llvm dev - Apr 2010 - [LLVMdev] Default behavior of DeadMachineInstructionElim deletes all instructions

I've recently sync'd to a newer version of LLVM(Apple branch 2326 from
Apple branch 2323.8) that changed the interface to addCommonCodeGenPasses which
caused the default implementation to be executed instead of my overriding
implementation. This default implementation has DeadMachineInstructionElim pass
enabled, which is causing havoc with my backend. Before entering this pass,
everything in my machine function is fine, after this pass, all instructions
that are not function calls are deleted. I've tracked this issue down to the
line:
BitVector NonAllocatableRegs = TRI->getAllocatableSet(MF); (In my case all
registers defined in RegisterInfo.td)

This function loops through all registers classes and sets all registers in the
bitset that are allocatable. It then inverts the registers that are set to get
the NonAllocatable registers and assigns that to the LivePhysRegs for each basic
block in a function. The function then loops through all instructions in a basic
block and checks to see if it is a dead instruction.  The check is whether it is
a physical register or not with the check:
TargetRegisterInfo::isPhysicalRegister(reg) ? LivePhysRegs[Reg] :
!MRI->use_nodbg_empty(Reg)

If the register is virtual, then MRI->use_nodbg_empty() returns false, if the
register is physical LivePhysRegs[Reg] returns false, since the bitvector was
inverted. So my instruction is considered dead and deleted.

So, what I am trying to figure out is why this behavior is occurring? I
don't see this happening on the x86 backend because getAllocatableSet does
not return all registers as set.

Since I'm working on virtual registers, how do I get
MRI->use_nodbg_empty() to return true? Is there something I should be setting
in my backend that affects this?

Thanks,
Micah
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On Apr 14, 2010, at 1:55 PMPDT, Villmow, Micah wrote:
> I’ve recently sync’d to a newer version of LLVM(Apple branch 2326 from
Apple branch 2323.8) that changed the interface to addCommonCodeGenPasses which
caused the default implementation to be executed instead of my overriding
implementation. This default implementation has DeadMachineInstructionElim pass
enabled, which is causing havoc with my backend. Before entering this pass,
everything in my machine function is fine, after this pass, all instructions
that are not function calls are deleted. I’ve tracked this issue down to the
line:
> 
> BitVector NonAllocatableRegs = TRI->getAllocatableSet(MF); (In my case
all registers defined in RegisterInfo.td)
> 
> This function loops through all registers classes and sets all registers in
the bitset that are allocatable. It then inverts the registers that are set to
get the NonAllocatable registers and assigns that to the LivePhysRegs for each
basic block in a function. The function then loops through all instructions in a
basic block and checks to see if it is a dead instruction.  The check is whether
it is a physical register or not with the check:
> 
> TargetRegisterInfo::isPhysicalRegister(reg) ? LivePhysRegs[Reg] :
!MRI->use_nodbg_empty(Reg)
> 
> If the register is virtual, then MRI->use_nodbg_empty() returns false,
if the register is physical LivePhysRegs[Reg] returns false, since the bitvector
was inverted. So my instruction is considered dead and deleted.
> 
Huh?  The code is

      if (TargetRegisterInfo::isPhysicalRegister(Reg) ?
          LivePhysRegs[Reg] : !MRI->use_nodbg_empty(Reg)) {
        // This def has a non-debug use. Don't delete the instruction!
        return false;

Returning false from MRI->use_nodbg_empty means the instruction has a use,
and it is not deleted. That's what's supposed to happen.

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Dale,
  Yeah that is correct, so that isn't the problem, not sure why I was
thinking it is. The !MRI->use_no_dbg_empty(Reg) seems to be correct and the
problem is LivePhysRegs[Reg] always returning false. I've looked into this
more and there is a part where I'm working with physical registers.
If there is a function call, there is a copy from virtual register space to the
functions that are expected to be passed into the register.
So it looks something like this:
... do some work ...
Mov %r0, %r1025
Mov %r1, %r1026
Mov %r2, %r1027
Call someFunc %r0, %r1, %r2 <-- this func stores to memory
Ret

The moves to the physical registers are being deleted as NonAllocatableRegs is
set to all one's and then inverted, and since these are deleted, every
instruction that they depended on is also deleted.

I have a workaround(disable the pass), but I'd much rather figure out what I
am doing incorrectly that is causing this behavior or if determining if the
default assumptions by this pass are correct.

Micah

From: Dale Johannesen [mailto:dalej at apple.com]
Sent: Wednesday, April 14, 2010 2:15 PM
To: Villmow, Micah
Cc: Dale Johannesen; llvmdev at cs.uiuc.edu
Subject: Re: [LLVMdev] Default behavior of DeadMachineInstructionElim deletes
all instructions


On Apr 14, 2010, at 1:55 PMPDT, Villmow, Micah wrote:


I've recently sync'd to a newer version of LLVM(Apple branch 2326 from
Apple branch 2323.8) that changed the interface to addCommonCodeGenPasses which
caused the default implementation to be executed instead of my overriding
implementation. This default implementation has DeadMachineInstructionElim pass
enabled, which is causing havoc with my backend. Before entering this pass,
everything in my machine function is fine, after this pass, all instructions
that are not function calls are deleted. I've tracked this issue down to the
line:
BitVector NonAllocatableRegs = TRI->getAllocatableSet(MF); (In my case all
registers defined in RegisterInfo.td)
This function loops through all registers classes and sets all registers in the
bitset that are allocatable. It then inverts the registers that are set to get
the NonAllocatable registers and assigns that to the LivePhysRegs for each basic
block in a function. The function then loops through all instructions in a basic
block and checks to see if it is a dead instruction.  The check is whether it is
a physical register or not with the check:
TargetRegisterInfo::isPhysicalRegister(reg) ? LivePhysRegs[Reg] :
!MRI->use_nodbg_empty(Reg)
If the register is virtual, then MRI->use_nodbg_empty() returns false, if the
register is physical LivePhysRegs[Reg] returns false, since the bitvector was
inverted. So my instruction is considered dead and deleted.
Huh?  The code is
      if (TargetRegisterInfo::isPhysicalRegister(Reg) ?
          LivePhysRegs[Reg] : !MRI->use_nodbg_empty(Reg)) {
        // This def has a non-debug use. Don't delete the instruction!
        return false;

Returning false from MRI->use_nodbg_empty means the instruction has a use,
and it is not deleted. That's what's supposed to happen.

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