https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109008
--- Comment #28 from Jakub Jelinek <jakub at gcc dot gnu.org> --- On IRC we've discussed this and I believe a possible fix could be before we do: return float_binary_op_range_finish (minus.fold_range (r, type, lhs, op2), r, type, lhs); etc. artificially extend the lhs range by 1ulp or .5ulp or whatever works in each direction (on a copy) and then just let it do its job. But I want to fixup above patch first, so that we have something to compare to. From --- gcc/range-op-float.cc.jj 2023-03-07 21:20:49.885225381 +0100 +++ gcc/range-op-float.cc 2023-03-08 09:13:10.063608296 +0100 @@ -2319,9 +2319,15 @@ float_range_extend (tree type, frange &r u = m; } gcc_checking_assert (l + 1 == u); +fprintf (stderr, "--\n"); +SET_REAL_EXP (&t, l); +real_arithmetic (&w, PLUS_EXPR, &v, &t); +real_convert (&w, mode, &w); +test (w, type, lhs, op2); SET_REAL_EXP (&t, u); real_arithmetic (&w, PLUS_EXPR, &v, &t); real_convert (&w, mode, &w); +test (w, type, lhs, op2); REAL_VALUE_TYPE lastw = w; for (int i = SIGNIFICAND_BITS - 2; i >= SIGNIFICAND_BITS - 2 - p; i--) { @@ -2338,7 +2344,10 @@ float_range_extend (tree type, frange &r lastw = w; } w = lastw; +fprintf (stderr, "---\n"); +test (w, type, lhs, op2); frange_nextafter (mode, w, upper ? dconstninf : dconstinf); +test (w, type, lhs, op2); goto update; } @@ -2367,7 +2376,15 @@ public: REAL_VALUE_TYPE res; frange_arithmetic (PLUS_EXPR, type, res, r, op2.lower_bound (), dconstinf); - return !real_less (&res, &lhs.lower_bound ()); +bool ret = !real_less (&res, &lhs.lower_bound ()); +char br[60], bop2[60], bres[60], blhs[60]; +real_to_hexadecimal (br, &r, sizeof (br), 0, 1); +real_to_hexadecimal (bop2, &op2.lower_bound (), sizeof (bop2), 0, 1); +real_to_hexadecimal (bres, &res, sizeof (bres), 0, 1); +real_to_hexadecimal (blhs, &lhs.lower_bound (), sizeof (blhs), 0, 1); +fprintf (stderr, "float_range_extend1 %s + %s = %s %s %s\n", br, bop2, bres, ret ? ">=" : "<", blhs); +return ret; +// return !real_less (&res, &lhs.lower_bound ()); }); float_range_extend (type, r, lhs, op2, true, [] (REAL_VALUE_TYPE &r, tree type, @@ -2376,7 +2393,15 @@ public: REAL_VALUE_TYPE res; frange_arithmetic (PLUS_EXPR, type, res, r, op2.upper_bound (), dconstninf); - return !real_less (&lhs.upper_bound (), &res); +bool ret = !real_less (&lhs.upper_bound (), &res); +char br[60], bop2[60], bres[60], blhs[60]; +real_to_hexadecimal (br, &r, sizeof (br), 0, 1); +real_to_hexadecimal (bop2, &op2.lower_bound (), sizeof (bop2), 0, 1); +real_to_hexadecimal (bres, &res, sizeof (bres), 0, 1); +real_to_hexadecimal (blhs, &lhs.lower_bound (), sizeof (blhs), 0, 1); +fprintf (stderr, "float_range_extend2 %s + %s = %s %s %s\n", br, bop2, bres, ret ? "<=" : ">", blhs); +return ret; +// return !real_less (&lhs.upper_bound (), &res); }); return true; } debugging hacks seems it is the loop that tries to narrow the mantissa bits that is wrong, as immediately before it the l and u values seem correct: float_range_extend1 -0x0.8p-53 + 0x0.8p+1 = 0x0.8p+1 >= 0x0.8p+1 float_range_extend1 -0x0.8p-52 + 0x0.8p+1 = 0x0.fffffffffffff8p+0 < 0x0.8p+1 where the first is for l where test passes and u doesn't (and the right answer here is -0x0.8p-53).