Re: [patch, libgfortran] PR48906 Wrong rounding results with -m32

[Thomas Henlich <thenlich@users.sourceforge.net>]

On Tue, Jun 14, 2011 at 06:51, jerry DeLisle <jvdelisle@charter.net> wrote:
>> It should be easy to implement:
>>
>> After the switch between F and E editing, we just need to shift the
>> decimal point and decrement the exponent. No new rounding is required,
>> because we keep the number of significant digits.
>>
>
> OK, after a little bit of experimentation, I have arrived at the updated
> patch attached.
>
> This has been regression tested and passes all test cases I am aware of.  I
> also have included a new test case gcc/testsuite/gfortran.dg/fmt_g.f90.
>
> OK for trunk?

I have reviewed your patch, and I noticed that you placed the
digit-shifting code quite at the top of output_float(), where the
final value of e is not even known. Due to rounding, e can be modified
after this point, so your code will generate invalid output in some
cases, for example:

print "(-2PG0)", nearest(0.1d0, -1.0d0) ! 1.0000000000000000E+001
expected .0099999999999999992E+001

Please put the code where at belongs, after the switch between F and E
editing (based on the final value of e).

The same applies to the scale factor in general, e.g.

print "(-2pg12.3)", 0.096    ! 1.00E+01 expected 0.001E+02
print "(-1pg12.3)", 0.0996   ! 1.00E+00 expected 0.010E+01
print "(-2pg12.3)", 0.09996  ! 1.00E+01 expected 0.100
print "(-1pg12.3)", 0.09996  ! 1.00E+00 expected 0.100
print "(1pg12.3)",  0.099996 ! 1.000E-01 expected 0.100
print "(2pg12.3)",  0.099996 ! 10.00E-02 expected 0.100
print "(-2pg12.3)",  999.6  ! 0.100E+04 expected 0.001E+06
print "(-1pg12.3)",  999.6  ! 0.100E+04 expected 0.010E+05
print "(1pg12.3)",  999.6  ! 0.100E+04 expected 9.996E+02
print "(2pg12.3)",  999.6  ! 0.100E+04 expected 99.96E+01

Please revise your code to fix this. A working approach I have outlined in
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48906#c28
and an (alpha) implementation is here:
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48906#c31

Thomas