Hi,
Thanks for your comments! Segher Boessenkool <seg...@kernel.crashing.org> writes: > Hi! > > On Wed, Jun 07, 2023 at 04:21:11PM +0800, Jiufu Guo wrote: >> This patch tries to optimize "(X - N * M) / N" to "X / N - M". >> For C code, "/" towards zero (trunc_div), and "X - N * M" maybe >> wrap/overflow/underflow. So, it is valid that "X - N * M" does >> not cross zero and does not wrap/overflow/underflow. > > Is it ever valid semi-generally when N does not divide X? It is valid only if there is no wrap/overflow/underflow, and the sign of "X" and "X-N*M" are the same. Under this condition, N,M and X can be any value. > > Say X=5, N=2, M=3. Then the original expression evaluates to 0, but the > new one to -1. Whenever one of the divisions rounds up and the other > rounds down you have this problem. You are right. Since '/' is always towards zero, so, 'X' and 'X-N*M' should have the same sign bit. Otherwise, one rounds up, the other rounds down, then the transform is invalid. BR, Jeff (Jiufu Guo) > > > Segher
