[cfe-commits] [llvm-commits] [Patch] Update to APSInt

Chandler Carruth chandlerc at google.com
Wed Jul 18 11:19:09 PDT 2012


On Wed, Jul 18, 2012 at 11:12 AM, John McCall <rjmccall at apple.com> wrote:

> On Jul 18, 2012, at 2:28 AM, Chandler Carruth wrote:
>
> On Tue, Jul 17, 2012 at 11:47 PM, Richard Smith <richard at metafoo.co.uk>wrote:
>
>> On Tue, Jul 17, 2012 at 11:26 PM, Chandler Carruth <chandlerc at google.com>wrote:
>>>
>>> I find the definition of APInt's operator== deeply troubling. Why
>>> *assert* if the bit widths aren't equal? That doesn't make a lot of sense
>>> to me. The function that it calls to actually implement it turns around and
>>> considers mismatched bitwdiths to cause *inequality*.
>>>
>>> However it seems that there is a very simple definition of equality we
>>> could use instead: zero-extended equality for APInt, and sign-extended
>>> equality for APSInt. I wonder if there would be general support for making
>>> APInt::operator== and APSInt::operator== work in this more rational model...
>>>
>>
>> APInt has no knowledge of whether its high bit is a sign bit, so always
>> zero-extending will be wrong in the case where it is in fact a sign bit.
>> APSInt does know this, so if we want to support heterogenous comparisons,
>> we should sign-extend if the APSInt is signed, and zero-extend if it is
>> unsigned. Heterogenous comparison on APInt is fundamentally unsafe, so
>> asserting there seems reasonable to me.
>>
>
> Well, Nick's comment may obviate the extension question, which leaves us
> with a simpler problem of comparing the same sizes for equality or
> inequality. I don't actually see any problems comparing same-sized APInts
> and APSInts for equality or inequality as-if they were both APInts. Given
> two APSInts, I think that the signedness should participate in the equality
> test though...
>
>
> It seems silly for APInt to treat bitwidth inequality as an illegal
> operation but APSInt to treat it as a semantic difference.  APInt's
> assertions *do* find bugs;  I would much rather extend those to APSInt than
> have it forge a new contract.
>

I never intended to suggest inconsistency between the two. I just didn't
understand the motivation for the assertion even at the APInt layer. Nick
provided that though, which was all I needed. =]
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