127 lines
7.1 KiB
Text
127 lines
7.1 KiB
Text
1) You should be aware that GNU-C, as with any other decent compiler,
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will do things when optimization is turned on that you may not expect.
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Sometimes intermediate results are not written to variables, if they are only
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used in one place, and sometimes variables that are not used at all will not be
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written to the symbol table. Also, parameters to inline functions are often
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inaccessible. You can see the assembly code equivalent by using KP7 in the
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debugger, and from this you can tell if in fact a variable should have the
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value that you expect. You can find out if a variable lives withing a register
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by doing a 'show symbol/addr'.
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2) Overly complex data types, such as:
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int (*(*(*(*(*(* sarr6)[1])[1])[2])[3])[4])[5];
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will not be debugged properly, since the debugging record overflows an internal
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debugger buffer. gcc-as will convert these to *void as far as the debugger
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symbol table is concerned, which will avoid any problems, and the assembler
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will give you a message informing you that this has happened.
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3) You must, of course, compile and link with /debug. If you link
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without debug, you still get traceback table in the executable, but there is no
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symbol table for variables.
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4) Included in the patches to VMS.C are fixes to two bugs that are
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unrelated to the changes that I have made. One of these made it impossible to
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debug small programs sometimes, and the other caused the debugger to become
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confused about which routine it was in, and give this incorrect info in
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tracebacks.
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5) If you are using the GNU-C++ compiler, you should modify the
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compiler driver file GNU_CC:[000000]GCC.COM (or GXX.COM). If you have a
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seperate GXX.COM, then you need to change one line in GXX.COM to:
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$ if f$locate("D",p2) .ne. P2_Length then Debug = " ""-G0"""
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Notice zero---> ^
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If you are using a GCC.COM that does both C and C++, add the following lines to
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GCC.COM:
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$!
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$! Use old style debugging records for VMS
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$!
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$ if (Debug.nes."" ).and. Plus then Debug = " ""-G0"""
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after the variables Plus and Debug are set. The reason for this, is that C++
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compiler by default generates debugging records that are more complex,
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with many new syntactical elements that allow for the new features of the
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language. The -G0 switch tells the C++ compiler to use the old style debugging
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records. Until the debugger understands C++ there is not any point to try and
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use the expanded syntax.
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6) When you have nested scopes, i.e.:
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main(){
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int i;
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{int i;
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{int i;
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};};}
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and you say "EXAM i" the debugger needs to figure out which variable you
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actually want to reference. I have arranged things to define a block to the
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debugger when you use brackets to enter a new scope, so in the example above,
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the variables would be described as:
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TEST\main\i
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TEST\main\$0\i
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TEST\main\$0\$0\i
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At each level, the block name is a number with a dollar sign prefix, the
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numbers start with 0 and count upward. When you say EXAM i, the debugger looks
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at the current PC, and decides which block it is currently in. It works from
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the innermost level outward until it finds a block that has the variable "i"
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defined. You can always specify the scope explicitly.
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7) With C++, there can be a lot of inline functions, and it would be
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rather restrictive to force the user to debug the program by converting all of
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the inline functions to normal functions. What I have done is to essentially
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"add" (with the debugger) source lines from the include files that contain the
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inline functions. Thus when you step into an inline function it appears as if
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you have called the function, and you can examine variables and so forth.
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There are several *very* important differences, however. First of all, since
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there is no function call involved, you cannot step over the inline function
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call - you always step into it. Secondly, since the same source lines are used
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in many locations, there is a seperate copy of the source for *each* usage.
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Without this, breakpoints do not work, since we must have a 1-to-1 mapping
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between source lines and PC.
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Since you cannot step over inline function calls, it can be a real pain
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if you are not really interested in what is going on for that function call.
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What I have done is to use the "-D" switch for the assembler to toggle the
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following behavior. With the "-D" switch, all inline functions are included in
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the object file, and you can debug everything. Without the "-D" switch
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(default case with VMS implementation), inline functions are included *only* if
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they did not come from system header files (i.e. from GNU_CC_INCLUDE: or
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GNU_GXX_INCLUDE:). Thus, without the switch the user only debugs his/her own
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inline functions, and not the system ones. (This is especially useful if you do
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a lot of stream I/O in C++). This probably will not provide enough granularity
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for many users, but for now this is still somewhat experimental, and I would
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like to reflect upon it and get some feedback before I go any further.
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Possible solutions include an interactive prompting, a logical name, or a new
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command line option in gcc.c (which is then passed through somehow to the guts
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of the assembler).
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The inline functions from header files appear after the source code
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for the source file. This has the advantage that the source file itself is
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numbered with the same line numbers that you get with an editor. In addition,
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the entire header file is not included, since the assembler makes a list of
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the min and max source lines that are used, and only includes those lines from
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the first to the last actually used. (It is easy to change it to include the
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whole file).
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8) When you are debugging C++ objects, the object "this" is refered to
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as "$this". Actually, the compiler writes it as ".this", but the period is
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not good for the debugger, so I have a routine to convert it to a $. (It
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actually converts all periods to $, but only for variables, since this was
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intended to allow us to access "this".
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9) If you use the asm("...") keyword for global symbols, you will not
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be able to see that symbol with the debugger. The reason is that there are two
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records for the symbol stored in the data structures of the assembler. One
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contains the info such as psect number and offset, and the other one contains
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the information having to do with the data type of the variable. In order to
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debug as symbol, you need to be able to coorelate these records, and the only
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way to do this is by name. The record with the storage attributes will take
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the name used in the asm directive, and the record that specifies the data type
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has the actual variable name, and thus when you use the asm directive to change
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a variable name, the symbol becomes invisible.
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10) Older versions of the compiler ( GNU-C 1.37.92 and earlier) place
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global constants in the text psect. This is unfortunate, since to the linker
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this appears to be an entry point. I sent a patch to the compiler to RMS,
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which will generate a .const section for these variables, and patched the
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assembler to put these variables into a psect just like that for normal
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variables, except that they are marked NOWRT. static constants are still
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placed in the text psect, since there is no need for any external access.
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