(g77) Known Bugs
(g77) Missing Features
Known Bugs In GNU Fortran
This section identifies bugs that `g77' *users* might run into in
the GCC-2.95 version of `g77'. This includes bugs that are actually in
the `gcc' back end (GBE) or in `libf2c', because those sets of code are
at least somewhat under the control of (and necessarily intertwined
with) `g77', so it isn't worth separating them out.
For information on bugs in *other* versions of `g77', see News
About GNU Fortran News. There, lists of bugs fixed in various
versions of `g77' can help determine what bugs existed in prior
An online, "live" version of this document (derived directly from
the mainline, development version of `g77' within `egcs') is available
via `http://www.gnu.org/software/gcc/onlinedocs/g77_bugs.html'. Follow
the "Known Bugs" link.
For information on bugs that might afflict people who configure,
port, build, and install `g77', see Problems Installing.
The following information was last updated on 1999-06-29:
* `g77' fails to warn about use of a "live" iterative-DO variable as
an implied-DO variable in a `WRITE' or `PRINT' statement (although
it does warn about this in a `READ' statement).
* Something about `g77''s straightforward handling of label
references and definitions sometimes prevents the GBE from
unrolling loops. Until this is solved, try inserting or removing
`CONTINUE' statements as the terminal statement, using the `END DO'
form instead, and so on.
* Some confusion in diagnostics concerning failing `INCLUDE'
statements from within `INCLUDE''d or `#include''d files.
* `g77' assumes that `INTEGER(KIND=1)' constants range from `-2**31'
to `2**31-1' (the range for two's-complement 32-bit values),
instead of determining their range from the actual range of the
type for the configuration (and, someday, for the constant).
Further, it generally doesn't implement the handling of constants
very well in that it makes assumptions about the configuration
that it no longer makes regarding variables (types).
Included with this item is the fact that `g77' doesn't recognize
that, on IEEE-754/854-compliant systems, `0./0.' should produce a
NaN and no warning instead of the value `0.' and a warning. This
is to be fixed in version 0.6, when `g77' will use the `gcc' back
end's constant-handling mechanisms to replace its own.
* `g77' uses way too much memory and CPU time to process large
aggregate areas having any initialized elements.
For example, `REAL A(1000000)' followed by `DATA A(1)/1/' takes up
way too much time and space, including the size of the generated
assembler file. This is to be mitigated somewhat in version 0.6.
Version 0.5.18 improves cases like this--specifically, cases of
*sparse* initialization that leave large, contiguous areas
uninitialized--significantly. However, even with the
improvements, these cases still require too much memory and CPU
(Version 0.5.18 also improves cases where the initial values are
zero to a much greater degree, so if the above example ends with
`DATA A(1)/0/', the compile-time performance will be about as good
as it will ever get, aside from unrelated improvements to the
Note that `g77' does display a warning message to notify the user
before the compiler appears to hang. Initialization of
Large Aggregate Areas Large Initialization, for information on
how to change the point at which `g77' decides to issue this
* `g77' doesn't emit variable and array members of common blocks for
use with a debugger (the `-g' command-line option). The code is
present to do this, but doesn't work with at least one debug
format--perhaps it works with others. And it turns out there's a
similar bug for local equivalence areas, so that has been disabled
As of Version 0.5.19, a temporary kludge solution is provided
whereby some rudimentary information on a member is written as a
string that is the member's value as a character string.
Options for Code Generation Conventions Code Gen Options,
for information on the `-fdebug-kludge' option.
* When debugging, after starting up the debugger but before being
able to see the source code for the main program unit, the user
must currently set a breakpoint at `MAIN__' (or `MAIN___' or
`MAIN_' if `MAIN__' doesn't exist) and run the program until it
hits the breakpoint. At that point, the main program unit is
activated and about to execute its first executable statement, but
that's the state in which the debugger should start up, as is the
case for languages like C.
* Debugging `g77'-compiled code using debuggers other than `gdb' is
likely not to work.
Getting `g77' and `gdb' to work together is a known
problem--getting `g77' to work properly with other debuggers, for
which source code often is unavailable to `g77' developers, seems
like a much larger, unknown problem, and is a lower priority than
making `g77' and `gdb' work together properly.
On the other hand, information about problems other debuggers have
with `g77' output might make it easier to properly fix `g77', and
perhaps even improve `gdb', so it is definitely welcome. Such
information might even lead to all relevant products working
together properly sooner.
* `g77' doesn't work perfectly on 64-bit configurations such as the
Digital Semiconductor ("DEC") Alpha.
This problem is largely resolved as of version 0.5.23. Version
0.6 should solve most or all remaining problems (such as
cross-compiling involving 64-bit machines).
* `g77' currently inserts needless padding for things like `COMMON
A,IPAD' where `A' is `CHARACTER*1' and `IPAD' is `INTEGER(KIND=1)'
on machines like x86, because the back end insists that `IPAD' be
aligned to a 4-byte boundary, but the processor has no such
requirement (though it is usually good for performance).
The `gcc' back end needs to provide a wider array of
specifications of alignment requirements and preferences for
targets, and front ends like `g77' should take advantage of this
when it becomes available.
* The `libf2c' routines that perform some run-time arithmetic on
`COMPLEX' operands were modified circa version 0.5.20 of `g77' to
work properly even in the presence of aliased operands.
While the `g77' and `netlib' versions of `libf2c' differ on how
this is accomplished, the main differences are that we believe the
`g77' version works properly even in the presence of *partially*
However, these modifications have reduced performance on targets
such as x86, due to the extra copies of operands involved.
(g77) Missing Features
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