Index: include/llvm/Transforms/IPO/Inliner.h
===================================================================
--- include/llvm/Transforms/IPO/Inliner.h
+++ include/llvm/Transforms/IPO/Inliner.h
@@ -96,6 +96,9 @@
   InlinerPass(InlineParams Params = getInlineParams())
       : Params(std::move(Params)) {}
 
+  InlinerPass(unsigned OptLevel, unsigned SizeLevel)
+      : InlinerPass(getInlineParams(OptLevel, SizeLevel)) {}
+
   PreservedAnalyses run(LazyCallGraph::SCC &C, CGSCCAnalysisManager &AM,
                         LazyCallGraph &CG, CGSCCUpdateResult &UR);
 
Index: lib/Passes/PassBuilder.cpp
===================================================================
--- lib/Passes/PassBuilder.cpp
+++ lib/Passes/PassBuilder.cpp
@@ -553,8 +553,9 @@
   // Run the inliner first. The theory is that we are walking bottom-up and so
   // the callees have already been fully optimized, and we want to inline them
   // into the callers so that our optimizations can reflect that.
-  // FIXME; Customize the threshold based on optimization level.
-  MainCGPipeline.addPass(InlinerPass());
+  unsigned OptLevel = Level > O3 ? 2 : Level;
+  unsigned SizeLevel = Level > O3 ? Level - O3 : 0;
+  MainCGPipeline.addPass(InlinerPass(OptLevel, SizeLevel));
 
   // Now deduce any function attributes based in the current code.
   MainCGPipeline.addPass(PostOrderFunctionAttrsPass());
@@ -863,7 +864,10 @@
   // valuable as the inliner doesn't currently care whether it is inlining an
   // invoke or a call.
   // Run the inliner now.
-  MPM.addPass(createModuleToPostOrderCGSCCPassAdaptor(InlinerPass()));
+  unsigned OptLevel = Level > O3 ? 2 : Level;
+  unsigned SizeLevel = Level > O3 ? Level - O3 : 0;
+  MPM.addPass(createModuleToPostOrderCGSCCPassAdaptor(
+      InlinerPass(OptLevel, SizeLevel)));
 
   // Optimize globals again after we ran the inliner.
   MPM.addPass(GlobalOptPass());