vsk added a comment.
In D73148#1832704 <https://reviews.llvm.org/D73148#1832704>, @clayborg wrote:
> Would it be better to just ensure that the buffer for DW_OP_piece is at least
> the size of the type instead? To do this right we would really need to track
> which bytes in the buffer are actually valid. This patch assumes we will
> always get the first N bytes of a value. Is it possible to describe bytes 4:7
> of a 16 bit value where bits 0:3 and 8:15 are not valid? In this case, with
> this patch, we would think that bytes 0:3 are valid when they are just not
> specified, 4:7 would be valid, and 8:15 would not display because our value
> doesn't contain the value.
It's not necessary for DW_OP_piece to produce a type-sized result buffer, as
the description of an object must start at byte offset 0, and we represent the
undefined bits within the Value either with 0 or by leaving them out. E.g.
`Evaluate({DW_OP_piece, 1, DW_OP_const1u, 0xff, DW_OP_piece, 1, DW_OP_piece,
1})` produces the 24-bit scalar `0x00ff00`, where the 0's correspond to the
undefined low bits and the undefined high bits are left out. It /is/ a bug that
we use 0 for undefined bits instead of `u` (or something), but we don't need a
type-sized buffer to fix that (a bitset would suffice). Finding a way to
improve the representation of undefined bits is something I'm interested in,
but it seems like a separate problem, and it's more urgent for me to fix the
memory smasher.
I'm also not sure that making DW_OP_piece produce a type-sized result would be
a sufficient fix, as there (theoretically) may be clients of
`Value::ResizeData()` other than the DW_OP_piece logic. I just count one,
though (`ExpressionVariable::GetValueBytes()`), and I'm not sure whether it can
resize the buffer to something other than the underlying type size.
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https://reviews.llvm.org/D73148/new/
https://reviews.llvm.org/D73148
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