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                Arm / Thumb Interworking

                ========================

The Cygnus GNU Pro Toolkit for the ARM7T processor supports function

calls between code compiled for the ARM instruction set and code

compiled for the Thumb instruction set and vice versa.  This document

describes how that interworking support operates and explains the

command line switches that should be used in order to produce working

programs.

Note:  The Cygnus GNU Pro Toolkit does not support switching between

compiling for the ARM instruction set and the Thumb instruction set

on anything other than a per file basis.  There are in fact two

completely separate compilers, one that produces ARM assembler

instructions and one that produces Thumb assembler instructions.  The

two compilers share the same assembler, linker and so on.

1. Explicit interworking support for C and C++ files

====================================================

By default if a file is compiled without any special command line

switches then the code produced will not support interworking.

Provided that a program is made up entirely from object files and

libraries produced in this way and which contain either exclusively

ARM instructions or exclusively Thumb instructions then this will not

matter and a working executable will be created.  If an attempt is

made to link together mixed ARM and Thumb object files and libraries,

then warning messages will be produced by the linker and a non-working

executable will be created.

In order to produce code which does support interworking it should be

compiled with the

        -mthumb-interwork

command line option.  Provided that a program is made up entirely from

object files and libraries built with this command line switch a

working executable will be produced, even if both ARM and Thumb

instructions are used by the various components of the program.  (No

warning messages will be produced by the linker either).

Note that specifying -mthumb-interwork does result in slightly larger,

slower code being produced.  This is why interworking support must be

specifically enabled by a switch.

2. Explicit interworking support for assembler files

====================================================

If assembler files are to be included into an interworking program

then the following rules must be obeyed:

        * Any externally visible functions must return by using the BX

        instruction.

        * Normal function calls can just use the BL instruction.  The

        linker will automatically insert code to switch between ARM

        and Thumb modes as necessary.

        * Calls via function pointers should use the BX instruction if

        the call is made in ARM mode:

                .code 32

                mov lr, pc

                bx  rX

        This code sequence will not work in Thumb mode however, since

        the mov instruction will not set the bottom bit of the lr

        register.  Instead a branch-and-link to the _call_via_rX

        functions should be used instead:

                .code 16

                bl  _call_via_rX

        where rX is replaced by the name of the register containing

        the function address.

        * All externally visible functions which should be entered in

        Thumb mode must have the .thumb_func pseudo op specified just

        before their entry point.  e.g.:

                        .code 16

                        .global function

                        .thumb_func

                function:

                        ...start of function....

        * All assembler files must be assembled with the switch

        -mthumb-interwork specified on the command line.  (If the file

        is assembled by calling gcc it will automatically pass on the

        -mthumb-interwork switch to the assembler, provided that it

        was specified on the gcc command line in the first place.)

3. Support for old, non-interworking aware code.

================================================

If it is necessary to link together code produced by an older,

non-interworking aware compiler, or code produced by the new compiler

but without the -mthumb-interwork command line switch specified, then

there are two command line switches that can be used to support this.

The switch

        -mcaller-super-interworking

will allow calls via function pointers in Thumb mode to work,

regardless of whether the function pointer points to old,

non-interworking aware code or not.  Specifying this switch does

produce slightly slower code however.

Note:  There is no switch to allow calls via function pointers in ARM

mode to be handled specially.  Calls via function pointers from

interworking aware ARM code to non-interworking aware ARM code work

without any special considerations by the compiler.  Calls via

function pointers from interworking aware ARM code to non-interworking

aware Thumb code however will not work.  (Actually under some

circumstances they may work, but there are no guarantees).  This is

because only the new compiler is able to produce Thumb code, and this

compiler already has a command line switch to produce interworking

aware code.

The switch

        -mcallee-super-interworking

will allow non-interworking aware ARM or Thumb code to call Thumb

functions, either directly or via function pointers.  Specifying this

switch does produce slightly larger, slower code however.

Note:  There is no switch to allow non-interworking aware ARM or Thumb

code to call ARM functions.  There is no need for any special handling

of calls from non-interworking aware ARM code to interworking aware

ARM functions, they just work normally.  Calls from non-interworking

aware Thumb functions to ARM code however, will not work.  There is no

option to support this, since it is always possible to recompile the

Thumb code to be interworking aware.

As an alternative to the command line switch

-mcallee-super-interworking, which affects all externally visible

functions in a file, it is possible to specify an attribute or

declspec for individual functions, indicating that that particular

function should support being called by non-interworking aware code.

