Tell gcc that a function call will not return

Question

I am using C99 under GCC.

I have a function declared static inline in a header that I cannot modify.

The function never returns but is not marked __attribute__((noreturn)).

How can I call the function in a way that tells the compiler it will not return?

I am calling it from my own noreturn function, and partly want to suppress the "noreturn function returns" warning but also want to help the optimizer etc.

I have tried including a declaration with the attribute but get a warning about the repeated declaration.

I have tried creating a function pointer and applying the attribute to that, but it says the function attribute cannot apply to a pointed function.

Answer 1

Several solutions:

redeclaring your function with the __attribute__

You should try to modify that function in its header by adding __attribute__((noreturn)) to it.

You can redeclare some functions with new attribute, as this stupid test demonstrates (adding an attribute to fopen) :

 #include <stdio.h>

 extern FILE *fopen (const char *__restrict __filename,
            const char *__restrict __modes)
   __attribute__ ((warning ("fopen is used")));

 void
 show_map_without_care (void)
 {
   FILE *f = fopen ("/proc/self/maps", "r");
   do
     {
       char lin[64];
       fgets (lin, sizeof (lin), f);
       fputs (lin, stdout);
     }
   while (!feof (f));
   fclose (f);
 }

---

overriding with a macro

At last, you could define a macro like

#define func(A) {func(A); __builtin_unreachable();}

(this uses the fact that inside a macro, the macro name is not macro-expanded).

If your never-returning func is declaring as returning e.g. int you'll use a statement expression like

#define func(A) ({func(A); __builtin_unreachable(); (int)0; })

Macro-based solutions like above won't always work, e.g. if func is passed as a function pointer, or simply if some guy codes (func)(1) which is legal but ugly.

---

redeclaring a static inline with the noreturn attribute

And the following example:

 // file ex.c
 // declare exit without any standard header
 void exit (int);

 // define myexit as a static inline
 static inline void
 myexit (int c)
 {
   exit (c);
 }

 // redeclare it as notreturn
 static inline void myexit (int c) __attribute__ ((noreturn));

 int
 foo (int *p)
 {
   if (!p)
     myexit (1);
   if (p)
     return *p + 2;
   return 0;
 }

when compiled with GCC 4.9 (from Debian/Sid/x86-64) as gcc -S -fverbose-asm -O2 ex.c) gives an assembly file containing the expected optimization:

         .type   foo, @function
 foo:
 .LFB1:
    .cfi_startproc
    testq   %rdi, %rdi      # p
    je      .L5     #,
    movl    (%rdi), %eax    # *p_2(D), *p_2(D)
    addl    $2, %eax        #, D.1768
    ret
.L5:
    pushq   %rax    #
    .cfi_def_cfa_offset 16
    movb    $1, %dil        #,
    call    exit    #
    .cfi_endproc
 .LFE1:
    .size   foo, .-foo

You could play with #pragma GCC diagnostic to selectively disable a warning.

---

Customizing GCC with MELT

Finally, you could customize your recent gcc using the MELT plugin and coding your simple extension (in the MELT domain specific language) to add the attribute noreturn when encoutering the desired function. It is probably a dozen of MELT lines, using register_finish_decl_first and a match on the function name.

^{Since I am the main author of MELT (free software GPLv3+) I could perhaps even code that for you if you ask, e.g. here or preferably on [e-mail]; give the concrete name of your never-returning function.}

Probably the MELT code is looking like:

  ;;file your_melt_mode.melt
  (module_is_gpl_compatible "GPLv3+")
  (defun my_finish_decl (decl)
     (let ( (tdecl (unbox :tree decl))
       )
     (match tdecl
        (?(tree_function_decl_named
            ?(tree_identifier ?(cstring_same "your_function_name")))
          ;;; code to add the noreturn attribute
          ;;; ....
        ))))
  (register_finish_decl_first my_finish_decl)

^{The real MELT code is slightly more complex. You want to define your_adding_attr_mode there. Ask me for more.}

Once you coded your MELT extension your_melt_mode.melt for your needs (and compiled that MELT extension into your_melt_mode.quicklybuilt.so as documented in the MELT tutorials) you'll compile your code with

  gcc -fplugin=melt \
      -fplugin-arg-melt-extra=your_melt_mode.quicklybuilt \
      -fplugin-arg-melt-mode=your_adding_attr_mode \
      -O2 -I/your/include -c yourfile.c

In other words, you just add a few -fplugin-* flags to your CFLAGS in your Makefile !

