Output register allocation constrain fix for HotSpot

[David Holmes]

Hi Deepak,

I've filed "7031385 incorrect register allocation in release_store_fence 
on linux x86" for this issue.

David Holmes

Deepak Bhole said the following on 03/26/11 12:59:
> Hi,
> 
> When trying to build IcedTea with GCC 4.6, we encountered an error that
> stems from incorrect register allocation in release_store_fence(volatile
> jbyte*  p, jbyte  v) in orderAccess_linux_x86.inline.hpp.
> 
> Specifically, the inline assembly call trying to do an atomic 8-bit
> exchange sets the output register constrain to r. The older versions GCC
> (by chance) ended up assigning the l-part of one of the a/b/c/d
> registers. However with 4.6, it attempts to do this with the
> %*bp register. This is fine on x86_64, but on i686 the bp register
> cannot be addressed in 8-bit (%bpl). This leads to a compilation
> failure:
> 
> /builddir/build/BUILD/icedtea6-1.10/openjdk-ecj/hotspot/src/os_cpu/linux_x86/vm/orderAccess_linux_x86.inline.hpp: Assembler messages:
> /builddir/build/BUILD/icedtea6-1.10/openjdk-ecj/hotspot/src/os_cpu/linux_x86/vm/orderAccess_linux_x86.inline.hpp:160: Error: bad register name `%bpl'
> /builddir/build/BUILD/icedtea6-1.10/openjdk-ecj/hotspot/src/os_cpu/linux_x86/vm/orderAccess_linux_x86.inline.hpp:160: Error: bad register name `%bpl'
> 
> It is possible to make it work on both 32 and 64-bit by constraining the
> output register further to only be one of a/b/c/d (=q).
> 
> Though it was only that function that caused an error, I think it'd be
> best to change both uses of xchgb in orderAccess_linux_x86.inline.hpp to
> use =q. Here is the webrev for it:
> http://cr.openjdk.java.net/~dbhole/register-allocation-fix/webrev.00/
> 
> If there are no issues, I'd be happy to push the above change.
> 
> Cheers,
> Deepak