[LLVMdev] Handling of KILL instructions.

[Quentin Colombet]

Hi Vasileios,

If you start moving instruction around, then I suggest you remove the kill instructions. Otherwise, if the liveness gets recomputed later on, you may have bad surprises, same thing with the MachineVerifier.

Cheers,
-Quentin
On Feb 11, 2015, at 4:38 AM, Vasileios Kalintiris <Vasileios.Kalintiris at imgtec.com> wrote:

> Ping.
> 
> To give some additional context, the KILL instructions are blocking
> candidates for the filling of delay slots in the Mips and the Sparc
> backends. Despite the fact that the delay slot filler pass is running
> immediately before machine code is emitted (registered with
> addPreEmitPass()), there are some backend specific machine function
> passes that have to run *after* the delay slot filler. This is why the
> handling of KILL instructions is relevant in our case.
> 
> -- Vasileios Kalintiris
> 
> ________________________________________
> From: Vasileios Kalintiris
> Sent: 04 February 2015 11:11
> To: llvmdev at cs.uiuc.edu
> Cc: ehostunreach at gmail.com; Daniel Sanders
> Subject: Handling of KILL instructions.
> 
> Hi all,
> 
> My understanding is that we keep around KILL instructions in order to keep
> the results of the various register liveness analysis passes valid.
> 
> Consider for example the following machine basic block:
> 
> BB#0: derived from LLVM BB %entry
>    Live Ins: %A0_64 %A1_64
>        %V0_64<def> = AND64 %A0_64<kill>, %A1_64<kill>
>        %V0<def> = KILL %V0, %V0_64<imp-use,kill>
>        PseudoReturn64 %RA_64
> 
> In this case we would like to move the AND64 instruction after the KILL
> instruction (generated from an identity COPY).
> 
> What is the right thing to do with the KILL instruction given the fact that
> the machine function pass calls TRI.invalidateLiveness()? Can we simply
> delete any KILL instruction we want? Or, can we ignore them as long as we
> don't request any register liveness analysis in subsequent machine function
> passes?
> 
> -- Vasileios Kalintiris
> 
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