为什么gcc不调用abs函数？

Question

I have some C code

#include<stdio.h>
#include<stdint.h>

int32_t abs(int32_t x) {

        int32_t mask = (x >> 31);
        printf("mask is %d\n",mask);
        return (x + mask) ^ mask;

}
int main(int argc, char *argv[])
{
        int32_t i=-5;
        printf("%d\n",abs(i));

        return 0;
}

and I compile it with GCC.

gcc -O0 -S -masm=intel

The asm code is in https://pastebin.com/4TGpSsdd

Why doesn't gcc call abs function but clang do.

The program doesn't call abs function and

execute code 「printf("mask is %d\\n",mask);」

My gcc version:

gcc (Ubuntu 4.8.4-2ubuntu1~14.04.3) 4.8.4

Is this a gcc bug?

Answer 1

abs is a builtin in GCC, but Clang replaces builtins with calls to their respective library functions.

To fix this, you can rename your function (which is recommended; see below for the reasoning) you can compile your code using -fno-builtin-abs , or you can mark your function as static .

Here's a quote from the linked page:

The ISO C90 functions abort , abs , acos , [...] and vsprintf are all recognized as built-in functions unless -fno-builtin is specified (or -fno-builtin- function is specified for an individual function). All of these functions have corresponding versions prefixed with __builtin_ .

The C standard states that redefining library functions with external is undefined behavior (C11 §7.1.3 1):

All identifiers with external linkage in any of the following subclauses (including the future library directions) and errno are always reserved for use as identifiers with external linkage.

If you define abs as static , the call will not be removed.

Answer 2

There is a function int abs(int x) in stdlib.h .

You need to rename your function.

Answer 3

1. do not use standard function name. Many functions (including abs ) are known well to the gcc compiler. They are called built-ins. So compiler knows what this function does and in many cases it does not have to call it.

If you rename it - it will genrate the call with the -O0 https://godbolt.org/z/dLg26U

if you want to have it called if you enable optimizations you need to tell the compiler to do not inline it. https://godbolt.org/z/aHaEik

 int32_t __attribute__((noinline)) myabs(int32_t x) {

Answer 4

Is this a gcc bug?

No.

abs is a function defined in the C standard, it's prototype is provided as int abs (int) by #include <stdlib.h> .

However, even without including that header, gcc has build-in knowledge about abs and knows what it is supposed to do. This applies to hosted C, which most users are using.

In non-hosted C however, the compiler must not make any assumptions wrt the semantics of functions declared in the standard headers. In gcc, you get non-hested resp. freestanding by -freestanding .

In addition, even without optimization turned on ( -O0 ), gcc will fold constants. This is because in

int x = 1 + 2;

you cannot do the initialization at run-time for x in static storage – as opposed to C++ which could initialize that at run-time in principle. In you case

abs (-5)

is folded to 5 .

As you very likely do not want to use -ffreestanding , you can use -fno-builtin-abs to take away built-in knowledge about abs , same for many other functions. Even with -fno-builtin-abs option, you can use __builtin_abs in your code if you prefer the optimized version of absolute value computation and no libcall to libc.a / libc.so .

---

Addendum ad freestanding due to a comment: C99 §4 Conformance, says:

6. The two forms of conforming implementation are hosted and freestanding. A conforming hosted implementation shall accept any strictly conforming program. A conforming freestanding implementation shall accept any strictly conforming program that does not use complex types and in which the use of the features specified in the library clause (clause 7) is confined to the contents of the standard headers <float.h> , <iso646.h> , <limits.h> , <stdarg.h> , <stdbool.h> , <stddef.h> , and <stdint.h> .