llvm dev - Sep 2011 - [LLVMdev] ARM CPSR liveness

All,

ARM LLVM afficianados: I'm trying to understand how LiveVariables computes
liveness for CPSR.  In particular,
I would like to tell whether a particular instruction is a use/kill/def for
doing some within block liveness checking I need.

Thanks in advance,

David
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Hi David,

I'm not too familiar with the LiveVariables pass in particular for how it
uses this information, but CPSR definition is handled on ARM instructions in two
ways:
a) As an OptionalDef explicit MachineOperand ("cc_out" in the .td
files) which is set to "zero_reg" if CPSR is not modified and
"CPSR" if CPSR is modified, or
b) as an implicit definition on the instruction class, which you'll see in
the .td file as a "let Defs = [CPSR]" on the instruction definition.

Uses of CPSR are handled like (b), with "let Uses = [CPSR]" on the
instruction definition in the .td file.

These uses and defs, including <def> and <kill> markers on the
operands, should be fairly reliable. If you find cases where they're not,
that's bugs we want to fix.

-Jim

On Sep 7, 2011, at 10:45 AM, David Sehr wrote:
> All,
> 
> ARM LLVM afficianados: I'm trying to understand how LiveVariables
computes liveness for CPSR.  In particular,
> I would like to tell whether a particular instruction is a use/kill/def for
doing some within block liveness checking I need.
> 
> Thanks in advance,
> 
> David
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev