Hi! _Fract types can't express 1, other spots that call build_one_cst already make sure that the type is integral or uses the !ALL_FRACT_MODE_P (TYPE_MODE (type)) check I've added in this patch.
Bootstrapped/regtested on x86_64-linux and i686-linux and tested with a cross to arm on the testcase. Ok for trunk? 2017-07-13 Jakub Jelinek <ja...@redhat.com> PR tree-optimization/81428 * match.pd (X / X -> one): Don't optimize _Fract divisions, as 1 can't be built for those types. * gcc.dg/fixed-point/pr81428.c: New test. --- gcc/match.pd.jj 2017-07-13 15:37:34.000000000 +0200 +++ gcc/match.pd 2017-07-13 15:46:11.194593051 +0200 @@ -243,8 +243,9 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT) /* X / X is one. */ (simplify (div @0 @0) - /* But not for 0 / 0 so that we can get the proper warnings and errors. */ - (if (!integer_zerop (@0)) + /* But not for 0 / 0 so that we can get the proper warnings and errors. + And not for _Fract types where we can't build 1. */ + (if (!integer_zerop (@0) && !ALL_FRACT_MODE_P (TYPE_MODE (type))) { build_one_cst (type); })) /* X / abs (X) is X < 0 ? -1 : 1. */ (simplify --- gcc/testsuite/gcc.dg/fixed-point/pr81428.c.jj 2017-07-13 15:49:52.980806440 +0200 +++ gcc/testsuite/gcc.dg/fixed-point/pr81428.c 2017-07-13 15:49:29.000000000 +0200 @@ -0,0 +1,9 @@ +/* PR tree-optimization/81428 */ +/* { dg-do compile } */ +/* { dg-options "-O2" } */ + +void +foo (long _Fract *a, long _Fract *b) +{ + *b = *a / *a; +} Jakub