0x7f454c46 / cprintf

Compile-time printf-like function's format string parsing

cprintf: Compile-time parsing for printf-like function's fmt

This is GCC plugin, made to eliminate cost for parsing printf-like format strings in runtime by doing this at compile time. It uses GCC plugin API to walk through gimple tree, get format-string value and insert calls to specified to plugin functions instead of printf-alike. Note: this can be done only to constant format-string parameters.

So, the idea is to change the function calls like this:

print_on_level(LOG_DEBUG, "cwd:%x swd:%x twd:%x fop:%x mxcsr:%x mxcsr_mask:%x\n",
                (int)i387->cwd, (int)i387->swd, (int)i387->twd,
                (int)i387->fop, (int)i387->mxcsr, (int)i387->mxcsr_mask);

To sequence of calls like:

print_str_level(LOG_DEBUG, "cwd:");
print_int_level(LOG_DEBUG, (int)i387->cwd);
print_str_level(LOG_DEBUG, " swd:");
print_int_level(LOG_DEBUG, (int)i387->swd);
print_str_level(LOG_DEBUG, " twd:");
print_int_level(LOG_DEBUG, (int)i387->twd);
print_str_level(LOG_DEBUG, " fop:");
print_int_level(LOG_DEBUG, (int)i387->fop);
print_str_level(LOG_DEBUG, " mxcsr:");
print_int_level(LOG_DEBUG, (int)i387->mxcsr);
print_str_level(LOG_DEBUG, " mxcsr_mask:");
print_int_level(LOG_DEBUG, (int)i387->mxcsr_mask);
print_str_level(LOG_DEBUG, "\n");

This will speed up the runtime by eliminating needless parsing of format string. All is done by GCC plugin during compilation. One should specify printf-alike function print_on_level and print_str_level, print_int_level as a plugin parameters.

TL;DR

To use this GCC compiler plugin, you need compile it with:

make

Then change your makefile so GCC will use plugin like this:

%.o: %.c
    gcc -o $@ $< -fplugin=./cprintf.so -fplugin-arg-cprintf-printf="printf(0): %c putchar %s putstring  \
                            %d putint %f putfloat %m puterrstr"

Where printf(0) means that format string is the first parameter. If you provide function like fprintf(1) which has some (one) parameters before format string, they will be passed to handlers in the same order.

Handlers: putchar function for %c specifier and so on. Note, specifier may be any length, ending with space symbol. I.e., %h$up is a valid specifier h$up.

Reserved specifiers

For some user-defined function foo(arg1, arg2, const char *fmt, ...) cprintf plugin expects that following specifiers have their special meaning if handler-functions have been specified in plugin args:

• %s is for strings (const char*). Cprintf will use it to output parts of format string between specifiers; Function prototype in example: bar(arg1, arg2, const char *str);
• %c is for chars (const char). Cprintf will use it to output single chars from format string; Function prototype in example: baz(arg1, arg2, const char s);
• %% special meaning for raw string output; Function prototype in example: quux(arg1, arg2, void *ptr, size_t size, size_t nmemb);

Tip: consider using %% specifier as fwrite() function as it will give great performance enhance.