On 10/22/18 4:17 PM, Alexander Monakov wrote: > 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.
Good observation, I'll fix that. > > 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. About this, using TYPE_OVERFLOW_WRAPS (which relies on flag_wrapv) should be enough? Martin > > Alexander >
