On Wed, May 11, 2011 at 6:14 PM, saeed bishara <saeed.bish...@gmail.com> wrote:
> On Sat, May 7, 2011 at 12:39 AM, Nicolas Pitre <nicolas.pi...@linaro.org>
> wrote:
>> On Fri, 6 May 2011, Ramana Radhakrishnan wrote:
>>
>>> On 6 May 2011 16:06, Ken Werner <ken.wer...@linaro.org> wrote:
>>> >
>>> > Currently the GCC ARM backend doesn't provide a pattern to inline 64bit
>>> > __sync_* functions but the compiler emits __sync_*_8 function calls [1].
>>> > The
>>> > libgcc does not provide these symbols via the usual thin wrapper around
>>> > the
>>> > kernel helper [2] because the ARM Linux __kernel_cmpxchg supports 32bit
>>> > only.
>>> > My understanding is that for ARMv7 the GCC backend could be enhanced to
>>> > inline
>>> > the __sync_* functions by using the LDREXD and STREXD instructions. But
>>> > for
>>> > ARMv5 we would still rely on a new kernel helper.
>>>
>>> It's a bit tricky with when you want to use the kernel helper for v5t,
>>> so we've got to find a way of turning this on only with special knobs
>>> and not by default and that's a bit tricky.
>>
>> What is the problem with v5t?
> Hi, when I run the following on my jaunty (for armv5), I get the following
>
>
> echo "void f(){__sync_synchronize();}" | gcc -O2 -x c -Wall -dA -S - -o -
> .arch armv6
> .eabi_attribute 27, 3
> .fpu vfp
> .eabi_attribute 20, 1
> .eabi_attribute 21, 1
> .eabi_attribute 23, 3
> .eabi_attribute 24, 1
> .eabi_attribute 25, 1
> .eabi_attribute 26, 2
> .eabi_attribute 30, 2
> .eabi_attribute 18, 4
> .file ""
> .text
> .align 2
> .global f
> .type f, %function
> f:
> @ args = 0, pretend = 0, frame = 0
> @ frame_needed = 0, uses_anonymous_args = 0
> @ link register save eliminated.
> @ basic block 2
> bx lr
> .size f, .-f
> .ident "GCC: (Ubuntu 4.4.1-4ubuntu9) 4.4.1"
> .section .note.GNU-stack,"",%progbits
>
> but I can see (with debugger) that the kernel helper functions are
> reached. any idea how?
There was a compiler bug some time ago which mistakenly generated no
code for __sync_synchronize():
https://bugs.launchpad.net/ubuntu/+source/gcc-4.4/+bug/491872
I suspect your Jaunty-era compiler may be affected by this -- you
probably need to upgrade it.
If you're observing calls to the helpers, they may be coming from
glibc etc. instead of your function.
You _should_ be seeing code something like this:
$ echo 'void f(void) { __sync_synchronize(); }' |
arm-linux-gnueabi-gcc -march=armv7-a -O2 -S -o - -x c -
[...]
f:
@ args = 0, pretend = 0, frame = 0
@ frame_needed = 0, uses_anonymous_args = 0
@ link register save eliminated.
dmb sy
bx lr
[...]
(If -march is set high enough, the dmb is explicit and inline.)
$ echo 'void f(void) { __sync_synchronize(); }' |
arm-linux-gnueabi-gcc -march=armv5t -O2 -S -o - -x c -
[...]
f:
push {r3, lr}
bl __sync_synchronize
@ sp needed for prologue
pop {r3, pc}
(-march=armv5t indicates an architecture which doesn't have the
barrier instructions, so the kuser helper is needed instead)
Cheers
---Dave
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