The function should be defined like this:

        int __attribute__((interfacearm)) function

        {

                ... body of function ...

        }

or

        int __declspec(interfacearm) function

        {

                ... body of function ...

        }

4. Interworking support in dlltool

==================================

It is possible to create DLLs containing mixed ARM and Thumb code.  It

is also possible to call Thumb code in a DLL from an ARM program and

vice versa.  It is even possible to call ARM DLLs that have been compiled

without interworking support (say by an older version of the compiler),

from Thumb programs and still have things work properly.

   A version of the `dlltool' program which supports the `--interwork'

command line switch is needed, as well as the following special

considerations when building programs and DLLs:

*Use `-mthumb-interwork'*

     When compiling files for a DLL or a program the `-mthumb-interwork'

     command line switch should be specified if calling between ARM and

     Thumb code can happen.  If a program is being compiled and the

     mode of the DLLs that it uses is not known, then it should be

     assumed that interworking might occur and the switch used.

*Use `-m thumb'*

     If the exported functions from a DLL are all Thumb encoded then the

     `-m thumb' command line switch should be given to dlltool when

     building the stubs.  This will make dlltool create Thumb encoded

     stubs, rather than its default of ARM encoded stubs.

     If the DLL consists of both exported Thumb functions and exported

     ARM functions then the `-m thumb' switch should not be used.

     Instead the Thumb functions in the DLL should be compiled with the

     `-mcallee-super-interworking' switch, or with the `interfacearm'

     attribute specified on their prototypes.  In this way they will be

     given ARM encoded prologues, which will work with the ARM encoded

     stubs produced by dlltool.

*Use `-mcaller-super-interworking'*

     If it is possible for Thumb functions in a DLL to call

     non-interworking aware code via a function pointer, then the Thumb

     code must be compiled with the `-mcaller-super-interworking'

     command line switch.  This will force the function pointer calls

     to use the _interwork_call_via_rX stub functions which will

     correctly restore Thumb mode upon return from the called function.

*Link with `libgcc.a'*

     When the dll is built it may have to be linked with the GCC

     library (`libgcc.a') in order to extract the _call_via_rX functions

     or the _interwork_call_via_rX functions.  This represents a partial

     redundancy since the same functions *may* be present in the

     application itself, but since they only take up 372 bytes this

     should not be too much of a consideration.

*Use `--support-old-code'*

     When linking a program with an old DLL which does not support

     interworking, the `--support-old-code' command line switch to the

     linker should be used.   This causes the linker to generate special

     interworking stubs which can cope with old, non-interworking aware

     ARM code, at the cost of generating bulkier code.  The linker will

     still generate a warning message along the lines of:

       "Warning: input file XXX does not support interworking, whereas YYY does."

     but this can now be ignored because the --support-old-code switch

     has been used.

5. How interworking support works

=================================

Switching between the ARM and Thumb instruction sets is accomplished

via the BX instruction which takes as an argument a register name.

Control is transfered to the address held in this register (with the

bottom bit masked out), and if the bottom bit is set, then Thumb

instruction processing is enabled, otherwise ARM instruction

processing is enabled.

When the -mthumb-interwork command line switch is specified, gcc

arranges for all functions to return to their caller by using the BX

instruction.  Thus provided that the return address has the bottom bit

correctly initialized to indicate the instruction set of the caller,

correct operation will ensue.

When a function is called explicitly (rather than via a function

pointer), the compiler generates a BL instruction to do this.  The

Thumb version of the BL instruction has the special property of

setting the bottom bit of the LR register after it has stored the

return address into it, so that a future BX instruction will correctly

return the instruction after the BL instruction, in Thumb mode.

The BL instruction does not change modes itself however, so if an ARM

function is calling a Thumb function, or vice versa, it is necessary

to generate some extra instructions to handle this.  This is done in

the linker when it is storing the address of the referenced function

into the BL instruction.  If the BL instruction is an ARM style BL

instruction, but the referenced function is a Thumb function, then the

linker automatically generates a calling stub that converts from ARM

mode to Thumb mode, puts the address of this stub into the BL

instruction, and puts the address of the referenced function into the

stub.  Similarly if the BL instruction is a Thumb BL instruction, and

the referenced function is an ARM function, the linker generates a

stub which converts from Thumb to ARM mode, puts the address of this

stub into the BL instruction, and the address of the referenced

function into the stub.

