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[binutils.git] / gas / README-vms-dbg

	1) You should be aware that GNU-C, as with any other decent compiler,
will do things when optimization is turned on that you may not expect. 
Sometimes intermediate results are not written to variables, if they are only
used in one place, and sometimes variables that are not used at all will not be
written to the symbol table.  Also, parameters to inline functions are often
inaccessible. You can see the assembly code equivalent by using KP7 in the
debugger, and from this you can tell if in fact a variable should have the
value that you expect.  You can find out if a variable lives withing a register
by doing a 'show symbol/addr'.

	2) Overly complex data types, such as:

int (*(*(*(*(*(* sarr6)[1])[1])[2])[3])[4])[5];

will not be debugged properly, since the debugging record overflows an internal
debugger buffer.  gcc-as will convert these to *void as far as the debugger
symbol table is concerned, which will avoid any problems, and the assembler
will give you a message informing you that this has happened.

	3) You must, of course, compile and link with /debug.  If you link
without debug, you still get traceback table in the executable, but there is no
symbol table for variables.

	4) Included in the patches to VMS.C are fixes to two bugs that are
unrelated to the changes that I have made.  One of these made it impossible to
debug small programs sometimes, and the other caused the debugger to become
confused about which routine it was in, and give this incorrect info in
tracebacks.

	5) If you are using the GNU-C++ compiler, you should modify the
compiler driver file GNU_CC:[000000]GCC.COM (or GXX.COM).  If you have a
seperate GXX.COM, then you need to change one line in GXX.COM to:
$ if f$locate("D",p2) .ne. P2_Length then Debug = " ""-G0"""
                                       Notice zero--->  ^
If you are using a GCC.COM that does both C and C++, add the following lines to
GCC.COM:

$!
$! Use old style debugging records for VMS
$!
$ if (Debug.nes."" ).and. Plus then Debug = " ""-G0"""

after the variables Plus and Debug are set.  The reason for this, is that C++
compiler by default generates debugging records that are more complex,
with many new syntactical elements that allow for the new features of the
language.  The -G0 switch tells the C++ compiler to use the old style debugging
records.  Until the debugger understands C++ there is not any point to try and
use the expanded syntax.

	6) When you have nested scopes, i.e.:
main(){
	int i;
	{int i;
		{int i;
};};}
and you say "EXAM i" the debugger needs to figure out which variable you
actually want to reference.  I have arranged things to define a block to the
debugger when you use brackets to enter a new scope, so in the example above,
the variables would be described as:
TEST\main\i
TEST\main\$0\i
TEST\main\$0\$0\i
At each level, the block name is a number with a dollar sign prefix, the
numbers start with 0 and count upward.  When you say EXAM i, the debugger looks
at the current PC, and decides which block it is currently in.  It works from
the innermost level outward until it finds a block that has the variable "i"
defined.  You can always specify the scope explicitly.

	7)  With C++, there can be a lot of inline functions, and it would be
rather restrictive to force the user to debug the program by converting all of
the inline functions to normal functions.  What I have done is to essentially
"add" (with the debugger) source lines from the include files that contain the
inline functions.  Thus when you step into an inline function it appears as if
you have called the function, and you can examine variables and so forth. 
There are several *very* important differences, however.  First of all, since
there is no function call involved, you cannot step over the inline function
call - you always step into it. Secondly, since the same source lines are used
in many locations, there is a seperate copy of the source for *each* usage. 
Without this, breakpoints do not work, since we must have a 1-to-1 mapping
between source lines and PC.
	Since you cannot step over inline function calls, it can be a real pain
if you are not really interested in what is going on for that function call.
What I have done is to use the "-D" switch for the assembler to toggle the
following behavior.  With the "-D" switch, all inline functions are included in
the object file, and you can debug everything.  Without the "-D" switch
(default case with VMS implementation), inline functions are included *only* if
they did not come from system header files (i.e. from GNU_CC_INCLUDE: or
GNU_GXX_INCLUDE:).  Thus, without the switch the user only debugs his/her own
inline functions, and not the system ones. (This is especially useful if you do
a lot of stream I/O in C++).  This probably will not provide enough granularity
for many users, but for now this is still somewhat experimental, and I would
like to reflect upon it and get some feedback before I go any further. 
Possible solutions include an interactive prompting, a logical name, or a new
command line option in gcc.c (which is then passed through somehow to the guts
of the assembler).
	The inline functions from header files appear after the source code
for the source file.  This has the advantage that the source file itself is
numbered with the same line numbers that you get with an editor.  In addition,
the entire header file is not included, since the assembler makes a list of
the min and max source lines that are used, and only includes those lines from
the first to the last actually used. (It is easy to change it to include the
whole file).

	8) When you are debugging C++ objects, the object "this" is refered to
as "$this".  Actually, the compiler writes it as ".this", but the period is
not good for the debugger, so I have a routine to convert it to a $.  (It
actually converts all periods to $, but only for variables, since this was
intended to allow us to access "this".

	9) If you use the asm("...") keyword for global symbols, you will not
be able to see that symbol with the debugger.  The reason is that there are two
records for the symbol stored in the data structures of the assembler.  One
contains the info such as psect number and offset, and the other one contains
the information having to do with the data type of the variable.  In order to
debug as symbol, you need to be able to coorelate these records, and the only
way to do this is by name.  The record with the storage attributes will take
the name used in the asm directive, and the record that specifies the data type
has the actual variable name, and thus when you use the asm directive to change
a variable name, the symbol becomes invisible.

	10) Older versions of the compiler ( GNU-C 1.37.92 and earlier) place
global constants in the text psect.  This is unfortunate, since to the linker
this appears to be an entry point.  I sent a patch to the compiler to RMS,
which will generate a .const section for these variables, and patched the
assembler to put these variables into a psect just like that for normal
variables, except that they are marked NOWRT.  static constants are still
placed in the text psect, since there is no need for any external access.