SIL: Add a PartialApplySimplification pass.

This will turn `partial_apply` instructions into explicit box construction and
extraction code sequences. To begin with, recognize when a private function
is only used in partial applications and directly modify the function to be
usable as a closure invocation function. This simplifies the lowering in IRGen
and avoids generating a "partial application forwarder" thunk.

Author: jckarter

include/swift/SILOptimizer/PassManager/Passes.def

@@ -446,6 +446,8 @@ PASS(MoveFunctionCanonicalization, "sil-move-function-canon",
      "function checking.")
 PASS(DebugInfoCanonicalizer, "sil-onone-debuginfo-canonicalizer",
      "Canonicalize debug info at -Onone by propagating debug info into coroutine funclets")
+PASS(PartialApplySimplification, "partial-apply-simplification",
+     "Transform partial_apply instructions into explicit closure box constructions")
 PASS(PruneVTables, "prune-vtables",
      "Mark class methods that do not require vtable dispatch")
 PASS_RANGE(AllPasses, AADumper, PruneVTables)

lib/SILOptimizer/PassManager/PassPipeline.cpp

@@ -825,9 +825,16 @@ SILPassPipelinePlan::getIRGenPreparePassPipeline(const SILOptions &Options) {
   SILPassPipelinePlan P(Options);
   P.startPipeline("IRGen Preparation");
   // Insert SIL passes to run during IRGen.
+  /*
+  // Simplify partial_apply instructions by expanding box construction into
+  // component operations.
+  P.addPartialApplySimplification();
+   */
   // Hoist generic alloc_stack instructions to the entry block to enable better
   // llvm-ir generation for dynamic alloca instructions.
   P.addAllocStackHoisting();
+  // Change large loadable types to be passed indirectly across function
+  // boundaries as required by the ABI.
   P.addLoadableByAddress();
 
   return P;

lib/SILOptimizer/Transforms/CMakeLists.txt

@@ -25,6 +25,7 @@ target_sources(swiftSILOptimizer PRIVATE
   Outliner.cpp
   ObjectOutliner.cpp
   AssemblyVisionRemarkGenerator.cpp
+  PartialApplySimplification.cpp
   PerformanceInliner.cpp
   PhiArgumentOptimizations.cpp
   PruneVTables.cpp