On Thu, Jun 12, 2008 at 01:10:10PM -0700, David Daney wrote:
>>> Among the versions of GCC that can build the current kernel, will any
>>> fail on this code because the "i" constraint cannot be matched when
>>> expanded to RTL?
>>
>> Someone will point this out if I don't, so for avoidance of doubt:
>> this needs to be always_inline. It also isn't guaranteed to work
>> with "bit" being a separate statement. I'm not truly sure it's
>> guaranteed to work even with:
>>
>> __asm__ __volatile__ (" foo %0, %1" : "=m" (*p) : "i" (nr & 5));
>>
>> but I think we'd try hard to make sure it does.
>>
>> I think Maciej said that 3.2 was the minimum current version.
>> Even with those two issues sorted out, I don't think you can
>> rely on this sort of thing with compilers that used RTL inlining.
>> (always_inline does go back to 3.2, in case you're wondering.)
>>
>
> Well I withdraw the patch. With the current kernel code we seem to always get good code generation. In the event that the compiler tries to put the shift amount (nr) in a register, the assembler will complain. I don't think it is possible to generate bad object code, so best to leave it alone.
>
> FYI, the reason that I stumbled on this several weeks ago is that if(__builtin_constant_p(nr)) in the trunk compiler was generating code for the asm even though nr was not constant.
How about I simply put your patch into the -queue tree, everybody gives it
a nice beating and then we'll how well it'll hold up in the real world?
Ralf
