SuperWord optimization

[John Rose]

On Dec 31, 2008, at 10:32 AM, James Walsh wrote:

> Is there some documentation of hdl used in x86_32.ad?  How would I
> describe a XMM register as a whole in that language?

The AD file describes machine registers in terms of 32-bit chunks.   
Each (named) chunk corresponds to a bit position in the register  
allocator's bitmasks.  Stack frame slots are numbered also in the  
same uniform scheme, so it doesn't make sense to have some chunks be  
128 bits, etc.  That is why the AD file talks about parts [abcd] of  
the 128-bit XMM regs.

The upside of this decision to standardize on a 32-bit unit is the  
spill and memory allocation logic (always a tricky part of register  
allocation) is simplified.  Stack slots do not need to be typed; they  
are just 32-bit words.  Another upside is that multi-purpose machine  
registers (e.g., two-floats-in-a-double or a long-in-two-ints) can  
also be represented readily.

The downside of this is that 64-bit entities must be represented  
using (contiguous, aligned) register pairs.  The register allocation  
logic is somewhat complicated by the need to work with such register  
pairs.  Generalizing the code further to 128-bit entities is a  
moderately tricky problem, but I think that's what has to be done.

The register allocation has an "ideal reg" query on each node which  
discloses what kind of register to allocate.  The upshot of this  
query is (a) a mask of possible hardware register resources where the  
result has to live, and (b) whether the ultimate allocation is a  
single 32-bit unit or a (contiguous, aligned) pair.  To cope with  
vectors, the system needs at least one new "ideal reg" type per  
vector size, perhaps an RegV4 (for XMM) or RegV8 (for YMM).  The RegL  
probably will continue to serve well as a RegV2.

> Obviously
> MOVAPS, ADDPS, etc and the like will need to reserve a whole XMM
> register, not just XMM0a etc.  Looking at the description of the XMM
> double register definitions I could probably guess but if there is  
> some
> official docs it would be nice to take a look at them.

Happy New Year!
-- John