Re: An unusual Performance approach using Synthetic registers

[Andy Walker <ja_walker at earthlink dot net>]

On Sunday 29 December 2002 02:34 am, Chris Lattner wrote:

<snip>

> However, the true problem is that GCC performs most of its scalar
> optimizations on the RTL representation, which operates on values in
> physical registers.  For an machine with few physical registers, such as
> X86, many scalar values are spilled to the stack, impeding optimization.
>
> The fix for this particular problem, IMHO, is not to increase the number
> of registers, for this is only a bandaid.  Instead, optimizations should
> be performed on a representation which is not limited by the number of
> registers in the target: in GCC this would be the tree representation, or
> hopefully soon the GENERIC and GIMPLE representations.  The idea here is
> that you perform optimizations on a representation exposing an infinite
> virtual register set, so scalar stay scalars, and optimizations are
> effective despite the number of physical registers in the target.

I keep in mind that at some point the theory hits the silicon.  As an old 
assembler programmer, I always have in the back of my head the question: 
"What will this look like in instructions or in memory?" 

My approach may allow for an effectively infinite register set.  For each 
subroutine, the compiler figures out how many registers are needed.  Right 
now, in GCC, the next step after adding up the number of registers is to call 
Daniel Berlin's implementation of Color-Graphing.  

Instead of Color-Graphing, just allocate that number of Synthetic registers 
as part of the frame for that subroutine.  If you need one hundred, use one 
hundred, and get on with it.  

I have some questions about whether an effectively infinite number of 
Synthetic registers is a good idea.  

I will not look into that until I have some timing and memory results using a 
statically fixed number of Synthetic registers.

Andy