fortran/89100: Default widths with -fdec-format-defaults

gcc/fortran ChangeLog:

2019-05-22  Jeff Law  <law@redhat.com>
            Mark Eggleston  <mark.eggleston@codethink.com>

        PR fortran/89100
        * gfortran.texi: Add Default widths for F, G and I format
        descriptors to Extensions section.
        * invoke.texi: Add -fdec-format-defaults
        * io.c (check_format): Use default widths for i, f and g when
        flag_dec_format_defaults is enabled.
        * lang.opt: Add new option.
        * options.c (set_dec_flags): Add SET_BITFLAG for
        flag_dec_format_defaults.


gcc/testsuite ChangeLog:

2019-05-22  Mark Eggleston  <mark.eggleston@codethink.com>

        PR fortran/89100
        * gfortran.dg/fmt_f_default_field_width_1.f90: New test.
        * gfortran.dg/fmt_f_default_field_width_2.f90: New test.
        * gfortran.dg/fmt_f_default_field_width_3.f90: New test.
        * gfortran.dg/fmt_g_default_field_width_1.f90: New test.
        * gfortran.dg/fmt_g_default_field_width_2.f90: New test.
        * gfortran.dg/fmt_g_default_field_width_3.f90: New test.
        * gfortran.dg/fmt_i_default_field_width_1.f90: New test.
        * gfortran.dg/fmt_i_default_field_width_2.f90: New test.
        * gfortran.dg/fmt_i_default_field_width_3.f90: New test.


libgfortran ChangeLog:

2019-05-22  Jeff Law  <law@redhat.com>

        PR fortran/89100
        * io/format.c (parse_format_list): set default width when the
        IOPARM_DT_DEC_EXT flag is set for i, f and g.
        * io/io.h: add default_width_for_integer, default_width_for_float
        and default_precision_for_float.
        * io/write.c (write_boz): extra parameter giving length of data
        corresponding to the type's kind.
        (write_b): pass data length as extra parameter in calls to
        write_boz.
        (write_o): pass data length as extra parameter in calls to
        write_boz.
        (write_z): pass data length as extra parameter in calls to
        write_boz.
        (size_from_kind): also set size is default width is set.
        * io/write_float.def (build_float_string): new paramter inserted
        before result parameter. If default width use values passed
        instead of the values in fnode.
        (FORMAT_FLOAT): macro modified to check for default width and
        calls to build_float_string to pass in default width.
        (get_float_string): set width and precision to defaults when
        needed.

From-SVN: r271511

gcc/fortran/ChangeLog

+2019-05-22  Jeff Law  <law@redhat.com>
+	    Mark Eggleston  <mark.eggleston@codethink.com>
+
+	PR fortran/89100
+	* gfortran.texi: Add Default widths for F, G and I format
+	descriptors to Extensions section.
+	* invoke.texi: Add -fdec-format-defaults
+	* io.c (check_format): Use default widths for i, f and g when
+	flag_dec_format_defaults is enabled.
+	* lang.opt: Add new option.
+	* options.c (set_dec_flags): Add SET_BITFLAG for
+	flag_dec_format_defaults.
+
 2019-05-21  Janne Blomqvist  <jb@gcc.gnu.org>
 
 	PR libfortran/90038

gcc/fortran/gfortran.texi

@@ -1576,6 +1576,7 @@ additional compatibility extensions along with those enabled by
 * X format descriptor without count field::
 * Commas in FORMAT specifications::
 * Missing period in FORMAT specifications::
+* Default widths for F@comma{} G and I format descriptors::
 * I/O item lists::
 * @code{Q} exponent-letter::
 * BOZ literal constants::
@@ -1782,6 +1783,22 @@ discouraged.
 10     FORMAT ('F4')
 @end smallexample
 
+@node Default widths for F@comma{} G and I format descriptors
+@subsection Default widths for @code{F}, @code{G} and @code{I} format descriptors
+
+To support legacy codes, GNU Fortran allows width to be omitted from format
+specifications if and only if @option{-fdec-format-defaults} is given on the
+command line.  Default widths will be used. This is considered non-conforming
+code and is discouraged.
+
+@smallexample
+       REAL :: value1
+       INTEGER :: value2
+       WRITE(*,10) value1, value1, value2
+10     FORMAT ('F, G, I')
+@end smallexample
+
+
 @node I/O item lists
 @subsection I/O item lists
 @cindex I/O item lists

