274 lines
10 KiB
Plaintext
274 lines
10 KiB
Plaintext
@c Copyright (C) 1995, 1996, 1997, 1998, 1999, 2000, 2001 Free Software Foundation, Inc.
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@c This is part of the G77 manual.
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@c For copying conditions, see the file g77.texi.
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@c The text of this file appears in the file BUGS
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@c in the G77 distribution, as well as in the G77 manual.
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@c Keep this the same as the dates above, since it's used
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@c in the standalone derivations of this file (e.g. BUGS).
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@set copyrights-bugs 1995,1996,1997,1998,1999,2000,2001
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@set last-update-bugs 2001-06-10
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@include root.texi
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@ifset DOC-BUGS
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@c The immediately following lines apply to the BUGS file
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@c which is derived from this file.
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@emph{Note:} This file is automatically generated from the files
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@file{bugs0.texi} and @file{bugs.texi}.
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@file{BUGS} is @emph{not} a source file,
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although it is normally included within source distributions.
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This file lists known bugs in the @value{which-g77} version
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of the GNU Fortran compiler.
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Copyright (C) @value{copyrights-bugs} Free Software Foundation, Inc.
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You may copy, distribute, and modify it freely as long as you preserve
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this copyright notice and permission notice.
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@node Top,,, (dir)
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@chapter Known Bugs In GNU Fortran
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@end ifset
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@ifset DOC-G77
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@node Known Bugs
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@section Known Bugs In GNU Fortran
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@end ifset
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This section identifies bugs that @code{g77} @emph{users}
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might run into in the @value{which-g77} version
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of @code{g77}.
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This includes bugs that are actually in the @code{gcc}
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back end (GBE) or in @code{libf2c}, because those
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sets of code are at least somewhat under the control
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of (and necessarily intertwined with) @code{g77},
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so it isn't worth separating them out.
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@ifset DOC-G77
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For information on bugs in @emph{other} versions of @code{g77},
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see @ref{News,,News About GNU Fortran}.
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There, lists of bugs fixed in various versions of @code{g77}
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can help determine what bugs existed in prior versions.
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@end ifset
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@ifset DOC-BUGS
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For information on bugs in @emph{other} versions of @code{g77},
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see @file{@value{path-g77}/NEWS}.
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There, lists of bugs fixed in various versions of @code{g77}
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can help determine what bugs existed in prior versions.
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@end ifset
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@ifset DEVELOPMENT
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@emph{Warning:} The information below is still under development,
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and might not accurately reflect the @code{g77} code base
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of which it is a part.
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Efforts are made to keep it somewhat up-to-date,
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but they are particularly concentrated
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on any version of this information
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that is distributed as part of a @emph{released} @code{g77}.
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In particular, while this information is intended to apply to
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the @value{which-g77} version of @code{g77},
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only an official @emph{release} of that version
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is expected to contain documentation that is
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most consistent with the @code{g77} product in that version.
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@end ifset
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An online, ``live'' version of this document
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(derived directly from the mainline, development version
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of @code{g77} within @code{gcc})
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is available via
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@uref{http://www.gnu.org/software/gcc/onlinedocs/g77_bugs.html}.
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Follow the ``Known Bugs'' link.
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The following information was last updated on @value{last-update-bugs}:
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@itemize @bullet
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@item
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@code{g77} fails to warn about
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use of a ``live'' iterative-DO variable
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as an implied-DO variable
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in a @code{WRITE} or @code{PRINT} statement
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(although it does warn about this in a @code{READ} statement).
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@item
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Something about @code{g77}'s straightforward handling of
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label references and definitions sometimes prevents the GBE
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from unrolling loops.
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Until this is solved, try inserting or removing @code{CONTINUE}
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statements as the terminal statement, using the @code{END DO}
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form instead, and so on.
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@item
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Some confusion in diagnostics concerning failing @code{INCLUDE}
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statements from within @code{INCLUDE}'d or @code{#include}'d files.
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@cindex integer constants
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@cindex constants, integer
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@item
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@code{g77} assumes that @code{INTEGER(KIND=1)} constants range
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from @samp{-2**31} to @samp{2**31-1} (the range for
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two's-complement 32-bit values),
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instead of determining their range from the actual range of the
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type for the configuration (and, someday, for the constant).
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Further, it generally doesn't implement the handling
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of constants very well in that it makes assumptions about the
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configuration that it no longer makes regarding variables (types).
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Included with this item is the fact that @code{g77} doesn't recognize
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that, on IEEE-754/854-compliant systems, @samp{0./0.} should produce a NaN
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and no warning instead of the value @samp{0.} and a warning.
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This is to be fixed in version 0.6, when @code{g77} will use the
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@code{gcc} back end's constant-handling mechanisms to replace its own.
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@cindex compiler speed
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@cindex speed, of compiler
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@cindex compiler memory usage
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@cindex memory usage, of compiler
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@cindex large aggregate areas
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@cindex initialization, bug
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@cindex DATA statement
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@cindex statements, DATA
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@item
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@code{g77} uses way too much memory and CPU time to process large aggregate
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areas having any initialized elements.
