[LLVMdev] Instruction Cost

[Jingyue Wu]

CostModule::getInstructionCost also consults TTI (
http://llvm.org/docs/doxygen/html/CostModel_8cpp_source.html#l00380). No?

Jingyue

On Wed, Jan 14, 2015 at 4:05 PM, Chandler Carruth <chandlerc at google.com>
wrote:

>
> On Wed, Jan 14, 2015 at 3:54 PM, Jingyue Wu <jingyue at google.com> wrote:
>
>> I'm looking for APIs that compute instruction costs, and noticed several
>> of them.
>>
>> 1. A series of APIs of TargetTransformInfo that compute the cost of
>> instructions of a particular type (e.g. getArithmeticInstrCost and
>> getShuffleCost)
>> 2. TargetTransformInfo::getOperationCost
>> 3. CostModel::getInstructionCost::getInstructionCost in
>> lib/Analysis/CostModel.cpp
>>
>> Only the first one is used extensively in LLVM's code base, but the
>> second and third one seems more recently added and more general, making me
>> think if they are designed to replace the first approach.
>>
>> So, what's the recommended way to compute instruction costs?
>>
>
> It depends on what for...
>
> #3 above is a *very* coarse, but target independent heuristic. It's used
> for basic things that don't need a lot of detail at all.
>
> #1 and #2 are both in TTI and useful to target-specific IR transforms such
> as the vectorizers and the partial unroller. I would expect #2 to be
> implemented in terms of #1 and generally just a wrapper that provides a
> generic interface.
>
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