gcc/fortran/invoke.texi

@@ -117,16 +117,16 @@ by type.  Explanations are in the following sections.
 @item Fortran Language Options
 @xref{Fortran Dialect Options,,Options controlling Fortran dialect}.
 @gccoptlist{-fall-intrinsics -fbackslash -fcray-pointer -fd-lines-as-code @gol
--fd-lines-as-comments @gol
--fdec -fdec-structure -fdec-intrinsic-ints -fdec-static -fdec-math @gol
--fdec-include -fdefault-double-8 -fdefault-integer-8 -fdefault-real-8 @gol
--fdefault-real-10 -fdefault-real-16 -fdollar-ok -ffixed-line-length-@var{n} @gol
--ffixed-line-length-none -fpad-source -ffree-form -ffree-line-length-@var{n} @gol
--ffree-line-length-none -fimplicit-none -finteger-4-integer-8 @gol
--fmax-identifier-length -fmodule-private -ffixed-form -fno-range-check @gol
--fopenacc -fopenmp -freal-4-real-10 -freal-4-real-16 -freal-4-real-8 @gol
--freal-8-real-10 -freal-8-real-16 -freal-8-real-4 -std=@var{std}
--ftest-forall-temp
+-fd-lines-as-comments -fdec -fdec-structure -fdec-intrinsic-ints @gol
+-fdec-static -fdec-math -fdec-include -fdec-format-defaults @gol
+-fdefault-double-8 -fdefault-integer-8 -fdefault-real-8 -fdefault-real-10 @gol
+-fdefault-real-16 -fdollar-ok -ffixed-line-length-@var{n} @gol
+-ffixed-line-length-none -fpad-source -ffree-form @gol
+-ffree-line-length-@var{n} -ffree-line-length-none @gol
+-fimplicit-none -finteger-4-integer-8 -fmax-identifier-length @gol
+-fmodule-private -ffixed-form -fno-range-check -fopenacc -fopenmp @gol
+-freal-4-real-10 -freal-4-real-16 -freal-4-real-8 -freal-8-real-10 @gol
+-freal-8-real-16 -freal-8-real-4 -std=@var{std} -ftest-forall-temp
 }
 
 @item Preprocessing Options
@@ -283,6 +283,11 @@ Enable parsing of INCLUDE as a statement in addition to parsing it as
 INCLUDE line.  When parsed as INCLUDE statement, INCLUDE does not have to
 be on a single line and can use line continuations.
 
+@item -fdec-format-defaults
+@opindex @code{fdec-format-defaults}
+Enable format specifiers F, G and I to be used without width specifiers,
+default widths will be used instead.
+
 @item -fdollar-ok
 @opindex @code{fdollar-ok}
 @cindex @code{$}

gcc/fortran/io.c

@@ -903,6 +903,13 @@ data_desc:
 
       if (u != FMT_POSINT)
 	{
+	  if (flag_dec_format_defaults)
+	    {
+	      /* Assume a default width based on the variable size.  */
+	      saved_token = u;
+	      break;
+	    }
+
 	  format_locus.nextc += format_string_pos;
 	  gfc_error ("Positive width required in format "
 			 "specifier %s at %L", token_to_string (t),
@@ -1027,6 +1034,13 @@ data_desc:
 	goto fail;
       if (t != FMT_ZERO && t != FMT_POSINT)
 	{
+	  if (flag_dec_format_defaults)
+	    {
+	      /* Assume the default width is expected here and continue lexing.  */
+	      value = 0; /* It doesn't matter what we set the value to here.  */
+	      saved_token = t;
+	      break;
+	    }
 	  error = nonneg_required;
 	  goto syntax;
 	}
@@ -1096,9 +1110,18 @@ data_desc:
 	goto fail;
       if (t != FMT_ZERO && t != FMT_POSINT)
 	{
+	  if (flag_dec_format_defaults)
+	    {
+	      /* Assume the default width is expected here and continue lexing.  */
+	      value = 0; /* It doesn't matter what we set the value to here.  */
+	      saved_token = t;
+	    }
+	  else
+	    {
 	      error = nonneg_required;
 	      goto syntax;
 	    }
+	}
       else if (is_input && t == FMT_ZERO)
 	{
 	  error = posint_required;
@@ -4369,7 +4392,7 @@ get_io_list:
 
   /* See if we want to use defaults for missing exponents in real transfers
      and other DEC runtime extensions. */
-  if (flag_dec)
+  if (flag_dec_format_defaults)
     dt->dec_ext = 1;
 
   /* A full IO statement has been matched.  Check the constraints.  spec_end is

gcc/fortran/lang.opt

@@ -452,6 +452,10 @@ fdec-include
 Fortran Var(flag_dec_include)
 Enable legacy parsing of INCLUDE as statement.
 
