More questions about "asm volatile"

[Ingo Rohloff]

Hello,

> The whole extended-asm section could use an intelligent rewrite.  This
> is difficult.  Patches welcome.

Please find attached a small diff for "extend.texi", trying to explain what I understood about "asm
volatile".
I am not sure if this mailing list is the best place for that (hints where instead to propose this
welcome).
Additionally I do not know if what I inserted is correct; especially since the semantics of
"volatiles" seem to be complicated (I just read about sequence points and so on).

I am also not a native speaker so the syntax might not be correct too; still I think it really would
help to include something similar as in the patch in the documentation.

so long
  Ingo


Ian Lance Taylor wrote:
> Ingo Rohloff <Ingo.Rohloff@lauterbach.com> writes:
> 
>> The document mentioned above says:
>> "Similarly, you can't expect a sequence of volatile asm instructions to remain perfectly
>> consecutive. If you want consecutive output, use a single asm."
>>
>> Does that mean that the order of "volatile asm" instructions might be swapped, or does this just
>> mean that the different "volatile asm" instructions might be interspersed with other instructions ?
> 
> Different non-volatile asm instructions might be interspersed with
> volatile asm instructions.
> 
>> If there is a guarantee that "volatile asm" instructions CANNOT be swapped, then I think the
>> "Extended-Asm.html" document should state this.
> 
> The whole extended-asm section could use an intelligent rewrite.  This
> is difficult.  Patches welcome.
> 
> 
>> The document mentioned above says
>> "Note that even a volatile asm instruction can be moved relative to other code, including across
>> jump instructions. "
>> and there are a number of old discussions about that behavior.
>>
>> Unfortunately neither the document above nor the discussions I have read make any statement if this
>> just means that "volatile" has no meaning or if there are circumstances in which there still is SOME
>> guarantee.
>>
>> What I am wondering in particular: If I have this code:
>>
>>
>> extern volatile int *gPtr;
>> void foo()
>> {
>>   int v;
>>   v=*gPtr;
>>   asm volatile ( <Something> );
>>   *gPtr=5;
>> }
>>
>> Is it possible that the "asm volatile" is swapped with the accesses to the "volatile" pointer, or
>> does the "asm volatile" interact with "volatile" pointer accesses, so that it is guaranteed that a
>> "asm volatile" will not be swapped with a "volatile" pointer access ?
>> Of course I am interested in the general case: Is there a guarantee that "asm volatile" will stay in
>> order with accesses to volatile pointers ?
> 
> A volatile asm is very much like a volatile load or store from memory.
> They can not be interchanged.
> 
> Ian
> 
> 
-------------- next part --------------
A non-text attachment was scrubbed...
Name: extended_asm.diff
Type: text/x-patch
Size: 1455 bytes
Desc: not available
URL: <https://gcc.gnu.org/pipermail/gcc-help/attachments/20110412/fd944f6c/attachment-0001.bin>