PATCH: TableGen: Report an error when defining more than 32 subreg indices

[Jakob Stoklund Olesen]

On Jan 31, 2013, at 11:16 AM, Tom Stellard <tom at stellard.net> wrote:

> On Thu, Jan 31, 2013 at 09:30:48AM -0800, Jakob Stoklund Olesen wrote:
>> 
>> On Jan 31, 2013, at 6:23 AM, Tom Stellard <tom at stellard.net> wrote:
>> 
>>> The maximum number of sub reg indices is limited to 32 by use of the
>>> LaneMask.  TableGen currently assigns all sub reg indices after the
>>> first 32 to the same LaneMask, which leads to bugs in the register
>>> allocator.  This patch prevents the user from defining more than 32
>>> sub reg indices.
>> 
>> Hi Tom,
>> 
>> That behavior is deliberate. It allows graceful degradation for crazy targets with more than 32 vector lanes by sharing the last available bit in the lane mask.
>> 
> 
> I'm not sure I understand exactly what the lane mask is supposed to
> represent.  Can you give a brief explanation of what it is used for?

Weird targets have weird register banks, making it necessary to have a somewhat abstract model for overlapping sub-registers. Just look at ARM and x86.

The lane masks are used to determine that two sub-registers are definitely not overlapping. If LaneMask(SubA) & LaneMask(SubB) is null, it is safe to assume that the sub-registers don't overlap.

The masks are not supposed to be used to determine if one sub-register is completely contained within another, and that could be the mistake the coalescer is making.

/jakob