+fdec-format-defaults
+Fortran Var(flag_dec_format_defaults)
+Enable default widths for i, f and g format specifiers.
+
 fdec-intrinsic-ints
 Fortran Var(flag_dec_intrinsic_ints)
 Enable kind-specific variants of integer intrinsic functions.

gcc/fortran/options.c

@@ -74,6 +74,7 @@ set_dec_flags (int value)
   SET_BITFLAG (flag_dec_static, value, value);
   SET_BITFLAG (flag_dec_math, value, value);
   SET_BITFLAG (flag_dec_include, value, value);
+  SET_BITFLAG (flag_dec_format_defaults, value, value);
 }
 
 /* Finalize DEC flags.  */

gcc/testsuite/ChangeLog

+2019-05-22  Mark Eggleston  <mark.eggleston@codethink.com>
+
+	PR fortran/89100
+	* gfortran.dg/fmt_f_default_field_width_1.f90: New test.
+	* gfortran.dg/fmt_f_default_field_width_2.f90: New test.
+	* gfortran.dg/fmt_f_default_field_width_3.f90: New test.
+	* gfortran.dg/fmt_g_default_field_width_1.f90: New test.
+	* gfortran.dg/fmt_g_default_field_width_2.f90: New test.
+	* gfortran.dg/fmt_g_default_field_width_3.f90: New test.
+	* gfortran.dg/fmt_i_default_field_width_1.f90: New test.
+	* gfortran.dg/fmt_i_default_field_width_2.f90: New test.
+	* gfortran.dg/fmt_i_default_field_width_3.f90: New test.
+
 2019-05-22  Martin Liska  <mliska@suse.cz>
 
 	PR testsuite/90564

gcc/testsuite/gfortran.dg/fmt_f_default_field_width_1.f90

+! { dg-do run }
+! { dg-options -fdec }
+!
+! Test case for the default field widths enabled by the -fdec-format-defaults flag.
+!
+! This feature is not part of any Fortran standard, but it is supported by the
+! Oracle Fortran compiler and others.
+!
+
+program test
+    character(50) :: buffer
+
+    real(4) :: real_4
+    real(8) :: real_8
+    real(16) :: real_16
+    integer :: len
+    character(*), parameter :: fmt = "(A, F, A)"
+
+    real_4 = 4.18
+    write(buffer, fmt) ':',real_4,':'
+    print *,buffer
+    if (buffer.ne.":      4.1799998:") stop 1
+
+    real_4 = 0.00000018
+    write(buffer, fmt) ':',real_4,':'
+    print *,buffer
+    if (buffer.ne.":      0.0000002:") stop 2
+
+    real_8 = 4.18
+    write(buffer, fmt) ':',real_8,':'
+    print *,buffer
+    len = len_trim(buffer)
+    if (len /= 27) stop 3
+
+    real_16 = 4.18
+    write(buffer, fmt) ':',real_16,':'
+    print *,buffer
+    len = len_trim(buffer)
+    if (len /= 44) stop 4
+end

gcc/testsuite/gfortran.dg/fmt_f_default_field_width_2.f90

+! { dg-do run }
+! { dg-options -fdec-format-defaults }
+!
+! Test case for the default field widths enabled by the -fdec-format-defaults flag.
+!
+! This feature is not part of any Fortran standard, but it is supported by the
+! Oracle Fortran compiler and others.
+!
+! Test case added by Mark Eggleston <mark.eggleston@codethink.com> to check
+! use of -fdec-format-defaults
+!
+
+program test
+    character(50) :: buffer
+
+    real(4) :: real_4
+    real(8) :: real_8
+    real(16) :: real_16
+    integer :: len
+    character(*), parameter :: fmt = "(A, F, A)"
+
+    real_4 = 4.18
+    write(buffer, fmt) ':',real_4,':'
+    print *,buffer
+    if (buffer.ne.":      4.1799998:") stop 1
+
+    real_4 = 0.00000018
+    write(buffer, fmt) ':',real_4,':'
+    print *,buffer
+    if (buffer.ne.":      0.0000002:") stop 2
+
+    real_8 = 4.18
+    write(buffer, fmt) ':',real_8,':'
+    print *,buffer
+    len = len_trim(buffer)
+    if (len /= 27) stop 3
+
+    real_16 = 4.18
+    write(buffer, fmt) ':',real_16,':'
+    print *,buffer
+    len = len_trim(buffer)
+    if (len /= 44) stop 4
+end

