How Can a C Pre-processor Eliminate Dead Code Based on Defined/Undefined Macros?

C Pre-processors for Eliminating Conditional Blocks

In the realm of C programming, the need often arises to eliminate dead code based on defined or undefined macros. While the standard C pre-processor provides limited conditional compilation capabilities, advanced requirements demand specialized tools. This article explores a C pre-processor capable of fulfilling such a need.

Consider the scenario where a codebase contains conditional blocks such as:

#ifdef NAME1
#define ALBUQUERQUE "ambidextrous"
#else
#define PHANTASMAGORIA "ghostly"
#endif

A pre-processor with the desired functionality would accept command-line arguments to specify defined or undefined macros, such as '-DNAME1' or '-UNAME1'. Running the command with '-DNAME1' would generate the following output:

#define ALBUQUERQUE "ambidextrous"

On the other hand, executing the command with '-UNAME1' would produce:

#define PHANTASMAGORIA "ghostly"

More complex scenarios with multiple conditions and nested blocks can also be handled accurately. Consider the following example:

#ifdef USE_VOID
#ifdef PLATFORM1
#define VOID void
#else
#undef VOID
typedef void    VOID;
#endif /* PLATFORM1 */
typedef void *  VOIDPTR;
#else
typedef mint     VOID;
typedef char *  VOIDPTR;
#endif /* USE_VOID */

Running the command with '-DUSE_VOID -UPLATFORM1' would result in the following output:

#undef VOID
typedef void    VOID;
typedef void *  VOIDPTR;

Finally, an example involving nested conditions:

#ifndef DOUBLEPAD
#if (defined NT) || (defined OLDUNIX)
#define DOUBLEPAD 8
#else
#define DOUBLEPAD 0
#endif /* NT */
#endif /* !DOUBLEPAD */

Passing the argument '-UOLDUNIX' would produce:

#ifndef DOUBLEPAD
#if (defined NT)
#define DOUBLEPAD 8
#else
#define DOUBLEPAD 0
#endif /* NT */
#endif /* !DOUBLEPAD */

Recommended Solution

Based on the recommendations provided in the original thread, sunifdef stands out as a robust and effective pre-processor capable of eliminating dead code based on defined/undefined macros. It has been extensively used with great success, eliminating thousands of unnecessary conditional blocks.

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