The GNU C Preprocessor Internals: Guard Macros

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## The Multiple-Include Optimization

Header files are often of the form

    #ifndef FOO
    #define FOO
    …
    #endif

to prevent the compiler from processing them more than once. The preprocessor notices such header files, so that if the header file appears in a subsequent `#include` directive and `FOO` is defined, then it is ignored and it doesn’t preprocess or even re-open the file a second time. This is referred to as the multiple include optimization.

Under what circumstances is such an optimization valid? If the file were included a second time, it can only be optimized away if that inclusion would result in no tokens to return, and no relevant directives to process. Therefore the current implementation imposes requirements and makes some allowances as follows:

1. There must be no tokens outside the controlling `#if`-`#endif` pair, but whitespace and comments are permitted.
2. There must be no directives outside the controlling directive pair, but the null directive (a line containing nothing other than a single ‘`#`’ and possibly whitespace) is permitted.
3. The opening directive must be of the form

       #ifndef FOO

   or

       #if !defined FOO     [equivalently, #if !defined(FOO)]

4. In the second form above, the tokens forming the `#if` expression must have come directly from the source file—no macro expansion must have been involved. This is because macro definitions can change, and tracking whether or not a relevant change has been made is not worth the implementation cost.
5. There can be no `#else` or `#elif` directives at the outer conditional block level, because they would probably contain something of interest to a subsequent pass.

First, when pushing a new file on the buffer stack, `_stack_include_file` sets the controlling macro `mi_cmacro` to `NULL`, and sets `mi_valid` to `true`. This indicates that the preprocessor has not yet encountered anything that would invalidate the multiple-include optimization. As described in the next few paragraphs, these two variables having these values effectively indicates top-of-file.

When about to return a token that is not part of a directive, `_cpp_lex_token` sets `mi_valid` to `false`. This enforces the constraint that tokens outside the controlling conditional block invalidate the optimization.

The `do_if`, when appropriate, and `do_ifndef` directive handlers pass the controlling macro to the function `push_conditional`. cpplib maintains a stack of nested conditional blocks, and after processing every opening conditional this function pushes an `if_stack` structure onto the stack. In this structure it records the controlling macro for the block, provided there is one and we’re at top-of-file (as described above). If an `#elif` or `#else` directive is encountered, the controlling macro for that block is cleared to `NULL`. Otherwise, it survives until the `#endif` closing the block, upon which `do_endif` sets `mi_valid` to true and stores the controlling macro in `mi_cmacro`.

`_cpp_handle_directive` clears `mi_valid` when processing any directive other than an opening conditional and the null directive. With this, and requiring top-of-file to record a controlling macro, and no `#else` or `#elif` for it to survive and be copied to `mi_cmacro` by `do_endif`, we have enforced the absence of directives outside the main conditional block for the optimization to be on.

Note that whilst we are inside the conditional block, `mi_valid` is likely to be reset to `false`, but this does not matter since the closing `#endif` restores it to `true` if appropriate.

Finally, since `_cpp_lex_direct` pops the file off the buffer stack at `EOF` without returning a token, if the `#endif` directive was not followed by any tokens, `mi_valid` is `true` and `_cpp_pop_file_buffer` remembers the controlling macro associated with the file. Subsequent calls to `stack_include_file` result in no buffer being pushed if the controlling macro is defined, effecting the optimization.

A quick word on how we handle the

    #if !defined FOO

case. `_cpp_parse_expr` and `parse_defined` take steps to see whether the three stages ‘`!`’, ‘`defined-expression`’ and ‘`end-of-directive`’ occur in order in a `#if` expression. If so, they return the guard macro to `do_if` in the variable `mi_ind_cmacro`, and otherwise set it to `NULL`. `enter_macro_context` sets `mi_valid` to false, so if a macro was expanded whilst parsing any part of the expression, then the top-of-file test in `push_conditional` fails and the optimization is turned off.

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