gcc/testsuite/gfortran.dg/fmt_f_default_field_width_3.f90

+! { dg-do compile }
+! { dg-options "-fdec -fno-dec-format-defaults" }
+!
+! Test case for the default field widths not enabled.
+!
+! Test case added by Mark Eggleston <mark.eggleston@codethink.com> to check
+! use of -fno-dec-format-defaults
+!
+
+program test
+    character(50) :: buffer
+
+    real*4 :: real_4
+    real*8 :: real_8
+    real*16 :: real_16
+    integer :: len
+    character(*), parameter :: fmt = "(A, F, A)"
+
+    real_4 = 4.18
+    write(buffer, fmt) ':',real_4,':' ! { dg-error "Nonnegative width required" }
+
+    real_4 = 0.00000018
+    write(buffer, fmt) ':',real_4,':' ! { dg-error "Nonnegative width required" }
+
+    real_8 = 4.18
+    write(buffer, fmt) ':',real_8,':' ! { dg-error "Nonnegative width required" }
+
+    real_16 = 4.18
+    write(buffer, fmt) ':',real_16,':' ! { dg-error "Nonnegative width required" }
+end

gcc/testsuite/gfortran.dg/fmt_g_default_field_width_1.f90

+! { dg-do run }
+! { dg-options -fdec }
+!
+! Test case for the default field widths enabled by the -fdec-format-defaults flag.
+!
+! This feature is not part of any Fortran standard, but it is supported by the
+! Oracle Fortran compiler and others.
+!
+
+program test
+    character(50) :: buffer
+
+    real(4) :: real_4
+    real(8) :: real_8
+    real(16) :: real_16
+    integer :: len
+    character(*), parameter :: fmt = "(A, G, A)"
+
+    real_4 = 4.18
+    write(buffer, fmt) ':',real_4,':'
+    print *,buffer
+    if (buffer.ne.":   4.180000    :") stop 1
+
+    real_4 = 0.00000018
+    write(buffer, fmt) ':',real_4,':'
+    print *,buffer
+    if (buffer.ne.":  0.1800000E-06:") stop 2
+
+    real_4 = 18000000.4
+    write(buffer, fmt) ':',real_4,':'
+    print *,buffer
+    if (buffer.ne.":  0.1800000E+08:") stop 3
+
+    real_8 = 4.18
+    write(buffer, fmt) ':',real_8,':'
+    print *,buffer
+    len = len_trim(buffer)
+    if (len /= 27) stop 4
+
+    real_16 = 4.18
+    write(buffer, fmt) ':',real_16,':'
+    print *,buffer
+    len = len_trim(buffer)
+    if (len /= 44) stop 5
+end

gcc/testsuite/gfortran.dg/fmt_g_default_field_width_2.f90

+! { dg-do run }
+! { dg-options -fdec-format-defaults }
+!
+! Test case for the default field widths enabled by the -fdec-format-defaults flag.
+!
+! This feature is not part of any Fortran standard, but it is supported by the
+! Oracle Fortran compiler and others.
+!
+! Test case added by Mark Eggleston <mark.eggleston@codethink.com> to check
+! use of -fdec-format-defaults
+!
+
+program test
+    character(50) :: buffer
+
+    real(4) :: real_4
+    real(8) :: real_8
+    real(16) :: real_16
+    integer :: len
+    character(*), parameter :: fmt = "(A, G, A)"
+
+    real_4 = 4.18
+    write(buffer, fmt) ':',real_4,':'
+    print *,buffer
+    if (buffer.ne.":   4.180000    :") stop 1
+
+    real_4 = 0.00000018
+    write(buffer, fmt) ':',real_4,':'
+    print *,buffer
+    if (buffer.ne.":  0.1800000E-06:") stop 2
+
+    real_4 = 18000000.4
+    write(buffer, fmt) ':',real_4,':'
+    print *,buffer
+    if (buffer.ne.":  0.1800000E+08:") stop 3
+
+    real_8 = 4.18
+    write(buffer, fmt) ':',real_8,':'
+    print *,buffer
+    len = len_trim(buffer)
+    if (len /= 27) stop 4
+
+    real_16 = 4.18
+    write(buffer, fmt) ':',real_16,':'
+    print *,buffer
+    len = len_trim(buffer)
+    if (len /= 44) stop 5
+end

