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3.3 C and C++ interworking and veneers

You can freely mix C and C++ code compiled for ARM and Thumb, but in ARM architecture v4T small code

segments called veneers are required between the ARM and Thumb code to carry out state changes. The ARM

linker generates these interworking veneers when it detects interworking calls.

3.3.1 Compiling code for interworking

The -apcs /interwork compiler option enables all ARM and Thumb C and C++ compilers to compile modules

containing routines that can be called by routines compiled for the other processor state:

tcc -apcs /interwork

armcc -apcs /interwork

tcpp -apcs /interwork

armcpp -apcs /interwork

Modules that are compiled for interworking on ARM architecture v4T generate slightly larger code, typically 2% larger

for Thumb and less than 1% larger for ARM. There is no difference for ARM architecture v5.

In a leaf function, that is a function whose body contains no function calls, the only change in the code generated by

the compiler is to replace MOV pc,lr with BX lr. The MOV instruction does not cause the necessary state change.

In nonleaf functions built for ARM architecture v4T, the Thumb compiler must replace, for example, the single

instruction:

POP {r4,r5,pc}

with the sequence:

POP {r4,r5}

POP {r3}

BX r3

This has a small effect on performance. Compile all source modules for interworking, unless you are sure they will

never be used with interworking.

The -apcs /interwork option also sets the interwork attribute for the code area the modules are compiled into.

The linker detects this attribute and inserts the appropriate veneer.

Note

ARM code compiled for interworking can only be used on ARM architecture v4T, or v5 and above, because other

processors do not implement the BX instruction.

Use the armlink -info veneers option to find the amount of space taken by the veneers.

C interworking example

Example 3-3 shows a Thumb routine that carries out an interworking call to an ARM subroutine. The ARM subroutine

call makes an interworking call to printf() in the Thumb library. These two modules are provided in

Examples\Interwork as thumbmain.c and armsub.c.

Example 3-3

/**********************

* thumbmain.c *

**********************/

#include <stdio.h>

extern void arm_function(void);

int main(void)

{

printf("Hello from Thumb World\n");

arm_function();

printf("And goodbye from Thumb World\n");

return (0);

}

/**********************

* armsub.c *

**********************/

#include <stdio.h>

void arm_function(void)

{

printf("Hello and Goodbye from ARM world\n");

}

Interworking ARM and Thumb