This file lists known bugs in the GNU Fortran compiler. Copyright (C) 1995, 1996 Free Software Foundation, Inc. You may copy, distribute, and modify it freely as long as you preserve this copyright notice and permission notice.

# Bugs in GNU Fortran

This section identifies bugs that `g77` users might run into. 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 that might afflict people who configure, port, build, and install `g77`, Problems Installing.

- `g77`'s version of `gcc`, and probably `g77` itself, cannot be reliably used with the `-O2` option (or higher) on Digital Semiconductor Alpha AXP machines. The problem is most immediately noticed in differences discovered by `make compare` following a bootstrap build using `-O2`. It also manifests itself as a failure to compile `DATA` statements such as `DATA R/7./` correctly; in this case, `R` might be initialized to `4.0`.

  Until this bug is fixed, use only `-O1` or no optimization.

- 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. (Probably improved, but not wholly fixed, in 0.5.21.)
- The `g77` command itself should more faithfully process options the way the `gcc` command does. For example, `gcc` accepts abbreviated forms of long options, `g77` generally doesn't.
- 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 time.

  (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 compiler.)

  Note that `g77` does display a warning message to notify the user before the compiler appears to hang. See Initialization of Large Aggregate Areas, for information on how to change the point at which `g77` decides to issue this warning.

- `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 well.

  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.

  See Options for Code Generation Conventions, 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` 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's good for performance).

  It is possible that this is not a real bug, and could be considered a performance feature, but it might be important to provide the ability to Fortran code to specify minimum padding for aggregate areas such as common blocks---and, certainly, there is the potential, with the current setup, for interface differences in the way such areas are laid out between `g77` and other compilers.

- `g77` doesn't work perfectly on 64-bit configurations such as the Alpha. This problem is expected to be largely resolved as of version 0.5.20, and further addressed by 0.5.21. Version 0.6 should solve most or all related problems (such as 64-bit machines other than Digital Semiconductor (``DEC'') Alphas).

  One known bug that causes a compile-time crash occurs when compiling code such as the following with optimization:

      SUBROUTINE CRASH (TEMP)
      INTEGER*2 HALF(2)
      REAL TEMP
      HALF(1) = NINT (TEMP)
      END

  It is expected that a future version of `g77` will have a fix for this problem, almost certainly by the time `g77` supports the forthcoming version 2.8.0 of `gcc`.

- Maintainers of gcc report that the back end definitely has ``broken'' support for `COMPLEX` types. Based on their input, it seems many of the problems affect only the more-general facilities for gcc's `__complex__` type, such as `__complex__ int` (where the real and imaginary parts are integers) that GNU Fortran does not use.

  Version 0.5.20 of `g77` works around this problem by not using the back end's support for `COMPLEX`. The new option `-fno-emulate-complex` avoids the work-around, reverting to using the same ``broken'' mechanism as that used by versions of `g77` prior to 0.5.20.

- There seem to be some problems with passing constants, and perhaps general expressions (other than simple variables/arrays), to procedures when compiling on some systems (such as i386) with `-fPIC`, as in when compiling for ELF targets. The symptom is that the assembler complains about invalid opcodes. This bug is in the gcc back end, and it apparently occurs only when compiling sufficiently complicated functions without the `-O` option.