ARM thumb-interwork libgcc asm functions have no bx

ARM thumb-interwork libgcc asm functions have no bx

[Giuliano Procida]

With:

../egcs-20000619/configure
--with-gcc-version-trigger=/u/gprocida/lad-tools/egcs-20000619/gcc/version.c
--host=i686-pc-linux --srcdir=../egcs-20000619 --target=arm-elf
--with-headers=../newlib-1.8.2/newlib/libc/include --enable-multilib
--without-fp --with-cpu=arm7tdmi --norecursion 

and the config/arm/t-arm-elf file edited to produce normal and interworking
libgcc variants (only).

Some of the gcc/libgcc/interwork/*.o use BX, some don't (rm-elf-objdump -d
$i | grep -q bx && echo $i). The ones that do not use BX seem to all come
from:

../egcs-20000619/gcc/config/arm/lib1funcs.asm

In any case, returning from __udivsi3 to thumb code is broken. Example:

bar.c, compiled with -mthumb -mthumb-interwork

int main () {
  volatile unsigned x = 20;
  volatile unsigned y = 35;
  unsigned z = y / x;
}

Relevant bits of objdump -d output:

bar:     file format elf32-littlearm

Disassembly of section .text:

000080e0 <main>:
    80e0:	b590      	push	{r4, r7, lr}
    80e2:	466f      	mov	r7, sp
    80e4:	b083      	sub	sp, #12
    80e6:	ff95f002 	bl	b014 <____gccmain_from_thumb>
    80ea:	46bc      	mov	ip, r7
    80ec:	1f3a      	sub	r2, r7, #4
    80ee:	2314      	mov	r3, #20
    80f0:	6013      	str	r3, [r2, #0]
    80f2:	46bc      	mov	ip, r7
    80f4:	1c3a      	mov	r2, r7		(add r2, r7, #0)
    80f6:	3a08      	sub	r2, #8
    80f8:	2323      	mov	r3, #35
    80fa:	6013      	str	r3, [r2, #0]
    80fc:	46bc      	mov	ip, r7
    80fe:	1c3c      	mov	r4, r7		(add r4, r7, #0)
    8100:	3c0c      	sub	r4, #12
    8102:	46bc      	mov	ip, r7
    8104:	1c3a      	mov	r2, r7		(add r2, r7, #0)
    8106:	3a08      	sub	r2, #8
    8108:	46bc      	mov	ip, r7
    810a:	1f3b      	sub	r3, r7, #4
    810c:	6810      	ldr	r0, [r2, #0]
    810e:	6819      	ldr	r1, [r3, #0]
    8110:	ff84f002 	bl	b01c <____udivsi3_from_thumb>
    8114:	1c03      	mov	r3, r0		(add r3, r0, #0)
    8116:	1c03      	mov	r3, r0		(add r3, r0, #0)
    8118:	6023      	str	r3, [r4, #0]
    811a:	2000      	mov	r0, #0
    811c:	46bd      	mov	sp, r7
    811e:	bc90      	pop	{r4, r7}
    8120:	bc02      	pop	{r1}
    8122:	4708      	bx	r1

00008124 <__udivsi3>:
    8124:	e3510000 	cmp	r1, #0	; 0x0
    8128:	0a00001f 	beq	81ac <Ldiv0>
    812c:	e3a03001 	mov	r3, #1	; 0x1
    8130:	e3a02000 	mov	r2, #0	; 0x0
    8134:	e1500001 	cmp	r0, r1
    8138:	3a000019 	bcc	81a4 <Lgot_result>

000081a4 <Lgot_result>:
    81a4:	e1a00002 	mov	r0, r2
    81a8:	e1a0f00e 	mov	pc, lr

000081ac <Ldiv0>:
    81ac:	e52de004 	str	lr, [sp, -#4]!
    81b0:	eb000001 	bl	81bc <__div0>
    81b4:	e3a00000 	mov	r0, #0	; 0x0
    81b8:	e8bd8000 	ldmia	sp!, {pc}

000081bc <__div0>:
    81bc:	e1a0f00e 	mov	pc, lr

0000b01c <____udivsi3_from_thumb>:
    b01c:	4778      	bx	pc
    b01e:	46c0      	nop			(mov r8,r8)

0000b020 <____udivsi3_change_to_arm>:
    b020:	eafff43f 	b	8124 <__udivsi3>

I hope this is pretty clear.

Giuliano.

Re: ARM thumb-interwork libgcc asm functions have no bx

[Richard Earnshaw]

Giuliano,

Can you please try the following patch.  You will need to rebuild the 
compiler and (at the very least) the interworking versions of libgcc.a

Richard.

