the number of bits].
> >
> > Best wishes,
> >
> > Duncan.
> 
>