Hi! The testcase in the PR constexpr bool a = __builtin_nan ("") > 0.0; constexpr bool b = __builtin_nans ("") > 0.0; constexpr bool c = __builtin_nan ("") < 0.0; constexpr bool d = __builtin_nans ("") < 0.0; constexpr bool e = __builtin_nan ("") >= 0.0; constexpr bool f = __builtin_nans ("") >= 0.0; constexpr bool g = __builtin_nan ("") <= 0.0; constexpr bool h = __builtin_nans ("") <= 0.0; has inconsistent behavior, we fold c and d initializers to 0 and don't fold any other comparisons to zero. Not including the testcase in the testsuite because I really don't know if it should be accepted or rejected (it is all accepted with -fno-trapping-math). The reason we optimize those < 0.0 comparisons to 0 is: /* Convert ABS_EXPR<x> < 0 to false. */ strict_overflow_p = false; if (code == LT_EXPR && (integer_zerop (arg1) || real_zerop (arg1)) && tree_expr_nonnegative_warnv_p (arg0, &strict_overflow_p)) and we return true for __builtin_nan ("") (but not for -__builtin_nan ("").
Now, it just feels wrong to me to say that NaN with sign bit clear is non-negative, but other than the above inconsistency I haven't been able to construct a miscompiled testcase, only questionable thing is that we fold away also comparisons of sNaN < 0.0 - but then while for sNaN >= 0.0 we don't fold that away during gimple optimizations, we fold it during RTL optimizations at least on x86_64. So, I really don't know if we want this or not, posting it for discussions. 2020-11-26 Jakub Jelinek <ja...@redhat.com> PR c++/97965 * fold-const.c (tree_single_nonnegative_warnv_p): Don't return true for NaNs with sign bit clear. --- gcc/fold-const.c.jj 2020-11-24 09:02:25.330419895 +0100 +++ gcc/fold-const.c 2020-11-25 15:37:14.426229476 +0100 @@ -14186,7 +14186,9 @@ tree_single_nonnegative_warnv_p (tree t, return tree_int_cst_sgn (t) >= 0; case REAL_CST: - return ! REAL_VALUE_NEGATIVE (TREE_REAL_CST (t)); + return (! REAL_VALUE_NEGATIVE (TREE_REAL_CST (t)) + /* Don't consider NaNs non-negative. */ + && ! REAL_VALUE_ISNAN (TREE_REAL_CST (t))); case FIXED_CST: return ! FIXED_VALUE_NEGATIVE (TREE_FIXED_CST (t)); Jakub