On Mon, 22 Oct 2018, Martin Liška wrote: > On 10/11/18 3:03 PM, Jakub Jelinek wrote: > > On Thu, Oct 11, 2018 at 02:56:14PM +0200, Martin Liška wrote: > >> As seen in the PR, switch conversion can do better when we return equal > >> numbers > >> based on index value. I implemented more than that, more precisely I > >> support all linear > >> transformation based on index value. It's the same what clang is capable > >> of. > > > > Not a review, just a question, do you check for overflows while computing > > it? Or force the arithmetics to be performed in a type with defined > > overflow. It would be bad to introduced UB... > > Very valid question. I hope as long as I calculate the linear function > values in wide_int (get via wi::to_wide (switch_element)), then it should > overflow in the same way as original tree type arithmetic. I have a test-case > with > overflow: gcc/testsuite/gcc.dg/tree-ssa/pr84436-4.c.
Sorry I missed this the first time around. Note the testcase should use 'signed char', otherwise it depends on whether plain 'char' is signed or not. I think this is the kind of invalid transformation Jakub was concerned about, the transformation introduces signed multiplication that overflows and assumes it would wrap around. Alexander
