Request for reviews (S): 6814842: Load shortening optimizations

[Christian Thalinger]

On Mon, 2009-03-09 at 12:56 -0700, Tom Rodriguez wrote:
> The adlc does tend for more verboseness than I would really like.  If  
> there was more uniformity between declarations in our ad files I could  
> imagine a shared ad file that would capture some of these special  
> matches at a high level and rely either on well known expand rules or  
> ins_encodings to do the final code emission though we might run up  
> against type problems.
> 
> In some cases it would be useful if a single instruct definition could  
> include mutiple match rules though in that case you run into issues  
> with type mismatches since some might use RegI and others RegL.  If we  
> could figure out a syntax for that I don't think it wouldn't be hard  
> to implement at the bottom by just making variants of the original  
> MachNode the way we do for commutative ops.
> 
> Maybe if it were possible to declare instructs without specifying the  
> register type and have the type be inferred from the types in the  
> match rule we could write that cleanly.  In general it would be good  
> if the adlc understood the real types of the nodes in the graph at  
> least so it could complain about mismatches. If it could use that to  
> find the real operands to use would could write something more  
> compact.  Alternatively we could just make up a placeholder syntax for  
> it.  Maybe something like:
> 
> instruct loadUB(<T> dst, memory mem) %{
>    match(Set <rRegI> dst (LoadUB mem));
>    match(Set <rRegL> dst (ConvI2L (LoadUB mem)));
> 
>    ...
> %}
> 
> This doesn't help with the load shortening on all platforms since some  
> will need extra offsets to deal with endianness but if the encoding  
> and format were the same you could use the same thing.

That sounds like a really good idea.  Having more than one match rule in
an instruct would definitely help.  I know very little about ADLC yet,
but if there is time and need to enhance it, I'd volunteer to do that.

-- Christian