[LLVMdev] disable llc optimizations

[John Criswell]

On 12/7/10 10:46 AM, Andrew Lukefahr wrote:
> 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.

>
> 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 at gmail.com <mailto:andrewlukefahr at gmail.com>
>
> Open Source, Open Minds
>
>
> On Mon, Dec 6, 2010 at 2:08 PM, Jim Grosbach <grosbach at apple.com 
> <mailto:grosbach at apple.com>> wrote:
>
>
>     On Dec 6, 2010, at 10:57 AM, John Criswell wrote:
>
>     > On 12/6/10 11:39 AM, Andrew Lukefahr wrote:
>     >> 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?
>     >
>
>
>     Perhaps the dead machine instruction elimination pass?
>     (DeadMachineInstructionElim.cpp) I don't think there's a way to
>     disable that.
>
>     -Jim
>
>

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