This is why it is necessary to mark Thumb functions with the

.thumb_func pseudo op when creating assembler files.  This pseudo op

allows the assembler to distinguish between ARM functions and Thumb

functions.  (The Thumb version of GCC automatically generates these

pseudo ops for any Thumb functions that it generates).

Calls via function pointers work differently.  Whenever the address of

a function is taken, the linker examines the type of the function

being referenced.  If the function is a Thumb function, then it sets

the bottom bit of the address.  Technically this makes the address

incorrect, since it is now one byte into the start of the function,

but this is never a problem because:

        a. with interworking enabled all calls via function pointer

           are done using the BX instruction and this ignores the

           bottom bit when computing where to go to.

        b. the linker will always set the bottom bit when the address

           of the function is taken, so it is never possible to take

           the address of the function in two different places and

           then compare them and find that they are not equal.

As already mentioned any call via a function pointer will use the BX

instruction (provided that interworking is enabled).  The only problem

with this is computing the return address for the return from the

called function.  For ARM code this can easily be done by the code

sequence:

        mov     lr, pc

        bx      rX

(where rX is the name of the register containing the function

pointer).  This code does not work for the Thumb instruction set,

since the MOV instruction will not set the bottom bit of the LR

register, so that when the called function returns, it will return in

ARM mode not Thumb mode.  Instead the compiler generates this

sequence:

        bl      _call_via_rX

(again where rX is the name if the register containing the function

pointer).  The special call_via_rX functions look like this:

        .thumb_func

_call_via_r0:

        bx      r0

        nop

The BL instruction ensures that the correct return address is stored

in the LR register and then the BX instruction jumps to the address

stored in the function pointer, switch modes if necessary.

6. How caller-super-interworking support works

==============================================

When the -mcaller-super-interworking command line switch is specified

it changes the code produced by the Thumb compiler so that all calls

via function pointers (including virtual function calls) now go via a

different stub function.  The code to call via a function pointer now

looks like this:

        bl _interwork_call_via_r0

Note: The compiler does not insist that r0 be used to hold the

function address.  Any register will do, and there are a suite of stub

functions, one for each possible register.  The stub functions look

like this:

        .code 16

        .thumb_func

_interwork_call_via_r0

        bx      pc

        nop

        .code 32

        tst     r0, #1

        stmeqdb r13!, {lr}

        adreq   lr, _arm_return

        bx      r0

The stub first switches to ARM mode, since it is a lot easier to

perform the necessary operations using ARM instructions.  It then

tests the bottom bit of the register containing the address of the

function to be called.  If this bottom bit is set then the function

being called uses Thumb instructions and the BX instruction to come

will switch back into Thumb mode before calling this function.  (Note

that it does not matter how this called function chooses to return to

its caller, since the both the caller and callee are Thumb functions,

and mode switching is necessary).  If the function being called is an

ARM mode function however, the stub pushes the return address (with

its bottom bit set) onto the stack, replaces the return address with

the address of the a piece of code called '_arm_return' and then

performs a BX instruction to call the function.

The '_arm_return' code looks like this:

        .code 32

_arm_return:

        ldmia   r13!, {r12}

        bx      r12

        .code 16

It simply retrieves the return address from the stack, and then

performs a BX operation to return to the caller and switch back into

Thumb mode.

7. How callee-super-interworking support works

==============================================

When -mcallee-super-interworking is specified on the command line the

Thumb compiler behaves as if every externally visible function that it

compiles has had the (interfacearm) attribute specified for it.  What

this attribute does is to put a special, ARM mode header onto the

function which forces a switch into Thumb mode:

  without __attribute__((interfacearm)):

                .code 16

                .thumb_func

        function:

                ... start of function ...

  with __attribute__((interfacearm)):

                .code 32

        function:

                orr     r12, pc, #1

                bx      r12

                .code 16

                .thumb_func

        .real_start_of_function:

                ... start of function ...

Note that since the function now expects to be entered in ARM mode, it

no longer has the .thumb_func pseudo op specified for its name.

Instead the pseudo op is attached to a new label .real_start_of_

(where  is the name of the function) which indicates the start

of the Thumb code.  This does have the interesting side effect in that

if this function is now called from a Thumb mode piece of code

outside of the current file, the linker will generate a calling stub

to switch from Thumb mode into ARM mode, and then this is immediately

overridden by the function's header which switches back into Thumb

mode.

In addition the (interfacearm) attribute also forces the function to

return by using the BX instruction, even if has not been compiled with

the -mthumb-interwork command line flag, so that the correct mode will

be restored upon exit from the function.