gcc/testsuite/gfortran.dg/fmt_g_default_field_width_3.f90

+! { dg-do compile }
+! { dg-options "-fdec -fno-dec-format-defaults" }
+!
+! Test case for the default field widths not enabled.
+!
+! Test case added by Mark Eggleston <mark.eggleston@codethink.com> to check
+! use of -fno-dec-format-defaults
+!
+
+program test
+    character(50) :: buffer
+
+    real(4) :: real_4
+    real(8) :: real_8
+    real(16) :: real_16
+    integer :: len
+    character(*), parameter :: fmt = "(A, G, A)"
+
+    real_4 = 4.18
+    write(buffer, fmt) ':',real_4,':' ! { dg-error "Positive width required" }
+
+    real_4 = 0.00000018
+    write(buffer, fmt) ':',real_4,':' ! { dg-error "Positive width required" }
+
+    real_4 = 18000000.4
+    write(buffer, fmt) ':',real_4,':' ! { dg-error "Positive width required" }
+
+    real_8 = 4.18
+    write(buffer, fmt) ':',real_8,':' ! { dg-error "Positive width required" }
+
+    real_16 = 4.18
+    write(buffer, fmt) ':',real_16,':' ! { dg-error "Positive width required" }
+end

gcc/testsuite/gfortran.dg/fmt_i_default_field_width_1.f90

+! { dg-do run }
+! { dg-options -fdec }
+!
+! Test case for the default field widths enabled by the -fdec-format-defaults flag.
+!
+! This feature is not part of any Fortran standard, but it is supported by the
+! Oracle Fortran compiler and others.
+
+program test
+    character(50) :: buffer
+    character(1) :: colon
+
+    integer(2) :: integer_2
+    integer(4) :: integer_4
+    integer(8) :: integer_8
+    character(*), parameter :: fmt = "(A, I, A)"
+
+    write(buffer, fmt) ':',12340,':'
+    print *,buffer
+    if (buffer.ne.":       12340:") stop 1
+
+    read(buffer, "(1A, I, 1A)") colon, integer_4, colon
+    if ((integer_4.ne.12340).or.(colon.ne.":")) stop 2
+
+    integer_2 = -99
+    write(buffer, fmt) ':',integer_2,':'
+    print *,buffer
+    if (buffer.ne.":    -99:") stop 3
+
+    integer_8 = -11112222
+    write(buffer, fmt) ':',integer_8,':'
+    print *,buffer
+    if (buffer.ne.":              -11112222:") stop 4
+
+! If the width is 7 and there are 7 leading zeroes, the result should be zero.
+    integer_2 = 789
+    buffer = '0000000789'
+    read(buffer, '(I)') integer_2
+    if (integer_2.ne.0) stop 5
+end

gcc/testsuite/gfortran.dg/fmt_i_default_field_width_2.f90

+! { dg-do run }
+! { dg-options -fdec-format-defaults }
+!
+! Test case for the default field widths enabled by the -fdec-format-defaults flag.
+!
+! This feature is not part of any Fortran standard, but it is supported by the
+! Oracle Fortran compiler and others.
+!
+! Test case added by Mark Eggleston <mark.eggleston@codethink.com> to check
+! use of -fdec-format-defaults
+!
+
+program test
+    character(50) :: buffer
+    character(1) :: colon
+
+    integer(2) :: integer_2
+    integer(4) :: integer_4
+    integer(8) :: integer_8
+    character(*), parameter :: fmt = "(A, I, A)"
+
+    write(buffer, fmt) ':',12340,':'
+    print *,buffer
+    if (buffer.ne.":       12340:") stop 1
+
+    read(buffer, '(A1, I, A1)') colon, integer_4, colon
+    if ((integer_4.ne.12340).or.(colon.ne.":")) stop 2
+
+    integer_2 = -99
+    write(buffer, fmt) ':',integer_2,':'
+    print *,buffer
+    if (buffer.ne.":    -99:") stop 3
+
+    integer_8 = -11112222
+    write(buffer, fmt) ':',integer_8,':'
+    print *,buffer
+    if (buffer.ne.":              -11112222:") stop 4
+
+! If the width is 7 and there are 7 leading zeroes, the result should be zero.
+    integer_2 = 789
+    buffer = '0000000789'
+    read(buffer, '(I)') integer_2
+    if (integer_2.ne.0) stop 5
+end

