[lldb-dev] Signedness of scalars built from APInt(s)

[Jonas Devlieghere via lldb-dev]

On Fri, Jan 4, 2019 at 3:13 PM Zachary Turner <zturner at google.com> wrote:

> I don't think #2 is a correct change.  Just because the sign bit is set
> doesn't mean it's signed.  Is the 4-byte value 0x10000000 signed or
> unsigned?  It's a trick question, because there's not enough information!
> If it was written "int x = 0x10000000" then it's signed (and negative).  If
> it was written "unsigned x = 0x10000000" then it's unsigned (and
> positive).  What about the 4-byte value 0x1?  Still a trick!  If it was
> written "int x = 1" then it's signed (and positive), and if it was written
> "unsigned x = 1" then it's unsigned (and positive).
>
> My point is that signedness of the *type* does not necessarly imply
> signedness of the value, and vice versa.
>
> APInt is purely a bit-representation and a size, there is no information
> whatsoever about whether type *type* is signed.  It doesn't make sense to
> say "is this APInt negative?" without additional information.
>
> With APSInt, on the other hand, it does make sense to ask that question.
> If you have an APSInt where isSigned() is true, *then* you can use the sign
> bit to determine whether it's negative.  And if you have an APSInt where
> isSigned() is false, then the "sign bit" is not actually a sign bit at all,
> it is just an extra power of 2 for the unsigned value.
>
> This is my understanding of the classes, someone correct me if I'm wrong.
>

> IIUC though, the way to fix this is by using APSInt throughout the class,
> and delete all references to APInt.
>

I think we share the same understanding. If we know at every call site
whether the type is signed or not then I totally agree, we should only use
APSInt. The reason I propose doing (2) first is for the first scenario you
described, where you don't know. Turning it into an explicit APSInt is as
bad as using an APInt and looking at the value. The latter has the
advantage that it conveys that you don't know, while the other may or may
not be a lie.


> On Fri, Jan 4, 2019 at 2:58 PM Jonas Devlieghere <jonas at devlieghere.com>
> wrote:
>
>> If I understand the situation correctly I think we should do both. I'd
>> start by doing (2) to improve the current behavior and add a constructor
>> for APSInt. We can then audit the call sites and migrate to APSInt where
>> it's obvious that the type is signed. That should match the semantics of
>> both classes?
>>
>> On Fri, Jan 4, 2019 at 2:00 PM Davide Italiano <dccitaliano at gmail.com>
>> wrote:
>>
>>> On Fri, Jan 4, 2019 at 1:57 PM Davide Italiano <dccitaliano at gmail.com>
>>> wrote:
>>> >
>>> > While adding support for 512-bit integers in `Scalar`, I figured I
>>> > could add some coverage.
>>> >
>>> > TEST(ScalarTest, Signedness) {
>>> >  auto s1 = Scalar(APInt(32, 12, false /* isSigned */));
>>> >  auto s2 = Scalar(APInt(32, 12, true /* isSigned */ ));
>>> >  ASSERT_EQ(s1.GetType(), Scalar::e_uint); // fails
>>> >  ASSERT_EQ(s2.GetType(), Scalar::e_sint); // pass
>>> > }
>>> >
>>> > The result of `s1.GetType()` is Scalar::e_sint.
>>> > This is because an APInt can't distinguish between "int patatino = 12"
>>> > and "uint patatino = 12".
>>> > The correct class in `llvm` to do that is `APSInt`.
>>> >
>>>
>>> Please note that this is also broken in the case where you have
>>> APInt(32 /* bitWidth */, -323);
>>> because of the way the constructor is implemented.
>>>
>>> --
>>> Davide
>>>
>>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/lldb-dev/attachments/20190104/206f08a9/attachment.html>