http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48047
Summary: Incorrect output rounding of double precision numbers
Product: gcc
Version: 4.6.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: libfortran
AssignedTo: [email protected]
ReportedBy: [email protected]
Created attachment 23603
--> http://gcc.gnu.org/bugzilla/attachment.cgi?id=23603
Test case
The Fortran library does not round real(8) numbers correctly on output if 39
decimal digits are requested and real(16) is supported. This violates IEEE Std
754-2008 which demands that the minimum of supported significant digits for
correct rounding of all supported binary formats (H) is at least H=39 if the
binary128 format is supported.
Thus, GCC misses this requirement by 1 digit.
The attached program fails because the exact value, as given by
quadmath_snprintf(..., (__float128)0.14285714285714285) is
0.142857142857142849212692681248881854116916656494...
IEEE Std 754-2008 says:
===
5.12.2 External decimal character sequences representing finite numbers
...
For the purposes of discussing the limits on correctly rounded conversion,
define the following quantities:
...
- for binary128, Pmin (binary128) = 36
...
- M = max(Pmin(bf)) for all supported binary formats bf
...
There might be an implementation-defined limit on the number of significant
digits that can be converted with correct rounding to and from supported binary
formats. That limit, H, shall be such that H >= M + 3 and it should be that H
is unbounded.
For all supported binary formats the conversion operations shall support
correctly rounded conversions to or from external character sequences for all
significant digit counts from 1 through H (that is, for all
expressible counts if H is unbounded).
...
===