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For example, @samp{REAL A(1000000)} followed by @samp{DATA A(1)/1/}
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takes up way too much time and space, including
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the size of the generated assembler file.
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This is to be mitigated somewhat in version 0.6.
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Version 0.5.18 improves cases like this---specifically,
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cases of @emph{sparse} initialization that leave large, contiguous
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areas uninitialized---significantly.
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However, even with the improvements, these cases still
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require too much memory and CPU time.
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(Version 0.5.18 also improves cases where the initial values are
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zero to a much greater degree, so if the above example
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ends with @samp{DATA A(1)/0/}, the compile-time performance
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will be about as good as it will ever get, aside from unrelated
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improvements to the compiler.)
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Note that @code{g77} does display a warning message to
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notify the user before the compiler appears to hang.
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@ifset DOC-G77
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A warning message is issued when @code{g77} sees code that provides
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initial values (e.g. via @code{DATA}) to an aggregate area (@code{COMMON}
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or @code{EQUIVALENCE}, or even a large enough array or @code{CHARACTER}
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variable)
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that is large enough to increase @code{g77}'s compile time by roughly
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a factor of 10.
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This size currently is quite small, since @code{g77}
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currently has a known bug requiring too much memory
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and time to handle such cases.
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In @file{@value{path-g77}/data.c}, the macro
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@code{FFEDATA_sizeTOO_BIG_INIT_} is defined
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to the minimum size for the warning to appear.
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The size is specified in storage units,
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which can be bytes, words, or whatever, on a case-by-case basis.
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After changing this macro definition, you must
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(of course) rebuild and reinstall @code{g77} for
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the change to take effect.
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Note that, as of version 0.5.18, improvements have
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reduced the scope of the problem for @emph{sparse}
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initialization of large arrays, especially those
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with large, contiguous uninitialized areas.
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However, the warning is issued at a point prior to
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when @code{g77} knows whether the initialization is sparse,
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and delaying the warning could mean it is produced
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too late to be helpful.
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Therefore, the macro definition should not be adjusted to
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reflect sparse cases.
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Instead, adjust it to generate the warning when densely
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initialized arrays begin to cause responses noticeably slower
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than linear performance would suggest.
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@end ifset
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@cindex code, displaying main source
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@cindex displaying main source code
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@cindex debugging main source code
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@cindex printing main source
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@item
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When debugging, after starting up the debugger but before being able
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to see the source code for the main program unit, the user must currently
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set a breakpoint at @code{MAIN__} (or @code{MAIN___} or @code{MAIN_} if
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@code{MAIN__} doesn't exist)
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and run the program until it hits the breakpoint.
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At that point, the
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main program unit is activated and about to execute its first
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executable statement, but that's the state in which the debugger should
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start up, as is the case for languages like C.
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@cindex debugger
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@item
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Debugging @code{g77}-compiled code using debuggers other than
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@code{gdb} is likely not to work.
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Getting @code{g77} and @code{gdb} to work together is a known
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problem---getting @code{g77} to work properly with other
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debuggers, for which source code often is unavailable to @code{g77}
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developers, seems like a much larger, unknown problem,
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and is a lower priority than making @code{g77} and @code{gdb}
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work together properly.
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On the other hand, information about problems other debuggers
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have with @code{g77} output might make it easier to properly
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fix @code{g77}, and perhaps even improve @code{gdb}, so it
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is definitely welcome.
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Such information might even lead to all relevant products
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working together properly sooner.
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@cindex Alpha, support
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@cindex support, Alpha
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@item
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@code{g77} doesn't work perfectly on 64-bit configurations
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such as the Digital Semiconductor (``DEC'') Alpha.
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This problem is largely resolved as of version 0.5.23.
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Version 0.6 should solve most or all remaining problems
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(such as cross-compiling involving 64-bit machines).
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@cindex padding
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@cindex structures
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@cindex common blocks
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@cindex equivalence areas
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@item
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@code{g77} currently inserts needless padding for things like
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@samp{COMMON A,IPAD} where @samp{A} is @code{CHARACTER*1} and @samp{IPAD}
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is @code{INTEGER(KIND=1)} on machines like x86,
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because the back end insists that @samp{IPAD}
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be aligned to a 4-byte boundary,
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but the processor has no such requirement
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(though it is usually good for performance).
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The @code{gcc} back end needs to provide a wider array
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of specifications of alignment requirements and preferences for targets,
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and front ends like @code{g77} should take advantage of this
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when it becomes available.
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@cindex complex performance
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@cindex aliasing
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@item
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The @code{libf2c} routines that perform some run-time
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arithmetic on @code{COMPLEX} operands
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were modified circa version 0.5.20 of @code{g77}
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to work properly even in the presence of aliased operands.
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While the @code{g77} and @code{netlib} versions of @code{libf2c}
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differ on how this is accomplished,
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the main differences are that we believe
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the @code{g77} version works properly
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even in the presence of @emph{partially} aliased operands.
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However, these modifications have reduced performance
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on targets such as x86,
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due to the extra copies of operands involved.
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@end itemize
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