llvm dev - Jan 2011 - [LLVMdev] Increasing TargetRegisterInfo::FirstVirtualRegister?

Would it be possible to increase TargetRegisterInfo::FirstVirtualRegister
to something quite a bit larger - such as 2^31?  I'm working on a target
where all SSA values need to be assigned physical registers, and the
current limit of 16384 may limit the size of programs that I'm able to
handle.  2^31 is a nice value since there are equal numbers of possible
physical and virtual registers, and registers can just inspect the high
bit to determine if it's a virtual register.  My only concern is that
there may be data structures using FirstVirtualRegister to size tables
rather than using the MachineRegisterInfo limit.

Along with this change, I would also propose subtracting
FirstVirtualRegister from the printed values of virtual registers, so that
what used to be "reg16384" would now show up as "reg0".

Andrew

On Jan 6, 2011, at 10:11 PM, andrew at sidefx.com wrote:
> Would it be possible to increase TargetRegisterInfo::FirstVirtualRegister
> to something quite a bit larger - such as 2^31?  I'm working on a
target
> where all SSA values need to be assigned physical registers, and the
> current limit of 16384 may limit the size of programs that I'm able to
> handle.  2^31 is a nice value since there are equal numbers of possible
> physical and virtual registers, and registers can just inspect the high
> bit to determine if it's a virtual register.  My only concern is that
> there may be data structures using FirstVirtualRegister to size tables
> rather than using the MachineRegisterInfo limit.
If you're interested in doing this, I'd suggest hacking up the backend
to not run register allocation, and represent all your "physical"
registers are virtual registers.  This gives you an unlimited number.

-Chris

> If you're interested in doing this, I'd suggest hacking up the
backend to
> not run register allocation, and represent all your "physical"
registers
> are virtual registers.  This gives you an unlimited number.
>
Yes it would be ideal to use all virtual registers, but a glance at the
code seemed to indicate the the use of physical registers, spilling, etc.
was pretty ingrained in all the register allocation algorithms, so it
seemed like it would be difficult to extract.  I also still need to
minimize the number of registers and minimize copies as if it were
generating code for more restrictive hardware.

Ideally I'd like something like the linear register allocator but that
never spilled or assigned physical registers, along with other goodies
like coalescing and phi elimination.

On the other hand, just increasing the allowable physical registers would
immediately solve most of my problems.

Andrew