gcc/testsuite/gfortran.dg/fmt_i_default_field_width_3.f90

+! { dg-do compile }
+! { dg-options "-fdec -fno-dec-format-defaults" }
+!
+! Test case for the default field widths enabled by the -fdec-format-defaults flag.
+!
+! This feature is not part of any Fortran standard, but it is supported by the
+! Oracle Fortran compiler and others.
+!
+! Test case added by Mark Eggleston <mark.eggleston@codethink.com> to check
+! use of -fdec-format-defaults
+!
+
+program test
+    character(50) :: buffer
+    character(1) :: colon
+
+    integer(2) :: integer_2
+    integer(4) :: integer_4
+    integer(8) :: integer_8
+    character(*), parameter :: fmt = "(A, I, A)"
+
+    write(buffer, fmt) ':',12340,':' ! { dg-error "Nonnegative width required" }
+
+    read(buffer, '(A1, I, A1)') colon, integer_4, colon ! { dg-error "Nonnegative width required" }
+    if (integer_4.ne.12340) stop 2
+
+    integer_2 = -99
+    write(buffer, fmt) ':',integer_2,':' ! { dg-error "Nonnegative width required" }
+
+    integer_8 = -11112222
+    write(buffer, fmt) ':',integer_8,':' ! { dg-error "Nonnegative width required" }
+
+! If the width is 7 and there are 7 leading zeroes, the result should be zero.
+    integer_2 = 789
+    buffer = '0000000789'
+    read(buffer, '(I)') integer_2 ! { dg-error "Nonnegative width required" }
+end

libgfortran/ChangeLog

+2019-05-22  Jeff Law  <law@redhat.com>
+
+	PR fortran/89100
+	* io/format.c (parse_format_list): set default width when the
+	IOPARM_DT_DEC_EXT flag is set for i, f and g.
+	* io/io.h: add default_width_for_integer, default_width_for_float
+	and default_precision_for_float.
+	* io/write.c (write_boz): extra parameter giving length of data
+	corresponding to the type's kind.
+	(write_b): pass data length as extra parameter in calls to
+	write_boz.
+	(write_o): pass data length as extra parameter in calls to
+	write_boz.
+	(write_z): pass data length as extra parameter in calls to
+	write_boz.
+	(size_from_kind): also set size is default width is set.
+	* io/write_float.def (build_float_string): new paramter inserted
+	before result parameter. If default width use values passed
+	instead of the values in fnode.
+	(FORMAT_FLOAT): macro modified to check for default width and
+	calls to build_float_string to pass in default width.
+	(get_float_string): set width and precision to defaults when
+	needed.
+
 2019-05-19  Janne Blomqvist  <jb@gcc.gnu.org>
 
         PR libfortran/90038

libgfortran/io/format.c

@@ -956,12 +956,33 @@ parse_format_list (st_parameter_dt *dtp, bool *seen_dd)
 	  *seen_dd = true;
 	  if (u != FMT_POSINT && u != FMT_ZERO)
 	    {
+	      if (dtp->common.flags & IOPARM_DT_DEC_EXT)
+		{
+		  tail->u.real.w = DEFAULT_WIDTH;
+		  tail->u.real.d = 0;
+		  tail->u.real.e = -1;
+		  fmt->saved_token = u;
+		  break;
+		}
 	      fmt->error = nonneg_required;
 	      goto finished;
 	    }
 	}
+      else if (u == FMT_ZERO)
+	{
+	  fmt->error = posint_required;
+	  goto finished;
+	}
       else if (u != FMT_POSINT)
 	{
+	  if (dtp->common.flags & IOPARM_DT_DEC_EXT)
+	    {
+	      tail->u.real.w = DEFAULT_WIDTH;
+	      tail->u.real.d = 0;
+	      tail->u.real.e = -1;
+	      fmt->saved_token = u;
+	      break;
+	    }
 	  fmt->error = posint_required;
 	  goto finished;
 	}
@@ -1100,6 +1121,13 @@ parse_format_list (st_parameter_dt *dtp, bool *seen_dd)
 	{
 	  if (t != FMT_POSINT)
 	    {
+	      if (dtp->common.flags & IOPARM_DT_DEC_EXT)
+		{
+		  tail->u.integer.w = DEFAULT_WIDTH;
+		  tail->u.integer.m = -1;
+		  fmt->saved_token = t;
+		  break;
+		}
 	      fmt->error = posint_required;
 	      goto finished;
 	    }
@@ -1108,6 +1136,13 @@ parse_format_list (st_parameter_dt *dtp, bool *seen_dd)
 	{
 	  if (t != FMT_ZERO && t != FMT_POSINT)
 	    {
+	      if (dtp->common.flags & IOPARM_DT_DEC_EXT)
+		{
+		  tail->u.integer.w = DEFAULT_WIDTH;
+		  tail->u.integer.m = -1;
+		  fmt->saved_token = t;
+		  break;
+		}
 	      fmt->error = nonneg_required;
 	      goto finished;
 	    }

