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

[Richard Trieu]

On Wed, Jul 18, 2012 at 11:19 AM, Chandler Carruth <chandlerc at google.com>wrote:

> 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. =]
>
> New patch.  Switched int parameters to int64_t.  APSInt::operator!= now
refers to APSInt::operator==
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