[RFC][mlir] Add profitability callback to the Inliner.

[vzakhari]

Discussion at https://discourse.llvm.org/t/inliner-cost-model/2992

This change adds a callback that reports whether inlining
of the particular call site (communicated via ResolvedCall argument)
is profitable or not. The default MLIR inliner pass behavior
is unchanged, i.e. the callback always returns true.
This callback may be used to customize the inliner behavior
based on the target specifics (like target instructions costs),
profitability of the inlining for further optimizations
(e.g. if inlining may enable loop optimizations or scalar optimizations
due to object shape propagation), optimization levels (e.g. -Os inlining
may be quite different from -Ofast inlining), etc.

One of the questions is whether the ResolvedCall entity represents
enough of the context for the custom inlining models to come up with
the profitability decision. I think we can start with this and
extend it as necessary.

[llvmbot]

@llvm/pr-subscribers-mlir-core

@llvm/pr-subscribers-mlir

Author: Slava Zakharin (vzakhari)

Changes

Discussion at https://discourse.llvm.org/t/inliner-cost-model/2992

This change adds a callback that reports whether inlining
of the particular call site (communicated via ResolvedCall argument)
is profitable or not. The default MLIR inliner pass behavior
is unchanged, i.e. the callback always returns true.
This callback may be used to customize the inliner behavior
based on the target specifics (like target instructions costs),
profitability of the inlining for further optimizations
(e.g. if inlining may enable loop optimizations or scalar optimizations
due to object shape propagation), optimization levels (e.g. -Os inlining
may be quite different from -Ofast inlining), etc.

One of the questions is whether the ResolvedCall entity represents
enough of the context for the custom inlining models to come up with
the profitability decision. I think we can start with this and
extend it as necessary.

---

Full diff: https://github.com/llvm/llvm-project/pull/84258.diff

3 Files Affected:

• (modified) mlir/include/mlir/Transforms/Inliner.h (+26-17)
• (modified) mlir/lib/Transforms/InlinerPass.cpp (+4-1)
• (modified) mlir/lib/Transforms/Utils/Inliner.cpp (+3)

diff --git a/mlir/include/mlir/Transforms/Inliner.h b/mlir/include/mlir/Transforms/Inliner.h
index 1fe61fb4bbe7d9..09b06f7c38cf55 100644
--- a/mlir/include/mlir/Transforms/Inliner.h
+++ b/mlir/include/mlir/Transforms/Inliner.h
@@ -69,19 +69,6 @@ class InlinerConfig {
 /// of inlining decisions from the leafs to the roots of the callgraph.
 class Inliner {
 public:
-  using RunPipelineHelperTy = std::function<LogicalResult(
-      Pass &pass, OpPassManager &pipeline, Operation *op)>;
-
-  Inliner(Operation *op, CallGraph &cg, Pass &pass, AnalysisManager am,
-          RunPipelineHelperTy runPipelineHelper, const InlinerConfig &config)
-      : op(op), cg(cg), pass(pass), am(am),
-        runPipelineHelper(std::move(runPipelineHelper)), config(config) {}
-  Inliner(Inliner &) = delete;
-  void operator=(const Inliner &) = delete;
-
-  /// Perform inlining on a OpTrait::SymbolTable operation.
-  LogicalResult doInlining();
-
   /// This struct represents a resolved call to a given callgraph node. Given
   /// that the call does not actually contain a direct reference to the
   /// Region(CallGraphNode) that it is dispatching to, we need to resolve them
@@ -94,7 +81,29 @@ class Inliner {
     CallGraphNode *sourceNode, *targetNode;
   };
 
-protected:
+  using RunPipelineHelperTy = std::function<LogicalResult(
+      Pass &pass, OpPassManager &pipeline, Operation *op)>;
+
+  /// Type of the callback answering if it is profitable
+  /// to inline a callable operation at a call site.
+  /// It might be the case that the ResolvedCall does not provide
+  /// enough context to make the profitability decision, so
+  /// this hook's interface might need to be extended in future.
+  using ProfitabilityCallbackTy = std::function<bool(ResolvedCall &)>;
+
+  Inliner(Operation *op, CallGraph &cg, Pass &pass, AnalysisManager am,
+          RunPipelineHelperTy runPipelineHelper, const InlinerConfig &config,
+          ProfitabilityCallbackTy isProfitableToInline)
+      : op(op), cg(cg), pass(pass), am(am),
+        runPipelineHelper(std::move(runPipelineHelper)), config(config),
+        isProfitableToInline(std::move(isProfitableToInline)) {}
+  Inliner(Inliner &) = delete;
+  void operator=(const Inliner &) = delete;
+
+  /// Perform inlining on a OpTrait::SymbolTable operation.
+  LogicalResult doInlining();
+
+private:
   /// An OpTrait::SymbolTable operation to run the inlining on.
   Operation *op;
   /// A CallGraph analysis for the given operation.
@@ -108,12 +117,12 @@ class Inliner {
   const RunPipelineHelperTy runPipelineHelper;
   /// The inliner configuration parameters.
   const InlinerConfig &config;
+  /// Returns true, if it is profitable to inline the callable operation
+  /// at the call site.
+  ProfitabilityCallbackTy isProfitableToInline;
 
-private:
   /// Forward declaration of the class providing the actual implementation.
   class Impl;
-
-public:
 };
 } // namespace mlir
 
diff --git a/mlir/lib/Transforms/InlinerPass.cpp b/mlir/lib/Transforms/InlinerPass.cpp
index c058e8050cd199..faa9a1aef9d353 100644
--- a/mlir/lib/Transforms/InlinerPass.cpp
+++ b/mlir/lib/Transforms/InlinerPass.cpp
@@ -100,9 +100,12 @@ void InlinerPass::runOnOperation() {
     return signalPassFailure();
   }
 
+  // By default, assume that any inlining is profitable.
+  auto profitabilityCb = [](Inliner::ResolvedCall &) { return true; };
+
   // Get an instance of the inliner.
   Inliner inliner(op, cg, *this, getAnalysisManager(), runPipelineHelper,
-                  config);
+                  config, profitabilityCb);
 
   // Run the inlining.
   if (failed(inliner.doInlining()))
diff --git a/mlir/lib/Transforms/Utils/Inliner.cpp b/mlir/lib/Transforms/Utils/Inliner.cpp
index 74776a73db9aaa..c24eff7353f6b9 100644
--- a/mlir/lib/Transforms/Utils/Inliner.cpp
+++ b/mlir/lib/Transforms/Utils/Inliner.cpp
@@ -733,6 +733,9 @@ bool Inliner::Impl::shouldInline(ResolvedCall &resolvedCall) {
   if (calleeHasMultipleBlocks && !callerRegionSupportsMultipleBlocks())
     return false;
 
+  if (!inliner.isProfitableToInline(resolvedCall))
+    return false;
+
   // Otherwise, inline.
   return true;
 }

[vzakhari]

Friendly ping.

[joker-eph]

LG, with some nits.

[joker-eph]

We don't test debug output, just remove this check and everything looks good otherwise.