libgfortran/io/io.h

@@ -1011,6 +1011,56 @@ memset4 (gfc_char4_t *p, gfc_char4_t c, int k)
     *p++ = c;
 }
 
+/* Used in width fields to indicate that the default should be used */
+#define DEFAULT_WIDTH -1
+
+/* Defaults for certain format field descriptors. These are decided based on
+ * the type of the value being formatted.
+ *
+ * The behaviour here is modelled on the Oracle Fortran compiler. At the time
+ * of writing, the details were available at this URL:
+ *
+ *   https://docs.oracle.com/cd/E19957-01/805-4939/6j4m0vnc3/index.html#z4000743746d
+ */
+
+static inline int
+default_width_for_integer (int kind)
+{
+  switch (kind)
+    {
+    case 1:
+    case 2:  return  7;
+    case 4:  return 12;
+    case 8:  return 23;
+    case 16: return 44;
+    default: return  0;
+    }
+}
+
+static inline int
+default_width_for_float (int kind)
+{
+  switch (kind)
+    {
+    case 4:  return 15;
+    case 8:  return 25;
+    case 16: return 42;
+    default: return  0;
+    }
+}
+
+static inline int
+default_precision_for_float (int kind)
+{
+  switch (kind)
+    {
+    case 4:  return 7;
+    case 8:  return 16;
+    case 16: return 33;
+    default: return 0;
+    }
+}
+
 #endif
 
 extern void

libgfortran/io/read.c

@@ -635,6 +635,12 @@ read_decimal (st_parameter_dt *dtp, const fnode *f, char *dest, int length)
 
   w = f->u.w;
 
+  /* This is a legacy extension, and the frontend will only allow such cases
+   * through when -fdec-format-defaults is passed.
+   */
+  if (w == DEFAULT_WIDTH)
+    w = default_width_for_integer (length);
+
   p = read_block_form (dtp, &w);
 
   if (p == NULL)

libgfortran/io/write.c

@@ -685,9 +685,8 @@ write_l (st_parameter_dt *dtp, const fnode *f, char *source, int len)
   p[wlen - 1] = (n) ? 'T' : 'F';
 }
 
-
 static void
-write_boz (st_parameter_dt *dtp, const fnode *f, const char *q, int n)
+write_boz (st_parameter_dt *dtp, const fnode *f, const char *q, int n, int len)
 {
   int w, m, digits, nzero, nblank;
   char *p;
@@ -720,6 +719,9 @@ write_boz (st_parameter_dt *dtp, const fnode *f, const char *q, int n)
   /* Select a width if none was specified.  The idea here is to always
      print something.  */
 
+  if (w == DEFAULT_WIDTH)
+    w = default_width_for_integer (len);
+
   if (w == 0)
     w = ((digits < m) ? m : digits);
 
@@ -846,6 +848,8 @@ write_decimal (st_parameter_dt *dtp, const fnode *f, const char *source,
 
   /* Select a width if none was specified.  The idea here is to always
      print something.  */
+  if (w == DEFAULT_WIDTH)
+    w = default_width_for_integer (len);
 
   if (w == 0)
     w = ((digits < m) ? m : digits) + nsign;
@@ -1206,13 +1210,13 @@ write_b (st_parameter_dt *dtp, const fnode *f, const char *source, int len)
   if (len > (int) sizeof (GFC_UINTEGER_LARGEST))
     {
       p = btoa_big (source, itoa_buf, len, &n);
-      write_boz (dtp, f, p, n);
+      write_boz (dtp, f, p, n, len);
     }
   else
     {
       n = extract_uint (source, len);
       p = btoa (n, itoa_buf, sizeof (itoa_buf));
-      write_boz (dtp, f, p, n);
+      write_boz (dtp, f, p, n, len);
     }
 }
 
