> On Thu, Dec 18, 2014 at 10:50:27AM -0800, David Daney wrote:
>
> > On 12/18/2014 07:09 AM, Markos Chandras wrote:
> > >MIPS R6 changed the opcodes for LL/SC instructions and reduced the
> > >offset field to 9-bits. This has some undesired effects with the "m"
> > >constrain since it implies a 16-bit immediate. As a result of which,
> > >add a register ("r") constrain as well to make sure the entire
> > >address is loaded to a register before the LL/SC operations. Also use
> > >macro to set the appropriate ISA for the asm blocks
> > >
> >
> > Has support for MIPS R6 been added to GCC?
> >
> > If so, that should include a proper constraint to be used with the new
> > offset restrictions. We should probably use that, instead of forcing
> > to a "r" constraint.
>
> In a non-public earlier discussion I've requested the same but somehow
> that was ignored.
I must have missed that comment or not been on the thread.
> We need suitable constraints or the alternatives will be very, very
> ugly.
We can certainly discuss and investigate such things but there is a
general problem of a growing list of different size displacement fields
in load/store instructions. Obviously you could just opt to keep things
the way they are for uMIPS today and leave the assembler to expand the
instruction but my opinion is that magic expanding assembler macros
are infuriating. We have however had to put support in binutils for many
of them, simply to keep enough software building to ease the transition.
So, all this patch does is highlight that magic assembler macros have
been hiding this issue since micromips was added.
>From your experiences will people invest the effort to look at the
size of a displacement field for all the memory operations in an inline
asm block and then choose an appropriate memory constraint?
I'm obviously wary of putting things into GCC that are either only used
in a handful of places (or not at all). The alternative to constraints
is of course to try and reduce the need for inline asm and offer builtins
for specific instructions or more complex operations.
Thanks,
Matthew
