[LLVMdev] LLVM Loop Vectorizer

[Villmow, Micah]

Why not just have a hook into the TargetInstrInfo to query for the cost of an instruction? This is already used in many places throughout the optimizers.

> -----Original Message-----
> From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu]
> On Behalf Of Das, Dibyendu
> Sent: Friday, October 05, 2012 2:00 AM
> To: Nadav Rotem; llvmdev at cs.uiuc.edu Mailing List
> Subject: Re: [LLVMdev] LLVM Loop Vectorizer
> 
> I think we should try to abstract the costs of instructions of various
> targets instead of trying to replicate them exactly. The coarser the
> costing infrastructure the more robust will be the vectorization pass.
> Also this eliminates/reduces the need of updating the costing
> infrastructure as and when new h/w reduces the cost(s) of existing
> instructions.
> - Dibyendu
> 
> -----Original Message-----
> From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu]
> On Behalf Of Nadav Rotem
> Sent: Friday, October 05, 2012 11:45 AM
> To: llvmdev at cs.uiuc.edu Mailing List
> Subject: [LLVMdev] LLVM Loop Vectorizer
> 
> Hi,
> 
> We are starting to work on an LLVM loop vectorizer. There's number of
> different projects that already vectorize LLVM IR. For example Hal's
> BB-Vectorizer, Intel's OpenCL Vectorizer, Polly, ISPC, AnySL, just to
> name a few. I think that it would be great if we could collaborate on
> the areas that are shared between the different projects. I think that
> refactoring LLVM in away that would expose target information to IR-
> level transformations would be a good way to start. Vectorizers, as
> well as other IR-level transformations, require target-specific
> information, such as the cost of different instruction or the
> availability of certain features. Currently, llvm-based vectorizers do
> not make use of this information, or just hard-code target information.
> A loop vectorizer would need target information. After we have some
> basic target information infrastructure in place we can start
> discussing the vectorizer itself.
> 
> I think that the first step would be to expose Target Lowering
> Interface (TLI) to OPT's IR-level passes. Currently TLI is only
> available in LLC. I suggest that we merge LLC and OPT into a single
> tool that will drive both IR-level passes and the codegen. LLC and OPT
> can remain as wrappers around the new tool. Please let me know if you
> can think of a good name for the new tool. I was thinking that "llvm-
> cli" may be a good name (cli = command line interface). OPT and LLC are
> only used by LLVM developers, so the impact of this change on the user
> community would be small.
> 
> Thanks,
> Nadav
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