On Wed, Jul 18, 2012 at 11:12 AM, John McCall <[email protected]> wrote:
> On Jul 18, 2012, at 2:28 AM, Chandler Carruth wrote: > > On Tue, Jul 17, 2012 at 11:47 PM, Richard Smith <[email protected]>wrote: > >> On Tue, Jul 17, 2012 at 11:26 PM, Chandler Carruth >> <[email protected]>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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