@@ -1227,13 +1231,13 @@ write_o (st_parameter_dt *dtp, const fnode *f, const char *source, int len)
   if (len > (int) sizeof (GFC_UINTEGER_LARGEST))
     {
       p = otoa_big (source, itoa_buf, len, &n);
-      write_boz (dtp, f, p, n);
+      write_boz (dtp, f, p, n, len);
     }
   else
     {
       n = extract_uint (source, len);
       p = otoa (n, itoa_buf, sizeof (itoa_buf));
-      write_boz (dtp, f, p, n);
+      write_boz (dtp, f, p, n, len);
     }
 }
 
@@ -1247,13 +1251,13 @@ write_z (st_parameter_dt *dtp, const fnode *f, const char *source, int len)
   if (len > (int) sizeof (GFC_UINTEGER_LARGEST))
     {
       p = ztoa_big (source, itoa_buf, len, &n);
-      write_boz (dtp, f, p, n);
+      write_boz (dtp, f, p, n, len);
     }
   else
     {
       n = extract_uint (source, len);
       p = gfc_xtoa (n, itoa_buf, sizeof (itoa_buf));
-      write_boz (dtp, f, p, n);
+      write_boz (dtp, f, p, n, len);
     }
 }
 
@@ -1491,7 +1495,7 @@ size_from_kind (st_parameter_dt *dtp, const fnode *f, int kind)
 {
   int size;
 
-  if (f->format == FMT_F && f->u.real.w == 0)
+  if ((f->format == FMT_F && f->u.real.w == 0) || f->u.real.w == DEFAULT_WIDTH)
     {
       switch (kind)
       {

libgfortran/io/write_float.def

@@ -113,7 +113,8 @@ determine_precision (st_parameter_dt * dtp, const fnode * f, int len)
 static void
 build_float_string (st_parameter_dt *dtp, const fnode *f, char *buffer,
 		    size_t size, int nprinted, int precision, int sign_bit,
-		    bool zero_flag, int npad, char *result, size_t *len)
+		    bool zero_flag, int npad, int default_width, char *result,
+		    size_t *len)
 {
   char *put;
   char *digits;
@@ -132,8 +133,17 @@ build_float_string (st_parameter_dt *dtp, const fnode *f, char *buffer,
   sign_t sign;
 
   ft = f->format;
+  if (f->u.real.w == DEFAULT_WIDTH)
+    /* This codepath can only be reached with -fdec-format-defaults. */
+    {
+      w = default_width;
+      d = precision;
+    }
+  else
+    {
       w = f->u.real.w;
       d = f->u.real.d;
+    }
   p = dtp->u.p.scale_factor;
   *len = 0;
 
@@ -960,6 +970,11 @@ determine_en_precision (st_parameter_dt *dtp, const fnode *f,
       int save_scale_factor;\
       volatile GFC_REAL_ ## x temp;\
       save_scale_factor = dtp->u.p.scale_factor;\
+      if (w == DEFAULT_WIDTH)\
+	{\
+	  w = default_width;\
+	  d = precision;\
+	}\
       switch (dtp->u.p.current_unit->round_status)\
 	{\
 	  case ROUND_ZERO:\
@@ -1035,7 +1050,8 @@ determine_en_precision (st_parameter_dt *dtp, const fnode *f,
 	  nprinted = FDTOA(y,precision,m);\
 	}\
       build_float_string (dtp, &newf, buffer, size, nprinted, precision,\
-				   sign_bit, zero_flag, npad, result, res_len);\
+				   sign_bit, zero_flag, npad, default_width,\
+				   result, res_len);\
       dtp->u.p.scale_factor = save_scale_factor;\
     }\
   else\
@@ -1045,7 +1061,8 @@ determine_en_precision (st_parameter_dt *dtp, const fnode *f,
       else\
 	nprinted = DTOA(y,precision,m);\
       build_float_string (dtp, f, buffer, size, nprinted, precision,\
-				   sign_bit, zero_flag, npad, result, res_len);\
+				   sign_bit, zero_flag, npad, default_width,\
+				   result, res_len);\
     }\
 }\
 
@@ -1059,6 +1076,16 @@ get_float_string (st_parameter_dt *dtp, const fnode *f, const char *source,
 {
   int sign_bit, nprinted;
   bool zero_flag;
+  int default_width = 0;
+
+  if (f->u.real.w == DEFAULT_WIDTH)
+    /* This codepath can only be reached with -fdec-format-defaults. The default
+     * values are based on those used in the Oracle Fortran compiler.
+     */
+    {
+      default_width = default_width_for_float (kind);
+      precision = default_precision_for_float (kind);
+    }
 
   switch (kind)
     {