llvm dev - Apr 2018 - llvm::PointerIntPair -- is this by design or a bug?

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<mailto: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<mailto: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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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
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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Rather than “fixing” it, it might be better to support a separate method for
signed extension. My reasoning is as follows:

int x = 7;
llvm::PointerIntPair<double*, 3, int> pip;
pip.setInt(x);

There could be code out there that expects pip.getInt() to return 7 and not -1.

So if you really want to set a negative and return a negative value, a separate
method setSignedInt and getSignedInt may be OK. Further, sign-extension would
need two shift instructions in X86 as opposed to no-sign extension where only
one ‘and’ with mask is needed for retrieving the int.

From: David Blaikie [mailto:dblaikie at gmail.com]
Sent: Wednesday, April 4, 2018 7:43 AM
To: Riyaz Puthiyapurayil <Riyaz.Puthiyapurayil at synopsys.com>
Cc: Florian Hahn <florian.hahn at arm.com>; llvm-dev <llvm-dev at
lists.llvm.org>; nd <nd at arm.com>
Subject: Re: [llvm-dev] llvm::PointerIntPair -- is this by design or a bug?

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<mailto: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<mailto: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<mailto: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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