8. Some examples

================

   Given these two test files:

             int arm (void) { return 1 + thumb (); }

             int thumb (void) { return 2 + arm (); }

   The following pieces of assembler are produced by the ARM and Thumb

version of GCC depending upon the command line options used:

   `-O2':

             .code 32                               .code 16

             .global _arm                           .global _thumb

                                                    .thumb_func

     _arm:                                    _thumb:

             mov     ip, sp

             stmfd   sp!, {fp, ip, lr, pc}          push    {lr}

             sub     fp, ip, #4

             bl      _thumb                          bl      _arm

             add     r0, r0, #1                      add     r0, r0, #2

             ldmea   fp, {fp, sp, pc}                pop     {pc}

   Note how the functions return without using the BX instruction.  If

these files were assembled and linked together they would fail to work

because they do not change mode when returning to their caller.

   `-O2 -mthumb-interwork':

             .code 32                               .code 16

             .global _arm                           .global _thumb

                                                    .thumb_func

     _arm:                                    _thumb:

             mov     ip, sp

             stmfd   sp!, {fp, ip, lr, pc}          push    {lr}

             sub     fp, ip, #4

             bl      _thumb                         bl       _arm

             add     r0, r0, #1                     add      r0, r0, #2

             ldmea   fp, {fp, sp, lr}               pop      {r1}

             bx      lr                             bx       r1

   Now the functions use BX to return their caller.  They have grown by

4 and 2 bytes respectively, but they can now successfully be linked

together and be expect to work.  The linker will replace the

destinations of the two BL instructions with the addresses of calling

stubs which convert to the correct mode before jumping to the called

function.

   `-O2 -mcallee-super-interworking':

             .code 32                               .code 32

             .global _arm                           .global _thumb

     _arm:                                    _thumb:

                                                    orr      r12, pc, #1

                                                    bx       r12

             mov     ip, sp                         .code 16

             stmfd   sp!, {fp, ip, lr, pc}          push     {lr}

             sub     fp, ip, #4

             bl      _thumb                         bl       _arm

             add     r0, r0, #1                     add      r0, r0, #2

             ldmea   fp, {fp, sp, lr}               pop      {r1}

             bx      lr                             bx       r1

   The thumb function now has an ARM encoded prologue, and it no longer

has the `.thumb-func' pseudo op attached to it.  The linker will not

generate a calling stub for the call from arm() to thumb(), but it will

still have to generate a stub for the call from thumb() to arm().  Also

note how specifying `--mcallee-super-interworking' automatically

implies `-mthumb-interworking'.

9. Some Function Pointer Examples

=================================

   Given this test file:

        int func (void) { return 1; }

        int call (int (* ptr)(void)) { return ptr (); }

   The following varying pieces of assembler are produced by the Thumb

version of GCC depending upon the command line options used:

   `-O2':

                .code   16

                .globl  _func

                .thumb_func

        _func:

                mov     r0, #1

                bx      lr

                .globl  _call

                .thumb_func

        _call:

                push    {lr}

                bl      __call_via_r0

                pop     {pc}

   Note how the two functions have different exit sequences.  In

particular call() uses pop {pc} to return, which would not work if the

caller was in ARM mode.  func() however, uses the BX instruction, even

though `-mthumb-interwork' has not been specified, as this is the most

efficient way to exit a function when the return address is held in the

link register.

   `-O2 -mthumb-interwork':

                .code   16

                .globl  _func

                .thumb_func

        _func:

                mov     r0, #1

                bx      lr

                .globl  _call

                .thumb_func

        _call:

                push    {lr}

                bl      __call_via_r0

                pop     {r1}

                bx      r1

   This time both functions return by using the BX instruction.  This

means that call() is now two bytes longer and several cycles slower

than the previous version.

   `-O2 -mcaller-super-interworking':

                .code   16

                .globl  _func

                .thumb_func

        _func:

                mov     r0, #1

                bx      lr

                .globl  _call

                .thumb_func

        _call:

                push    {lr}

                bl      __interwork_call_via_r0

                pop     {pc}

   Very similar to the first (non-interworking) version, except that a

different stub is used to call via the function pointer.  This new stub

will work even if the called function is not interworking aware, and

tries to return to call() in ARM mode.  Note that the assembly code for

call() is still not interworking aware itself, and so should not be

called from ARM code.

