gcc -E -dM/-dD

[George2]

Hello everyone,


I am learning some other's code -- build files. Here are two samples which I am confused.

gcc -E -o ListDef -dM -P YourSrcCode.c

gcc -E -o ListDef -dD -P YourSrcCode.c

I have tried that in ListDef file (new generated after either of the two commands) and it contains several hundred lines of #define, typedef and some other definitions. Could anyone help to answer what is the function of the two commands? Differences between -dM and -dD?


thanks in advance,
George

[Salem]

They're basically "debug" modes of the compiler to help you track down certain problems.
In this instance, it would seem to dump all the #defines.
Read the manual.

[matsp]

I don't think most people will be able to use -dM, but -dD is useful to show what macros was used during the pre-compilation process.

The -E option tells the compiler to just generate the precompiled output, rather than the usual "full compile". So your list file will be exactly how the compiler will see the code (on large file with all the stuff from various include files etc, all comments removed, macros replaced with their text replacement, all #if/#else etc removed (and any code that was determined not to be "needed" due to said #if/#else).

--
Mats

[George2]

Thanks Salem,

They're basically "debug" modes of the compiler to help you track down certain problems.
In this instance, it would seem to dump all the #defines.
Read the manual.

I noticed a lot of information are dumped. It means my program will use all of the dumped information?

I can not believe my application will use so many #define and typedef. :-)


regards,
George

[George2]

Thanks Mats,

I don't think most people will be able to use -dM, but -dD is useful to show what macros was used during the pre-compilation process.

The -E option tells the compiler to just generate the precompiled output, rather than the usual "full compile". So your list file will be exactly how the compiler will see the code (on large file with all the stuff from various include files etc, all comments removed, macros replaced with their text replacement, all #if/#else etc removed (and any code that was determined not to be "needed" due to said #if/#else).

--
Mats

In what situation do we need to use -dM and in what situation do we need -dD?


regards,
George

[matsp]

 -dM
           -fdump-rtl-mach
               Dump after performing the machine dependent reorganization
               pass, to file.35.mach.

The machine dependant re-organization is done as part of the optimisation during the compile phase. Since your original command line had "-E" as part of the command line, it will not get to this step, so there won't be any output different from "no -dM" option.

I'm not sure what format it is, but I believe the rtl is the internal "assembler" format of gcc, and it's certainly not very useful unless you are debugging gcc's code generation for a particular processor [and unless you actually are capable of FIXING it - in other cases, it's usually sufficient to provide a small C example of something broken (after gcc -E perhaps), and a full list of which gcc argumentns give the problem. E.g. gcc -O2 err.c makes the variable x be overwritten by y because the load instruction is moved above the store instruction. And send that to [email protected] or whatever the e-mail address is]


--
Mats

[George2]

Thanks Mats,


This is what I found from gcc manual,

--------------------
-dCHARS CHARS is a sequence of one or more of the following characters, and must
not be preceded by a space. Other characters are interpreted by the compiler
proper, or reserved for future versions of GCC, and so are silently ignored. If
you specify characters whose behavior conflicts, the result is undefined.
‘M’ Instead of the normal output, generate a list of ‘#define’ directives
for all the macros defined during the execution of the preprocessor,
including predefined macros. This gives you a way of finding out
what is predefined in your version of the preprocessor. Assuming
you have no file ‘foo.h’, the command
touch foo.h; cpp -dM foo.h
will show all the predefined macros.
--------------------

seems different from your points? :-)

I have tried that gcc -E -dM and gcc -E (without -dM) will have different output.

I am using gcc version 3.4.5.

Any comments?


regards,
George

The machine dependant re-organization is done as part of the optimisation during the compile phase. Since your original command line had "-E" as part of the command line, it will not get to this step, so there won't be any output different from "no -dM" option.

I'm not sure what format it is, but I believe the rtl is the internal "assembler" format of gcc, and it's certainly not very useful unless you are debugging gcc's code generation for a particular processor [and unless you actually are capable of FIXING it - in other cases, it's usually sufficient to provide a small C example of something broken (after gcc -E perhaps), and a full list of which gcc argumentns give the problem. E.g. gcc -O2 err.c makes the variable x be overwritten by y because the load instruction is moved above the store instruction. And send that to [email protected] or whatever the e-mail address is]


--
Mats

[matsp]

So, it appears that those have changed. In the current gcc (4.x) it means different things than in 3.x obviously.

--
Mats

[George2]

Thanks Mats,

So, it appears that those have changed. In the current gcc (4.x) it means different things than in 3.x obviously.

--
Mats

In my context, in what situation should we use -dM and in what situation should we use -dD?


regards,
George

[matsp]

-dM will list any macros that are defined by the compiler itself (e.g. __GNUC__, __ANSI__, __DATE__ etc)
-dD will list all macros that are defined during the compile, which includes those supplied by the compiler and those defined by header files, C files and on the command-line.

--
Mats