[llvm-dev] TargetTransformInfo getOperationCost uses

[Hal Finkel via llvm-dev]

Hi Matt, 

So the problem here is that, as you imply, TTI has two cost models: One is used for vectorization (and has received a fair amount of attention), and one is used for inlining/unrolling, and has received less attention. 

The vectorization cost model is, generally speaking, concerned with instruction throughputs, and is used to estimate the relative speed of a vectorized loop vs. the scalar one (it assumes that a proper amount of ILP is available, or created by interleaving, such that the throughputs matter much more than the latencies). 

The "user" code model, used by the inliner and unroller, is concerned with estimating something more-closely related to code size (although there is obviously a desired correlation with performance). It has primarily been customized to let the inliner/unroller, on a target specific basis, understand that certain zext/sexts are free, etc. 

As you also imply. there has been a lot more specific target customization work on the vectorization cost model. To be honest, I don't think this situation is ideal (we could have one cost model that returns information on size, latency and throughput that is used by all clients). Nevertheless, hopefully this makes things somewhat clearer. 

-Hal 

----- Original Message -----

> From: "Matthew via llvm-dev Arsenault" <llvm-dev at lists.llvm.org>
> To: "llvm-dev" <llvm-dev at lists.llvm.org>
> Sent: Tuesday, January 5, 2016 10:25:46 AM
> Subject: [llvm-dev] TargetTransformInfo getOperationCost uses

> Hi,

> I'm trying to implement the TTI hooks for AMDGPU to avoid unrolling
> loops for operations with huge expansions (i.e. integer division).

> The values that are ultimately reported by opt -cost-model -analyze
> (the actual cost model tests) seem to not matter for this. The huge
> cost I've assigned division doesn't prevent the loop from being
> unrolled, because it isn't actually considered when loop unrolling.

> The loop unrolled uses CodeMetrics, which via getUserCost ultimately
> uses TargetTransformInfoImplBase::getOperationCost(), which returns
> various fixed values (4 for division (TCC_Expensive, but this isn't
> nearly expensive enough)).

> getOperationCost only uses the type and opcode to estimate, so it
> doesn't require a value. No target overrides this. The hooks that
> targets do really implement, like getArithmeticInstrCost, use some
> information about the operands so require a value. These don't
> appear to be used at all by the cost model, and instead in specific
> places in some passes.

> Why don't any targets override getOperationCost or why aren't there
> any other hooks that change its behavior? Having these two parallel
> path for operation costs is confusing, especially since I expected
> to be able to use the -cost-model output for testing all of the
> computed costs.

> -Matt
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-- 

Hal Finkel 
Assistant Computational Scientist 
Leadership Computing Facility 
Argonne National Laboratory 
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