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. John.
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