gnu arm assembler command line macro fails with "Invalid identifier for .ifdef"

Question

My toolchain is a recent version of arm-gcc.

I have a piece of code in an assembly file which must be conditionally included/assembled.

.ifdef MACRO_FROM_CMDLINE
 Assembly instr1
 Assembly instr2
.endif

Encapsulated code is a recent addition.

I have tried both:

gcc -x assembler-with-cpp --defsym MACRO_FROM_CMDLINE=1 <along with other necessary options> 

gcc -x assembler-with-cpp -D MACRO_FROM_CMDLINE=1 <along with other necessary options>   

The -D results in "Invalid identifier for .ifdef " and ".endif without .if" errors.

The --defsym results in "MACRO_FROM_CMDLINE=1 : No such file or directory", "unrecognized option --defsym" errors.

Answer 1

The gcc binary drives the compilation process by invoking a number of other programs in sequence to actually perform the various stages of work (compiling, assembling, linking).

When you say:

gcc -x assembler-with-cpp -D MACRO_FROM_CMDLINE=1 ...

you are asking it to run the source through the C preprocessor, and then run the result through the assembler.

The C preprocessor step will turn:

.ifdef MACRO_FROM_CMDLINE

into:

.ifdef 1

before passing it to the assembler, which then can't make sense of it. This is why you get the "invalid identifier" error. It also explains why using C preprocessor #ifdef fixes the problem.

---

--defsym doesn't work because it's an option to the assembler, not the gcc driver program. (The gcc driver does understand and pass through some options to some of the programs it invokes, but not all.)

You can, however, pass arbitrary options through to the assembler using the

-Wa,option[,option...]

syntax, which tells the gcc driver to pass those option(s) through to the assembler (as a list of space-separated options).

For example:

gcc -x assembler-with-cpp -Wa,--defsym,MACRO_FROM_CMDLINE=1 ...

adds

--defsym MACRO_FROM_CMDLINE=1

to the list of options passed to as when gcc invokes it, and that's how to make your original .ifdef example work.

---

You can see the individual programs invoked by gcc, and the options it actually passes to them, by adding the -v option.

In this case, you should see something called cc1 (the actual GCC C compiler binary) invoked with the -E flag (preprocess only) to preprocess the input to a temporary file, and then as invoked on the temporary file to assemble it.