Hi Albert,
On Saturday 18 February 2012 03:43 PM, Albert ARIBAUD wrote:
Hi Aneesh,
Le 17/02/2012 12:09, Aneesh V a écrit :
Hi Albert,
On Wednesday 15 February 2012 07:27 PM, Aneesh V wrote:
This is done using the following directive preceding
each function definition:
.type<func-name>, %function
This marks the symbol as a function in the object
header which in turn helps the linker in some cases.
In particular this was found needed for resolving ARM/Thumb
calls correctly in a build with Thumb interworking enabled.
This solves the following problem I had reported earlier:
"When U-Boot/SPL is built using the Thumb instruction set the
toolchain has a potential issue with weakly linked symbols.
If a function has a weakly linked default implementation in C
and a real implementation in assembly GCC is confused about the
instruction set of the assembly implementation. As a result
the assembly function that is built in ARM is executed as
if it is Thumb. This results in a crash"
Signed-off-by: Aneesh V<ane...@ti.com>
Does this look good to you. I was a bit nervous about touching so many
files. Please let me know if you would prefer to change only the OMAP
function that was creating the ARM/Thumb problem. I did a "MAKEALL -a
arm" and didn't see any new errors.
Let me know if this is an acceptable solution to the problem.
Regarding the solution: it is quite ok to me. I would just like to
understand the exact effect of the .function directive, what its options
are and if some of these should not be explicitly specified.
Regarding touching many files: I won't be worried as long as you check
that the first three patches have no effect on existing boards. This can
be verified as follows -- if you haven't done so already:
- build your OMAP target without the patch set and do a hex dump of
u-boot.bin;
- apply the first three patches of your set, rebuild your OMAP target
without the patch set and do a hex dump of u-boot.bin;
- compare both dumps. Normally you should only see one difference, in
the build version and date -- if .function does not actually alter the
assembly code, which I hope it indeed does not when building for ARM.
If there are more changes than build version and date, then they might
be due to .function requiring some yet unknown additional option, or to
some change in patch 1 or 3 not being completely conditioned on
CONFIG_SYS_THUMB_BUILD.
I can reproduce the problem with a simple test program.
Note: I can reproduce this with Sourcery G++ Lite 2010q1-202 (GCC 4.4.1
- Binutils 2.19.51.20090709)
But I *can not* reproduce reproduce this with Linaro GCC 2012.01 (GCC
4.6.3 , Binutils 2.22)
So apparently the issue has been fixed recently. Unfortunately Linaro
GCC 2012.01 creates a new Thumb problem that I am investigating now.
Somehow I missed this when I tested earlier. So, my Thumb build is
not working with Linaro GCC 2012.01. But this one is not reproduced on
Sourcery G++ Lite 2010q1-202!
Here is the program I used to reproduce the problem in Sourcery G++
Lite 2010q1-202 that this patch is addressing
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 build it and take the assembly dump using the following commands:
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
arm-none-linux-gnueabi-objdump -S --reloc a.out
You will get something like this in the assembly dump:
00008094 <call_foo>:
8094: fa000006 blx 80b4 <foo>
8098: e12fff1e bx lr
The blx is wrong as we are jumping to an ARM function from ARM.
Now if you change b.S like this:
.text
.align 2
+.type foo, %function
.global foo
foo:
push {r7}
And compile it again in the same way you will see:
00008094 <call_foo>:
8094: eb000006 bl 80b4 <foo>
8098: e12fff1e bx lr
Please note that the branch to foo is correct now.
I hope this convinces you that %function indeed has an effect.
I will get back with more details on the Linaro GCC 2012.01 later.
br,
Aneesh
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