[PATCH] D24338: [InlineCost] Remove CallPenalty and change MinSizeThreshold to 5

Chandler Carruth via llvm-commits llvm-commits at lists.llvm.org
Fri Nov 4 11:07:28 PDT 2016

chandlerc added inline comments.

Comment at: include/llvm/Analysis/TargetTransformInfo.h:188
+  /// and InlineCost changed to delegate to getCallCost.
+  int getInliningCallPenalty() const;
davidxl wrote:
> I think it is a mistake to conflate two different things here. One is the penalty of 'not' inlining a callsite (aka the benefit of inlining it). This penalty models the branch(call)/return, function prologue and epilogue etc. It can also model the size impact of call instruction. The other is the overhead of a newly exposed callsite from the inline instance. The latter models the potential caller save register spills etc.  The former is the one that needs to defined in TTI.  For the latter, I think it should stay in Inliner proper. In the future, it can be replaced by target dependent register pressure analysis.  In other words, this interface should be getCallPenalty().  
Sorry I'm a bit late to the thread, but I'm confused by this and some of the other comments.

There are definitely two concepts here: the cost of having a call in the instruction stream, and the expected "benefit" of inlining. But I don't think `CallPenalty` is really being used to model the latter concept these days. The *threshold* is what models the latter concept.

So originally, in early 2010, we started using `CallPenalty` scaled by the number of arguments to track the the "caller cost of having a call". Then later in 2010, we replaced this usage of `CallPenalty` with a new and completely unrelated usage to model the fact that "big" functions are "slow". Even though it was applied to a threshold that is primarily size based. Then a few years later we *completely changed how we even think about inline cost*. Since then, the idea of functions being either "big" or "slow" and modeling that with `CallPenalty`, IMO, no longer makes sense in the inline cost analysis *at all*. So I suspect that the current usage of `CallPenalty` is just wrong. If it is doing anything, it is augmenting the basic call cost to fix a failure for it to account for caller (size) cost of *having* a call instruction.

My only real concern with adding an API is understanding why we can't just make this what the existing `getCallCost` do exactly this. In fact, it already does most of this. Is there just some way we can adjust it to allow us to transition from `CallPenalty` to `getCallCost` to model the caller-side cost? If the name is too confusing, we could rename it `getCallerCallCost` or some such...

However, I would *not* merge `getInliningThresholdMultiplier` into this. That is the target's tool to adjust the *threshold* which models the other thing you are talking about David. I agree they should remain separate. If we need more granular control of the threshold in a target, we should just add an API that has that granularity (with an understanding of why it is needed).

And I still think it is reasonable to factor the caller cost *into* the threshold (after it is scaled), as it does model one aspect of the "benefit" of inlining. The threshold itself models the rest, and the multiplier can scale it appropriately.

Comment at: lib/Analysis/InlineCost.cpp:110
   int VectorBonus;
+  int CallPenalty;
davidxl wrote:
> This is a good parameter to be added to InlineParams struct
See above, but as a consequence I disagree. I think this is just a cost modeling issue, and not a threshold issue. The InlineParams should be concerned with setting up the threshold to model the "benefit".

Comment at: lib/Analysis/InlineCost.cpp:1209
+  else
+    CallPenalty = (TTI.getInliningCallPenalty() * InlineConstants::InstrCost) /
+                  TargetTransformInfo::TCC_Basic;
davidxl wrote:
> Why not directly using the TTI returned value?
Agreed. I think this will become much more straight-forward when it is expressed as a cost and thus can use normal cost values to configure itself.



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