llvm dev - Mar 2012 - [LLVMdev] Register coalescing

Need some guidance about the right way to model this -- how would you model
a backend with a handful of read-only physical registers that are passed as
arguments to a function? I was emitting copyFromReg nodes in the
LowerFormalArgument() routine, but then the register allocator and
coalescer are resisting coalescing the COPY MI's for various reasons - for
example, the read-only register class contains too few registers and the
live range threshold cancels the coalescing.

A simple example (post-ISEL):

%vreg2<def> = COPY %C1; GPReg:%vreg2
...
%vreg11<def> = MUL %vreg7, %vreg2; GPreg:%vreg11,%vreg7,%vreg2

I'd want it to propagate %C1 into the MUL, replacing %vreg2. How is this
supposed to work? Is there a DAG operation or MF pass that should handle
this before regalloc, or some other means?

Thanks,
Joe
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On Mar 8, 2012, at 3:52 PM, Joe Matarazzo <joe.matarazzo at gmail.com>
wrote:
> Need some guidance about the right way to model this -- how would you model
a backend with a handful of read-only physical registers that are passed as
arguments to a function? I was emitting copyFromReg nodes in the
LowerFormalArgument() routine, but then the register allocator and coalescer are
resisting coalescing the COPY MI's for various reasons - for example, the
read-only register class contains too few registers and the live range threshold
cancels the coalescing.
> 
> A simple example (post-ISEL):
> 
> %vreg2<def> = COPY %C1; GPReg:%vreg2
> ...
> %vreg11<def> = MUL %vreg7, %vreg2; GPreg:%vreg11,%vreg7,%vreg2
> 
> I'd want it to propagate %C1 into the MUL, replacing %vreg2. How is
this supposed to work? Is there a DAG operation or MF pass that should handle
this before regalloc, or some other means?
You should model the live-in registers like other targets do, let
MachineRegisterInfo::EmitLiveInCopies() produce the copies.

Make sure the constant registers belong to the register class you are using
(GPReg), otherwise coalescing is impossible.

Mark the constant registers as reserved, and RegisterCoalescer should take care
of the rest.

/jakob