-fno-builtin

When compiling without -fno-builtin, the compiler can replace calls to certain standard library functions with inline code or with calls to other library functions. The Run-time ABI for the Arm^® Architecture lists library functions that the compiler can use. This means that your re-implementations of the standard library functions might not be used, and might be removed by the linker.

Default

-fno-builtin is disabled by default.

Example without -fno-builtin

The following test program uses the printf() function:

#include "stdio.h"

void foo( void )
{
    printf("Hello\n");
}

1. Compile the program without -fno-builtin:

   armclang -c -O2 -g --target=arm-arm-none-eabi -mcpu=cortex-a9 -mfpu=none foo.c -o foo.o

2. Run the following fromelf command to show the disassembled output:

   fromelf --disassemble foo.o -o foo.s
   ...
   ||foo|| PROC
           ADR      r0,|L3.8|
           B        puts
   |L3.8|
           DCD      0x6c6c6548
           DCD      0x0000006f
           ENDP
   ...

   Note:

   The compiler has replaced the printf() function with the puts() function.

Example with -fno-builtin

1. Compile the same test program with -fno-builtin:

   armclang -c -O2 -g --target=arm-arm-none-eabi -mcpu=cortex-a9 -mfpu=none -fno-builtin foo.c -o foo.o

2. Run the following fromelf command to show the disassembled output:

   fromelf --disassemble foo.o -o foo.s
   ...
   ||foo|| PROC
           ADR      r0,|L3.8|
           B        __2printf
   |L3.8|
           DCD      0x6c6c6548
           DCD      0x00000a6f
           ENDP
   ...

   Note:

   The compiler has not replaced the printf() function with the puts() function when using the -fno-builtin option. Instead, it has replaced it with __2printf from the Arm standard C library. In order to prevent that, you need to use the -nostdlib option as well.

Example with -fno-builtin and -nostdlib

1. Compile the test program with -fno-builtin and -nostdlib:

   armclang -c -O2 -g --target=arm-arm-none-eabi -mcpu=cortex-a9 -mfpu=none -fno-builtin -nostdlib foo.c -o foo.o

2. Run the following fromelf command to show the disassembled output:

   fromelf --disassemble foo.o -o foo.s
   						...
   						||foo|| PROC
   						ADR      r0,|L3.8|
   						B        printf
   						|L3.8|
   						DCD      0x6c6c6548
   						DCD      0x00000a6f
   						ENDP
   						...

   Note:

   The compiler has now not replaced the printf() function at all.