	* arm.h (CPP_SPEC): Use sub-spec cpp_interwork.
	(CPP_INTERWORK_SPEC, CPP_INTEWORK_DEFAULT_SPC): New sub-specs.
	(EXTRA_SPECS): Add them.
	* arm/lib1funcs.asm: Support builds for interworking.

RE: ARM thumb-interwork libgcc asm functions have no bx

[Giuliano Procida]

Nick Clifton wrote:

> It sounds like you have a broken build of libgcc.a.

> All the functions in lib1funcs.asm come in two varieties, an ARM
> encoded version and a Thumb encoded version.

Agreed.

> The versions in your
> gcc/libgcc/interwork/*.o should all be the Thumb encoded versions not
> the ARM encoded ones.

[omitted discussion about possible (non-)interworking/Thumb/ARM libgccs]

In any case, libgcc has been compiled for ARM with thumb-interwork and this
has produced a bad library. It should be possible to either compile both ARM
(interworking) and Thumb (interworking) libgccs, or the former should be
detected as a configuration error.

> The building of the Thumb version of the lib1funcs.asm functions is
> controlled by the presence of the __thumb__ define when the library is
> built.  I would guess that this value is not being defined for some
> reason.

I would imagine so.

> The definition of __thumb__  is normally controlled by the
> CPP_ISA_SPEC macro in gcc/config/arm/arm.h which gets added into the
> EXTRA_SPECS macro.  I would check your spec strings to see if this is
> being defined properly.

This was a basically unmodified snapshot with the configuration I gave. I
have checked the file you mention and it seems to do the right thing for
CPP_ISA_SPEC. The problem is that xgcc is not being invoked with -mthumb
(this is removing one of the .o files, then remaking):

make GCC_FOR_TARGET="/omitted/build-gcc/gcc/xgcc -B/omitted/build-gcc/gcc/
-B/usr/local/arm-elf/bin/ -I/usr/local/arm-elf/include" \
  HOST_PREFIX="" HOST_PREFIX_1="loser-" \
  AR_FOR_TARGET="arm-elf-ar" \
  AR_CREATE_FOR_TARGET="arm-elf-ar  rc" \
  AR_FLAGS_FOR_TARGET="" \
  OLDCC="cc" CCLIBFLAGS="-O" CFLAGS="-g -O2" \
  RANLIB_FOR_TARGET="arm-elf-ranlib" \
  RANLIB_TEST_FOR_TARGET="[ -f arm-elf-ranlib ] || ( [ "i686-pc-linux-gnu" =
"arm-unknown-elf" ] && [ -f /usr/bin/ranlib -o -f /bin/ranlib ] )" \
  LIBGCC2_CFLAGS="-O2  -DCROSS_COMPILE -DIN_GCC    `echo -g -O2|sed -e
's/-pedantic//g' -e 's/-Wtraditional//g'` -isystem ./include  -Dinhibit_libc
-fno-inline -g1  -DIN_LIBGCC2 -D__GCC_FLOAT_NOT_NEEDED " \
  INCLUDES="-I. -I../../egcs-20000619/gcc -I../../egcs-20000619/gcc/config
-I../
../egcs-20000619/gcc/../include" MAYBE_USE_COLLECT2="" \
  CONFIG_H="" MACHMODE_H="machmode.h machmode.def" \
  LIB1ASMSRC='arm/lib1funcs.asm' \
  -f libgcc.mk all
make[2]: Entering directory `/omitted/build-gcc/gcc'
for d in libgcc interwork libgcc/interwork; do \
  if [ -d $d ]; then true; else mkdir $d; fi \
done
if [ -f stmp-dirs ]; then true; else touch stmp-dirs; fi
/omitted/build-gcc/gcc/xgcc -B/omitted/build-gcc/gcc/
-B/usr/local/arm-elf/bin/ -I/usr/local/arm-elf/include -O2  -DCROSS_COMPILE
-DIN_GCC    -g -O2 -isystem ./include  -Dinhibit_libc -fno-inline -g1
-DIN_LIBGCC2 -D__GCC_FLOAT_NOT_NEEDED  -I. -I../../egcs-20000619/gcc
-I../../egcs-20000619/gc
c/config -I../../egcs-20000619/gcc/../include -mthumb-interwork -DL_udivsi3
-xassembler-with-cpp -c ../../egcs-20000619/gcc/config/arm/lib1funcs.asm -o
libgcc/interwork/_udivsi3.o
cpp: -lang-asm: linker input file unused since linking not done
rm -rf interwork/libgcc.a
[snip]

Note the lack of -mthumb! By the way. is this t-arm-elf incorrect?