BTW, I'm just coding in the MELT monitor (on github: https://github.com/bstarynk/melt-monitor ..., file meltmom-process.melt something quite similar.

With a MELT extension, you won't get any additional warning, since the MELT extension would alter the internal GCC AST (a GCC Tree) of the declared function on the fly!

Customizing GCC with MELT is probably the most bullet-proof solution, since it is modifying the GCC internal AST. Of course, it is probably the most costly solution (and it is GCC specific and might need -small- changes when GCC is evolving, e.g. when using the next version of GCC), but as I am trying to show it is quite easy in your case.

Answer 2

From the function you defined, and which calls the external function, add a call to __builtin_unreachable which is built into at least GCC and Clang compilers and is marked noreturn. In fact, this function does nothing else and should not be called. It's only here so that the compiler can infer that program execution will stop at this point.

static inline external_function() // lacks the noreturn attribute
{ /* does not return */ }

void your_function() __attribute__((noreturn)) {
    external_function();     // the compiler thinks execution may continue ...
    __builtin_unreachable(); // ... and now it knows it won't go beyond here
}

Edit: Just to clarify a few points raised in the comments, and generally give a bit of context:

• A function has has only two ways of not returning: loop forever, or short-circuit the usual control-flow (e.g. throw an exception, jump out of the function, terminate the process, etc.)
• In some cases, the compiler may be able to infer and prove through static analysis that a function will not return. Even theoretically, this is not always possible, and since we want compilers to be fast only obvious/easy cases are detected.
• __attribute__((noreturn)) is an annotation (like const) which is a way for the programmer to inform the compiler that he's absolutely sure a function will not return. Following the trust but verify principle, the compiler tries to prove that the function does indeed not return. If may then issue an error if it proves the function may return, or a warning if it was not able to prove whether the function returns or not.
• __builtin_unreachable has undefined behaviour because it is not meant to be called. It's only meant to help the compiler's static analysis. Indeed the compiler knows that this function does not return, so any following code is provably unreachable (except through a jump).

Once the compiler has established (either by itself, or with the programmer's help) that some code is unreachable, it may use this information to do optimizations like these:

• Remove the boilerplate code used to return from a function to its caller, if the function never returns
• Propagate the unreachability information, i.e. if the only execution path to a code points is through unreachable code, then this point is also unreachable. Examples:
  • if a function does not return, any code following its call and not reachable through jumps is also unreachable. Example: code following __builtin_unreachable() is unreachable.
  • in particular, it the only path to a function's return is through unreachable code, the function can be marked noreturn. That's what happens for your_function.
  • any memory location / variable only used in unreachable code is not needed, therefore settings/computing the content of such data is not needed.
  • any computations which is probably (1) unnecessary (previous bullet) and (2) has no side effects (such as pure functions) may be removed.

Illustration: - The call to external_function cannot be removed because it might have side-effects. In fact, it probably has at least the side effect of terminating the process! - The return boiler plate of your_function may be removed

Here's another example showing how code before the unreachable point may be removed

int compute(int) __attribute((pure)) { return /* expensive compute */ }
if(condition) {
    int x = compute(input); // (1) no side effect => keep if x is used
                            // (8) x is not used  => remove
    printf("hello ");       // (2) reachable + side effect => keep
    your_function();        // (3) reachable + side effect => keep
                            // (4) unreachable beyond this point
    printf("word!\n");      // (5) unreachable => remove
    printf("%d\n", x);      // (6) unreachable => remove
                            // (7) mark 'x' as unused
} else {
                            // follows unreachable code, but can jump here
                            // from reachable code, so this is reachable
   do_stuff();              // keep
}