[LLVMdev] register scavenging

[reed kotler]

I ran into a case in Mips16 where I need two registers.

The problem arises from the fact that SP is not a mips16 register.
You can implicitly use it in load/store and, move it to/from a mips16 
register and add small immediate values to it but that's it.

It's not in general a problem for me because there are a bunch of mips32 
registers that are hard to use in mips16 so at this time, I only use 
them in special situations. So they are convenient for temporarily 
storing mips16 registers.

mips16 registers is a subset of the full mips32 register set.

so i'm guessing that I can still use the RegScavenger class for this.

I'm thinking of something [not sure if this is a correct understanding 
of this API]

RegScavenger rs;
rs.enterBasicBlock(MBB);
forward(II);

and then I can get and set available registers????

Ideally I could use usual way to get the first register I need by 
implementing the virtual function for register scavenging in 
Mips16RegisterInfo [which I already do] and just roll my own for this 
one moment to get an additional register. If there are no free 
registers, I can save one in an unused mips32 only register.

What I need is somewhere to move SP so that I can use it in another 
instruction.

mov unused mips32 register, temp mips16 register; if temp is being used
mov temp mips16 register, SP
add.....temp mips16 register....
; temp is no longer needed
mov temp mips16 register, unused mip32 register ; if temp was being used

This is my main problem.

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There is another issue from the point of view of optimization and that 
is that if I'm going to be
doing this a lot, then this copy of SP is a CSE and should be treated as 
such. Of course, we don't
know to do this until the end.

I think there is an earlier llvm pass which gives a better estimate of 
local stack size so I could
make this decision earlier and allocate an alias for SP in a mips16 
register.