CROSS_LIBGCC1 = libgcc1-asm.a
LIB1ASMSRC = arm/lib1funcs.asm
LIB1ASMFUNCS = _udivsi3 _divsi3 _umodsi3 _modsi3 _dvmd_tls _bb_init_func
_call_via_rX _interwork_call_via_rX

FPBIT = fp-bit.c
DPBIT = dp-bit.c

fp-bit.c: $(srcdir)/config/fp-bit.c
	echo '#define FLOAT' > fp-bit.c
	echo '#ifndef __ARMEB__' >> fp-bit.c
	echo '#define FLOAT_BIT_ORDER_MISMATCH' >> fp-bit.c
	echo '#endif' >> fp-bit.c
	cat $(srcdir)/config/fp-bit.c >> fp-bit.c

dp-bit.c: $(srcdir)/config/fp-bit.c
	echo '#ifndef __ARMEB__' > dp-bit.c
	echo '#define FLOAT_BIT_ORDER_MISMATCH' >> dp-bit.c
	echo '#define FLOAT_WORD_ORDER_MISMATCH' >> dp-bit.c
	echo '#endif' >> dp-bit.c
	cat $(srcdir)/config/fp-bit.c >> dp-bit.c

# these lines added
MULTILIB_OPTIONS    += mno-thumb-interwork/mthumb-interwork
MULTILIB_DIRNAMES   += normal interwork
MULTILIB_EXCEPTIONS += *mapcs-26/*mthumb-interwork*

MULTILIB_MATCHES     = mbig-endian=mbe mlittle-endian=mle
EXTRA_MULTILIB_PARTS = crtbegin.o crtend.o

LIBGCC = stmp-multilib
INSTALL_LIBGCC = install-multilib

TARGET_LIBGCC2_CFLAGS = -Dinhibit_libc -fno-inline

> I hope that this help syou to track down your problem.

Well, I just need to work out what to do to get libgcc built with -mthumb.

Giuliano.

ps apologies for the long cc: list

Re: ARM thumb-interwork libgcc asm functions have no bx

[Richard Earnshaw]

> Hi Giuliano,
> 
> : Some of the gcc/libgcc/interwork/*.o use BX, some don't (rm-elf-objdump -d
> : $i | grep -q bx && echo $i). The ones that do not use BX seem to all come
> : from:
> : 
> : ../egcs-20000619/gcc/config/arm/lib1funcs.asm
> 
> It sounds like you have a broken build of libgcc.a.
> 

Nick,

I think Giuliano is right.  Consider the following code snippets.

file1.c:
int foo(a, b)
{
  return a / b;
}

file2.c:
int bar(a, b)
{
  return a / b;
}

file3.c:

int main()
{
  foo (55, 2);
  bar (55, 2);
  return 0;
}

gcc -O -c -minterwork file1.c
gcc -O -c -mthumb -minterwork file2.c

Now, lets link with the ARM interworking libraries:

gcc -minterwork file3.c file1.o file2.o -o arm-code

We then get the division routine from the ARM+interworking library (ARM 
code): but this doesn't use a bx instruction to return, so the call from 
within bar will fail to switch back to thumb mode on return.

So, lets link with the thumb interworking libraries:

gcc -minterwork -mthumb file3.c file1.o file2.o -o thumb-code

We then get the division routine from the Thumb+interworking library 
(Thumb code):  but this doesn't use a bx instruction either, so the call 
from within foo will fail.

I'm having a think about the problem.  It isn't entirely obvious what is 
the best solution is (I'm not convinced we should be switching in and out 
of thumb/ARM mode for such low-level routines).

Re: ARM thumb-interwork libgcc asm functions have no bx

[Nick Clifton]

Hi Giuliano,

: Some of the gcc/libgcc/interwork/*.o use BX, some don't (rm-elf-objdump -d
: $i | grep -q bx && echo $i). The ones that do not use BX seem to all come
: from:
: 
: ../egcs-20000619/gcc/config/arm/lib1funcs.asm

It sounds like you have a broken build of libgcc.a.

All the functions in lib1funcs.asm come in two varieties, an ARM
encoded version and a Thumb encoded version.  The versions in your
gcc/libgcc/interwork/*.o should all be the Thumb encoded versions not
the ARM encoded ones.  (And hence there should be no need for the
linker to generate interworking stubs to switch into ARM mode to call
the _udivsi3 function).

The building of the Thumb version of the lib1funcs.asm functions is
controlled by the presence of the __thumb__ define when the library is
built.  I would guess that this value is not being defined for some
reason.

The definition of __thumb__  is normally controlled by the
CPP_ISA_SPEC macro in gcc/config/arm/arm.h which gets added into the
EXTRA_SPECS macro.  I would check your spec strings to see if this is
being defined properly.

I hope that this help syou to track down your problem.

Cheers
	Nick