[LLVMdev] ARM assembler's syntax in clang

[Ashi]

> And be warned that the PC doesn't point at the next instruction when you
use it like this - I believe you don't need to modify it at all if you swap
the pop and the .long.
Bernie, is it related to ARM pipeline? I'm interesting in this, is there
any other additional information?

On Fri, Mar 8, 2013 at 4:59 AM, Tim Northover <t.p.northover at gmail.com>wrote:

> Hi Ashi,
>
> > ld: illegal text-relocation to _data_table in table.o from foo in
> > use_table.o for architecture armv7
>
> It looks like you're using iOS. I'm not familiar with the exact
> workings of that platform, but I think a similar message would occur
> in ELF-land.
>
> If iOS *is* comparable, your issue is that symbols in dynamically
> loaded objects can't (usually) be referenced directly because the code
> section (and any embedded litpools) need to be identical no matter
> where everything is loaded in memory. This is most of the point of
> .dylib files: the instructions only have to exist once in memory and
> can be shared. If the bytes would be different for each user that's
> not possible --  the linker spots places where that happens and gives
> an error.
>
> My advice would be to compile a .c file  (with "-S" to view the
> assembly) which makes accesses like you're trying to do and copy its
> code. There's no shame in it: I had a good idea of what ELF did but
> still got the syntactic details wrong when I tried to write similar
> code off the top of my head.
>
> In this case, for example, I'd compile the C code (with "-fPIC" in
> principle, though it seems to make no difference here. Actually, I'm
> rather disturbed that you're trying to compile the C part of a .dylib
> without "-fPIC". In the Linux world that's a no-no. Could be valid for
> iOS, but I'd suggest you make sure if you don't know):
>
> extern int data_table[];
>
> int *wheres_data_table(void) {
>     return &data_table[0];
> }
>
> (N.b. "armv7" uses a movw/movt pair which is perfectly valid. If you
> want to see how to do it using litpools, you need to compile for
> "armv6" which doesn't have those instructions).
>
> Cheers.
>
> Tim.
>
Thanks for all your great reply! Finally, I got it work by several ways,
compiler's assembly output would help a lot(thanks Tim) and a linker
option:-Wl,-read_only_relocs,suppress would also help.
And here is a similar problem under powerPC :
http://lists.apple.com/archives/unix-porting/2008/Jan/msg00027.html

here is my summary:
*** problem ***
LDR Rx, =Label is not supported under Clang

*** solution ***
replace LDR pseudo-instruction by manually loading Label. 2 methods are
used, they are shown in use_table_m1.s and use_table_m2.s respectively.
There are 7 targets in my Makefile, include several different option
test(details are all in Makefile).
Test environment is Xcode4.6. The test_d_m1_1 can't be generated due to
illegal text-relocation error and others can be generated, they are 3
dynamic targets and 3 static targets, the test_s_m1_2 have a warning due to
-mdynamic-no-pic option. however I haven't test them on iOS device, I'm not
sure whether the "-Wl,-read_only_relocs,suppress" option would really work.



/* ==begin table.c== */
int data_table[] = {0xff, 0xff};
/* ==end table.c== */

/* ==begin use_table_m1.s==
 * use table by LDR
 */
    .text
    .syntax unified
    .align   4
    .global   foo
    .thumb
    .thumb_func

foo:
    LDR         r0,[PC,#4]
    BX    lr
    .long _data_table
/* ==end use_table_m1.s== */

/* ==begin use_table_m2.s==
 * use table by code from compiler's assembly out put
 */
    .text
    .syntax unified
    .align   4
    .global   foo
    .thumb
    .thumb_func

foo:
    /* these lines are from compiler's assembly output($(CC) -S):
     * extern int data_table[];
     * int *wheres_data_table(void) {
     * return &data_table[0];
     * }
     */
    movw    r1, :lower16:(L_data_table$non_lazy_ptr-(LPC0_0+4))
    movt    r1, :upper16:(L_data_table$non_lazy_ptr-(LPC0_0+4))
LPC0_0:
    add    r1, pc
    ldr     r1, [r1]
    bx    lr

    .section        __DATA,__nl_symbol_ptr,non_lazy_symbol_pointers
    .align  2
L_data_table$non_lazy_ptr:
    .indirect_symbol        _data_table
    .long   0

    .subsections_via_symbols
/* ==end use_table_m2.s== */

/* ==begin test.c ==*/
extern int data_table[];
int main(void)
{
  int a = data_table[0];
  return 0;
}
/* ==end test.c ==*/


/* ==Makefile== */

CC = /Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang
AR = /Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/ar
CFLAG = -fPIC -Wall -arch armv7 -mcpu=cortex-a9 -isysroot
/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS6.1.sdk


all:dynamic static

dynamic:test_d_m1_1 test_d_m1_2 test_d_m2_1 test_d_m2_2

static:test_s_m1_1 test_s_m1_2 test_s_m2_1

test_d_m1_1:test.c libtest_m1_1.dylib
    $(CC) $(CFLAG) test.c ./libtest_m1_1.dylib -o $@

test_d_m1_2:test.c libtest_m1_2.dylib
    $(CC) $(CFLAG) test.c ./libtest_m1_2.dylib -o $@

test_d_m2_1:test.c libtest_m2_2.dylib
    $(CC) $(CFLAG) test.c ./libtest_m2_2.dylib -o $@

test_d_m2_2:test.c libtest_m2_2.dylib
    $(CC) $(CFLAG) test.c ./libtest_m2_2.dylib -o $@

test_s_m1_1:test.c libtest_m1_1.a
    $(CC) $(CFLAG) $^ -o $@

test_s_m1_2:test.c libtest_m1_1.a
    $(CC) $(CFLAG) -mdynamic-no-pic $^ -o $@

test_s_m2_1:test.o libtest_m2_1.a
    $(CC) $(CFLAG) $^ -o $@

libtest_m1_1.dylib:table.o use_table_m1.o
    $(CC) -dynamiclib $(CFLAG) $^ -o $@

libtest_m1_2.dylib:table.o use_table_m1.o
    $(CC) -dynamiclib -Wl,-read_only_relocs,suppress $(CFLAG) $^ -o $@

libtest_m2_1.dylib:table.o use_table_m2.o
    $(CC) -dynamiclib $(CFLAG) $^ -o $@

libtest_m2_2.dylib:table.o use_table_m2.o
    $(CC) -dynamiclib -Wl,-read_only_relocs,suppress $(CFLAG) $^ -o $@

libtest_m1_1.a:table.o use_table_m1.o
    $(AR) rcs $@ $^

libtest_m2_1.a:table.o use_table_m2.o
    $(AR) rcs $@ $^

table.o:table.c
    $(CC) -c $(CFLAG) $^ -o $@

test.o:test.c
    $(CC) -c $(CFLAG) $^ -o $@

use_table_m1.o:use_table_m1.s
    $(CC) -c -integrated-as $(CFLAG) $^ -o $@

use_table_m2.o:use_table_m2.s
    $(CC) -c -integrated-as $(CFLAG) $^ -o $@

clean:
    rm -f *.o libtest* test_d_m* *~ test_s_m*

/* == end Makefile == */

(all of these files are also available at
http://dl.dropbox.com/u/13855163/ill_text_relocation_test.tar.gz)

Best Regards,
Ashi
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