rjmccall added inline comments.
================ Comment at: clang/lib/CodeGen/CGExprScalar.cpp:3444 case BO_NE: + return Builder.CreateICmpNE(FullLHS, FullRHS); + case BO_Mul: ---------------- leonardchan wrote: > rjmccall wrote: > > Are padding bits guaranteed zero or unspecified? Or are we just not really > > supporting padding bits all the way to IRGen at this time? > I believe the consensus was leaving them unspecified, so operations that can > cause overflow into them would result in undefined behavior. If I'm understanding you correctly, you're saying that (1) we are assuming that inputs are zero-padded and (2) we are taking advantage of the non-saturating-overflow-is-UB rule to avoid clearing the padding bits after arithmetic. That's actually what I meant by "guaranteed zero", so we have our labels reversed, but at least we understand each other now. (I suppose you're thinking of this as "unspecified" because non-saturating arithmetic can leave an unspecified value there. I think of this as a "guarantee" because it's basically a representational invariant: it's a precondition for correctness that the bit is zero, and all operations guarantee that the bit will be zero in their well-defined cases (but overflow is not well-defined). Just a difference in perspective, I guess.) Is this written down somewhere? Are there targets that use the opposite ABI rule that we might need to support someday? At any rate, I think it's worth adding a short comment here explaining why it's okay to do a normal comparison despite the possibility of padding bits. Along those lines, is there any value in communicating to the backend that padded-unsigned comparisons could reasonably be done with either a signed or unsigned comparison? Are there interesting targets where one or the other is cheaper? Repository: rC Clang CHANGES SINCE LAST ACTION https://reviews.llvm.org/D57219/new/ https://reviews.llvm.org/D57219 _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
