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

Wed Apr 4 11:34:32 PDT 2018

I'd argue that bitfield sign extensions are surprising and are usually a
source of bugs. It would be much more explicit and less error prone for the
user to write the sign extension if they want it.

By extension, it seems good that PointerIntPair doesn't do sign extension
when the type happens to be signed.

On Wed, Apr 4, 2018 at 9:47 AM David Blaikie via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> I think it'd be reasonable to model this on the same behavior as int to
> short to int round-tripping & not to speculate that there might be code
> relying on the existing behavior until there's evidence of it.
>
> I'd suggest changing the behavior & testing to see if anything breaks -
> and if nothing does, moving to the behavior rather than supporting both.
>
> On Wed, Apr 4, 2018 at 9:27 AM Riyaz Puthiyapurayil <
> Riyaz.Puthiyapurayil at synopsys.com> wrote:
>
>> 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> 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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