On Sun, Sep 3, 2023 at 7:16 PM Torbjörn Granlund <t...@gmplib.org> wrote: > > Andrew Teylu <andrewvaugh...@gmail.com> writes: > > When I run `multiply.c` from gmpbench [https://gmplib.org/gmpbench], > I'm seeing an unsigned integer overflow in `toom_eval_pm2.c` on this > line: > > neg ^= ((k & 1) - 1) > > I fully appreciate that unsigned integer overflow is implementation > defined, but I am not sure if this is the intended behaviour of > `mpn_toom_eval_pm2` or not. > > In C, unsigned arithmetic is completely defined as computing mod 2^k, > where k is the bit size of the corresponding type. > > I am not sure the arithmetic on unsigned types is what clang is unhappy > about, though. Perhaps it dislikes the xor with "neg", which is a > signed variable. > > Arithmetic on signed types as well as assignments between signed and > unsigned is not well-defined for certain operand ranges. >
I guess maybe a different question is: what is that code supposed to do? Is the intent to `xor` with `0xFFFFFFFF` if the `k` is even and `xor` with `0` if the `k` is odd (i.e., it either flips all the bits in the even case or leaves them in the odd case)? Cheers, Andrew _______________________________________________ gmp-bugs mailing list gmp-bugs@gmplib.org https://gmplib.org/mailman/listinfo/gmp-bugs
