[llvm-dev] llvm::PointerIntPair -- is this by design or a bug?

[David Blaikie via llvm-dev]

I'd suggest someone try fixing this & see if it breaks anything that can't
reasonably be fixed (before we go assuming this is by design/shouldn't be
fixed just because it's the way it is today).

On Wed, Apr 4, 2018 at 7:16 AM Riyaz Puthiyapurayil via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> It won't move the sign bit, so negative values won't fit, unless you have
> a 3 bit signed type ;)
>
>
> Note that if you assign negative values to and then read from a signed
> bit-field, you would do sign extension. So 3-bit signed types do exist in
> C++.
>
> It begs the question why PointerIntPair supports signed int types if it
> always loses the sign. Is it just to avoid signed/unsigned comparison when
> comparing the return value of getInt with signed types? Or to use enums
> that default to a signed type? In any case, this should be clearly
> documented if there is no intention to fix it.
>
>
> / Riyaz
>
> On Apr 4, 2018, at 4:04 AM, Florian Hahn <florian.hahn at arm.com> wrote:
>
>
>
> On 04/04/2018 11:15, David Chisnall wrote:
>
> On 4 Apr 2018, at 11:01, Florian Hahn via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>
> Hi,
>
>
> On 04/04/2018 05:34, Riyaz Puthiyapurayil via llvm-dev wrote:
>
> llvm::PointerIntPair<double*, 3, signed> P;
>
> P.setInt(-4);
>
> Ideally, the value range for a 3-bit signed integer should be [-4,3]. But
> the above call to setInt will fail. Essentially, the signed int field in
> PointerIntPair is behaving the same as an 3-bit unsigned field which has
> the legal value range of [0,7]. Is this by design? Are negative values not
> allowed in PointerIntPair?
>
>
> I think that's by design. setInt only allows you to set integer values
> that fit into the available bits. It won't move the sign bit, so negative
> values won't fit, unless you have a 3 bit signed type ;)
>
> That doesn’t sound right (for any computer made in the last few decades),
> the representation of -3 will be 1111…1111101.  Storing the low bits will
> yield 101, which is a 3-bit negative three.  When you then sign extend this
> to any other signed type, you will get -3 in that representation.  It
> sounds as if the signed specialisation of PointerIntPair is simply not
> doing the sign extension.
>
>
> Yep, I meant it looks like it currently does not do a sign extension, it
> expects only the available bits to be set, no others. In any case, it is
> probably worth documenting the behaviour.
>
> Cheers,
> Florian
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