[llvm-dev] RFC: Adding bit to register MachineOperands to allow post-RA register renaming

[Geoff Berry via llvm-dev]

Hi All,

Currently, changing register assignments of definitions after register 
allocation is not safe because there is no way to know which register 
definitions were physical registers before RA (e.g. to meet ABI or ISA 
constraints) and thus should not be changed.  I'd like to propose adding 
a bit to MachineOperand (by overloading the meaning of the IsKill bit 
for defs, so no extra storage would be required), that tracks whether a 
given register definition was a virtual register before RA.  I'll throw 
out 'IsRenameable' for a potential name.

Register definitions created with virtual registers would have this bit 
set.  This bit should be verifiable until after RA.  Register 
definitions created after RA (presumably with physical registers) would 
not have this bit set.  I believe the only potential for this bit to be 
set incorrectly (and not be caught be verification) would be if a 
post-RA pass was already renaming a register definition from a 
previously virtual register to a previously non-virtual register, which 
would arguably be a bug already.

We have encountered several potential uses for this bit.  For example, 
the MachineCopyPropagation changes I have been working on to forward 
register COPYs would likely be greatly simplified if this bit were 
available.  Other passes, like AArch64LoadStoreOptimizer, which run 
post-RA so as not to overly-restrict the register allocator, could be 
made to catch more cases if renaming of load instructions could be done 
safely.

-- 
Geoff Berry
Employee of Qualcomm Datacenter Technologies, Inc.
  Qualcomm Datacenter Technologies, Inc. as an affiliate of Qualcomm 
Technologies, Inc.  Qualcomm Technologies, Inc. is a member of the Code 
Aurora Forum, a Linux Foundation Collaborative Project.