+ linaro-toolchain
Hello Ulrich,
I want to revisit this old thread. Sorry for the sloppy follow-up. But,
this time around I have more data.
On Tue, Mar 15, 2011 at 9:00 PM, Ulrich Weigand
wrote:
> Aneesh V wrote:
>
>> I was trying to build u-boot in Thumb2 for OMAP4. Everything was fine
>> until I added some patches recently. One of these patches introduced an
>> API (let's say foo()) that has a weakly linked alias(let's say
>> __foo()) and a strongly linked implementation(the real foo()) in an
>> assembly file.
>>
>> Although I give -mthumb and -mthumb-interwork for all the files,
>> apparently GCC generates ARM code for assembly files. In the final
>> image foobar() calls foo() using a BL. Since foobar() is in Thumb and
>> foo() in ARM, it ends up crashing. Looks like foobar() assumed foo()
>> to be Thumb because __foo() is Thumb.
>
> I'm unable to reproduce this. Do you have a complete test case?
>
> I've tried with the following small example:
>
> foo1.c:
>
> extern void foo (void) __attribute__ ((weak, alias ("__foo")));
>
> void __foo (void)
> {
> }
>
> int main (void)
> {
> foo ();
> }
>
> foo2.S:
> .text
> .align 2
> .global foo
> .type foo, %function
> foo:
> push {r7}
> add r7, sp, #0
> mov sp, r7
> pop {r7}
> bx lr
> .size foo, .-foo
>
> When building just "gcc foo1.c", I get:
>
> 835c <__foo>:
> 835c: b480 push {r7}
> 835e: af00 add r7, sp, #0
> 8360: 46bd mov sp, r7
> 8362: bc80 pop {r7}
> 8364: 4770 bx lr
> 8366: bf00 nop
>
> 8368 :
> 8368: b580 push {r7, lr}
> 836a: af00 add r7, sp, #0
> 836c: f7ff fff6 bl 835c <__foo>
> 8370: 4618 mov r0, r3
> 8372: bd80 pop {r7, pc}
>
> When building both files "gcc foo1.c foo2.S", I get instead:
>
> 8368 :
> 8368: b580 push {r7, lr}
> 836a: af00 add r7, sp, #0
> 836c: f000 e802 blx 8374
> 8370: 4618 mov r0, r3
> 8372: bd80 pop {r7, pc}
>
> 8374 :
> 8374: e92d0080 push {r7}
> 8378: e28d7000 add r7, sp, #0
> 837c: e1a0d007 mov sp, r7
> 8380: e8bd0080 pop {r7}
> 8384: e12fff1e bx lr
>
>
> So it seems to me the linker is handling this correctly ...
>
> (This is on Ubuntu Natty using system gcc and binutils.)
I could reproduce the problem on older tool-chain(Sourcery G++ Lite
2010q1-202) [1] with a modified version of your sample code:
a.c:
extern void foo (void) __attribute__ ((weak, alias ("__foo")));
void __foo (void)
{
}
extern void call_foo(void);
int main (void)
{
call_foo ();
}
b.S:
.text
.align 2
.global foo
foo:
push{r7}
add r7, sp, #0
mov sp, r7
pop {r7}
bx lr
.size foo, .-foo
c.S
.text
.align 2
.global call_foo
call_foo:
bl foo
bx lr
.global __aeabi_unwind_cpp_pr0
__aeabi_unwind_cpp_pr0:
bx lr
Now, I build them using the following commands, which is similar to
what U-Boot does:
arm-none-linux-gnueabi-gcc -mthumb -mthumb-interwork -c a.c
arm-none-linux-gnueabi-gcc -mthumb -mthumb-interwork -c b.S
arm-none-linux-gnueabi-gcc -mthumb -mthumb-interwork -c c.S
arm-none-linux-gnueabi-ld -r a.o -o alib.o
arm-none-linux-gnueabi-ld -r b.o -o blib.o
arm-none-linux-gnueabi-ld -r c.o -o clib.o
arm-none-linux-gnueabi-ld --start-group clib.o alib.o blib.o
--end-group -o a.out
armobjdump -S --reloc a.out
You will get something like:
8094 :
8094: fa06blx 80b4
8098: e12fff1ebx lr
Please note that that the 'blx' is not correct. Now, do the following change:
diff --git a/b.S b/b.S
index e0f2de9..96dba1f 100644
--- a/b.S
+++ b/b.S
@@ -1,5 +1,6 @@
.text
.align 2
+.type foo, %function
.global foo
foo:
push{r7}
And build it again the same way and you will see:
8094 :
8094: eb06bl 80b4
8098: e12fff1ebx lr
I can't reproduce this on Linaro GCC 2012.01, so looks like the problem
is solved in recent tool-chains. However, sadly I could reproduce a
different but similar problem with Linaro GCC 2012.01. This time the
call is from C(Thumb) to assembly(ARM) and no weakly linked symbols are
involved.
a.c:
int main (void)
{
foo ();
}
b.S:
.text
.align 2
.global foo
foo:
push{r7}
add r7, sp, #0
mov sp, r7
pop {r7}
bx lr
.size foo, .-foo
.global __aeabi_unwind_cpp_pr0
__aeabi_unwind_cpp_pr0:
bx lr
arm-linux-gnueabi-gcc -mthumb -mthumb-interwork -c
