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agbcc/gcc_arm/config/arm/README-interworking
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agbcc/gcc_arm/config/arm/README-interworking
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|
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Arm / Thumb Interworking
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========================
|
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The Cygnus GNU Pro Toolkit for the ARM7T processor supports function
|
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calls between code compiled for the ARM instruction set and code
|
||||
compiled for the Thumb instruction set and vice versa. This document
|
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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
|
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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.
|
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|
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|
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1. Explicit interworking support for C and C++ files
|
||||
====================================================
|
||||
|
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By default if a file is compiled without any special command line
|
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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
|
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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:
|
||||
|
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* Any externally visible functions must return by using the BX
|
||||
instruction.
|
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|
||||
* 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
|
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bx rX
|
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|
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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:
|
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|
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.code 16
|
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bl _call_via_rX
|
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|
||||
where rX is replaced by the name of the register containing
|
||||
the function address.
|
||||
|
||||
* All externally visible functions which should be entered in
|
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Thumb mode must have the .thumb_func pseudo op specified just
|
||||
before their entry point. eg:
|
||||
|
||||
.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.)
|
||||
|
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|
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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.
|
||||
|
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The switch
|
||||
|
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-mcaller-super-interworking
|
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|
||||
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.
|
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|
||||
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:
|
||||
|
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int __attribute__((interfacearm)) function
|
||||
{
|
||||
... body of function ...
|
||||
}
|
||||
|
||||
or
|
||||
|
||||
int __declspec(interfacearm) function
|
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{
|
||||
... body of function ...
|
||||
}
|
||||
|
||||
|
||||
|
||||
4. Interworking support in dlltool
|
||||
==================================
|
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|
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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 initialised 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_<name>
|
||||
(where <name> 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
|
||||
outsside 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.
|
||||
Reference in New Issue
Block a user