https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93567
--- Comment #8 from Dominique d'Humieres <dominiq at lps dot ens.fr> ---
Simpler patch with comments
--- ../_clean/libgfortran/io/write_float.def 2020-06-13 03:11:55.000000000
+0200
+++ libgfortran/io/write_float.def 2020-07-21 23:03:08.000000000 +0200
@@ -987,16 +987,19 @@ determine_en_precision (st_parameter_dt
w = default_width;\
d = precision;\
}\
+ /* The switch between FMT_E and FMT_F is based on the absolute value. \
+ Set r=0 for rounding toward zero and r = 1 otherwise. \
+ If (exp_d - m) == 1 there is no rounding needed. */\
switch (dtp->u.p.current_unit->round_status)\
{\
case ROUND_ZERO:\
- r = sign_bit ? 1.0 : 0.0;\
+ r = 0.0;\
break;\
case ROUND_UP:\
- r = 1.0;\
+ r = sign_bit ? 0.0 : 1.0;\
break;\
case ROUND_DOWN:\
- r = 0.0;\
+ r = sign_bit ? 1.0 : 0.0;\
break;\
default:\
break;\
@@ -1004,7 +1007,8 @@ determine_en_precision (st_parameter_dt
exp_d = calculate_exp_ ## x (d);\
r_sc = (1 - r / exp_d);\
temp = 0.1 * r_sc;\
- if ((m > 0.0 && ((m < temp) || (r >= (exp_d - m))))\
+ if ((m > 0.0 && ((m < temp) || (r < 1 && r >= (exp_d - m))\
+ || (r == 1 && 1 > (exp_d - m))))\
|| ((m == 0.0) && !(compile_options.allow_std\
& (GFC_STD_F2003 | GFC_STD_F2008)))\
|| d == 0)\
