llvm dev - Jun 2012 - [LLVMdev] Best way to replace LLVM IR operation with code containing control flow?

Hi,

-Does anyone know where a backend-specific optimization can be added to replace
an instruction with code containing control flow?

I'm interested in adding an optimization for the DIV instruction (x86-atom)
which replace the IDIV/DIV with code containing control flow to select between
the intended IDIV/DIV and an 8-bit DIV with movzx, as described in the Intel
Atom Optimization Guide. My first attempt was to add this with a custom inserter
in X86ISelLowering (see EmitInstrWithCustomInserter). However, the isel already
done in X86ISelDAGToDAG, line 2200, adds multiple instructions which makes
adding control flow in X86ISelLowering difficult. Also, it looks like control
flow cannot be added in X86ISelDAGToDAG.

Any suggestions are appreciated.

Thanks,

Tyler Nowicki
Intel
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Hi Tyler,
> -Does anyone know where a backend-specific optimization can be added to
replace
> an instruction with code containing control flow?
I think the backend lowering of atomic intrinsics generates control flow
(loops), so that may give you a clue.

Ciao, Duncan.

Thanks for the response Duncan.

Inserting this code using a customer inserter (as used by atomics) would require
the instructions added by X86ISelDAGToDAG to be found and removed which is
counterproductive and difficult due to optimization. Inserting the code earlier,
perhaps before the code becomes a DAG, would allow X86ISelDAGToDAG to insert the
instructions it wants to and the optimizer to move and transform the
instructions for efficiency.

To reiterate, I'm interested in adding an optimization for the DIV
instruction (x86-atom) which replaces the IDIV/DIV with code containing control
flow to select between the intended IDIV/DIV and an 8-bit DIV, as described in
the Intel Atom Optimization Guide.

-Is there an earlier pass in which an instruction can be replaced by code
containing control flow? Is this something I should add?

Thanks,

Tyler Nowicki
Intel

-----Original Message-----
From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu] On
Behalf Of Duncan Sands
Sent: Tuesday, June 19, 2012 3:45 AM
To: llvmdev at cs.uiuc.edu
Subject: Re: [LLVMdev] Best way to replace LLVM IR operation with code
containing control flow?

Hi Tyler,
> -Does anyone know where a backend-specific optimization can be added 
> to replace an instruction with code containing control flow?
I think the backend lowering of atomic intrinsics generates control flow
(loops), so that may give you a clue.

Ciao, Duncan.
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