   `-O2 -mcallee-super-interworking':

                .code   32

                .globl  _func

        _func:

                orr     r12, pc, #1

                bx      r12

                .code   16

                .globl .real_start_of_func

                .thumb_func

        .real_start_of_func:

                mov     r0, #1

                bx      lr

                .code   32

                .globl  _call

        _call:

                orr     r12, pc, #1

                bx      r12

                .code   16

                .globl .real_start_of_call

                .thumb_func

        .real_start_of_call:

                push    {lr}

                bl      __call_via_r0

                pop     {r1}

                bx      r1

   Now both functions have an ARM coded prologue, and both functions

return by using the BX instruction.  These functions are interworking

aware therefore and can safely be called from ARM code.  The code for

the call() function is now 10 bytes longer than the original, non

interworking aware version, an increase of over 200%.

   If a prototype for call() is added to the source code, and this

prototype includes the `interfacearm' attribute:

        int __attribute__((interfacearm)) call (int (* ptr)(void));

   then this code is produced (with only -O2 specified on the command

line):

                .code   16

                .globl  _func

                .thumb_func

        _func:

                mov     r0, #1

                bx      lr

                .globl  _call

                .code   32

        _call:

                orr     r12, pc, #1

                bx      r12

                .code   16

                .globl .real_start_of_call

                .thumb_func

        .real_start_of_call:

                push    {lr}

                bl      __call_via_r0

                pop     {r1}

                bx      r1

   So now both call() and func() can be safely called via

non-interworking aware ARM code.  If, when such a file is assembled,

the assembler detects the fact that call() is being called by another

function in the same file, it will automatically adjust the target of

the BL instruction to point to .real_start_of_call.  In this way there

is no need for the linker to generate a Thumb-to-ARM calling stub so

that call can be entered in ARM mode.

10. How to use dlltool to build ARM/Thumb DLLs

==============================================

   Given a program (`prog.c') like this:

             extern int func_in_dll (void);

             int main (void) { return func_in_dll(); }

   And a DLL source file (`dll.c') like this:

             int func_in_dll (void) { return 1; }

   Here is how to build the DLL and the program for a purely ARM based

environment:

*Step One

     Build a `.def' file describing the DLL:

             ; example.def

             ; This file describes the contents of the DLL

             LIBRARY     example

             HEAPSIZE    0x40000, 0x2000

             EXPORTS

                          func_in_dll  1

*Step Two

     Compile the DLL source code:

            arm-pe-gcc -O2 -c dll.c

*Step Three

     Use `dlltool' to create an exports file and a library file:

            dlltool --def example.def --output-exp example.o --output-lib example.a

*Step Four

     Link together the complete DLL:

            arm-pe-ld dll.o example.o -o example.dll

*Step Five

     Compile the program's source code:

            arm-pe-gcc -O2 -c prog.c

*Step Six

     Link together the program and the DLL's library file:

            arm-pe-gcc prog.o example.a -o prog

   If instead this was a Thumb DLL being called from an ARM program, the

steps would look like this.  (To save space only those steps that are

different from the previous version are shown):

*Step Two

     Compile the DLL source code (using the Thumb compiler):

            thumb-pe-gcc -O2 -c dll.c -mthumb-interwork

*Step Three

     Build the exports and library files (and support interworking):

            dlltool -d example.def -z example.o -l example.a --interwork -m thumb

*Step Five

     Compile the program's source code (and support interworking):

            arm-pe-gcc -O2 -c prog.c -mthumb-interwork

   If instead, the DLL was an old, ARM DLL which does not support

interworking, and which cannot be rebuilt, then these steps would be

used.

*Step One

     Skip.  If you do not have access to the sources of a DLL, there is

     no point in building a `.def' file for it.

*Step Two

     Skip.  With no DLL sources there is nothing to compile.

*Step Three

     Skip.  Without a `.def' file you cannot use dlltool to build an

     exports file or a library file.

*Step Four

     Skip.  Without a set of DLL object files you cannot build the DLL.

     Besides it has already been built for you by somebody else.

*Step Five

     Compile the program's source code, this is the same as before:

            arm-pe-gcc -O2 -c prog.c

*Step Six

     Link together the program and the DLL's library file, passing the

     `--support-old-code' option to the linker:

            arm-pe-gcc prog.o example.a -Wl,--support-old-code -o prog

     Ignore the warning message about the input file not supporting

     interworking as the --support-old-code switch has taken care if this.

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Copying and distribution of this file, with or without modification,

are permitted in any medium without royalty provided the copyright

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