disable llc optimizations

Hi,

How would I disable dead code elimination in llc? Can that be done via the command line or do I need to modify llc’s source?

Thanks

Andrew Lukefahr
andrewlukefahr@gmail.com

Open Source, Open Minds

Hi Andrew,

How would I disable dead code elimination in llc? Can that be done via the
command line or do I need to modify llc's source?

I don't think llc does non-trivial dead code elimination (though I could be
wrong). Did you mean "opt"?

Ciao, Duncan.

Hi,

How would I disable dead code elimination in llc? Can that be done via the command line or do I need to modify llc’s source?

You can use llc --help-hidden to see a list of options. There are several -disable-xxx options that disable various code generator optimizations.

That said, I, like Duncan, wasn’t aware that llc did any dead-code elimination (beyond peephole optimization). Are you sure llc is removing the instructions you care about?

– John T.

Perhaps the dead machine instruction elimination pass? (DeadMachineInstructionElim.cpp) I don't think there's a way to disable that.

-Jim

Ok,

I’m trying to get llvm to decide when to turn the multiplier “on” and “off” for an ARM simulator. These instructions are just to let the simulator know when to power on/off the multiplier. I need to insert an instruction before the first multiply and another one after the last multiply in a basic block. I’m currently trying to figure out the best way to do that without having to add a new instruction to llvm.

My plan was to insert useless instructions, let llc generate an assembly (.s) file, then use sed to replace the useless instructions with my special instructions. However, llc eliminates my useless instructions. ( Allocas and binary operators go away, volatile loads and stores change the register allocation. )

%mul_en = alloca i32
%2 = mul nsw i32 %y, %x
%mul_dis = alloca i32

becomes…

mul r12, r12, r0

Next I tried using llvm-dis and llvm-as to manually replace my useless instructions with a inline no-op in the .bc file. However, llc then moves the volatile no-op below the multiply.

tail call void asm sideeffect “NOP”, “”() nounwind
%2 = mul nsw i32 %y, %x
tail call void asm sideeffect “NOP”, “”() nounwind

becomes…

mul r12, r12, r0

@APP
NOP
@NO_APP
@APP
NOP
@NO_APP

Andrew Lukefahr
andrewlukefahr@gmail.com

Open Source, Open Minds

Ok,

I’m trying to get llvm to decide when to turn the multiplier “on” and “off” for an ARM simulator. These instructions are just to let the simulator know when to power on/off the multiplier. I need to insert an instruction before the first multiply and another one after the last multiply in a basic block. I’m currently trying to figure out the best way to do that without having to add a new instruction to llvm.

Can you use inline assembly to insert your instructions? I think the LLVM IR optimizers will leave inline assembly alone. The only trick then is to use an inline assembly sequence that doesn’t appear as dead code to the code generator.

– John T.