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PR fortran/40539 Document LOGICAL representation

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From-SVN: r158080
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Janne Blomqvist 2010-04-07 21:01:24 +03:00
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6
gcc/fortran/ChangeLog
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@ -1,3 +1,9 @@
2010-04-07 Janne Blomqvist <jb@gcc.gnu.org>
PR fortran/40539
* gfortran.texi: Add section about representation of
LOGICAL variables.
2010-04-07 Simon Baldwin <simonb@google.com>
* cpp.c (cb_cpp_error): Add warning reason argument, set a value

37
gcc/fortran/gfortran.texi
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@ -181,7 +181,7 @@ Part I: Invoking GNU Fortran
Part II: Language Reference
* Fortran 2003 and 2008 status:: Fortran 2003 and 2008 features supported by GNU Fortran.
* Compiler Characteristics:: KIND type parameters supported.
* Compiler Characteristics:: User-visible implementation details.
* Mixed-Language Programming:: Interoperability with C
* Extensions:: Language extensions implemented by GNU Fortran.
* Intrinsic Procedures:: Intrinsic procedures supported by GNU Fortran.
@ -962,14 +962,13 @@ about the current Fortran 2008 implementation status.
@node Compiler Characteristics
@chapter Compiler Characteristics
@c TODO: Formulate this introduction a little more generally once
@c there is more here than KIND type parameters.
This chapter describes certain characteristics of the GNU Fortran compiler,
namely the KIND type parameter values supported.
This chapter describes certain characteristics of the GNU Fortran
compiler, that are not specified by the Fortran standard, but which
might in some way or another become visible to the programmer.
@menu
* KIND Type Parameters::
* Internal representation of LOGICAL variables::
@end menu
@ -1013,6 +1012,32 @@ imaginary part are a real value of the given size). It is recommended to use
the @code{SELECT_*_KIND} intrinsics instead of the concrete values.
@node Internal representation of LOGICAL variables
@section Internal representation of LOGICAL variables
@cindex logical, variable representation
The Fortran standard does not specify how variables of @code{LOGICAL}
type are represented, beyond requiring that @code{LOGICAL} variables
of default kind have the same storage size as default @code{INTEGER}
and @code{REAL} variables. The GNU Fortran internal representation is
as follows.
A @code{LOGICAL(KIND=N)} variable is represented as an
@code{INTEGER(KIND=N)} variable, however, with only two permissible
values: @code{1} for @code{.TRUE.} and @code{0} for
@code{.FALSE.}. Any other integer value results in undefined behavior.
Note that for mixed-language programming using the
@code{ISO_C_BINDING} feature, there is a @code{C_BOOL} kind that can
be used to create @code{LOGICAL(KIND=C_BOOL)} variables which are
interoperable with the C99 _Bool type. The C99 _Bool type has an
internal representation described in the C99 standard, which is
identical to the above description, i.e. with 1 for true and 0 for
false being the only permissible values. Thus the internal
representation of @code{LOGICAL} variables in GNU Fortran is identical
to C99 _Bool, except for a possible difference in storage size
depending on the kind.
@c ---------------------------------------------------------------------
@c Extensions
@c ---------------------------------------------------------------------