Make FSO thunks always_inline.

This forces the callsites to be rewritten by the inliner.

we have the issue that the thunk changes from the time the its created to
the time its reread to figure out what we have done to the original function

This results in missed opportunities.

This solution solves the problem gracefully, because the thunk carries the information
on how to set up the call to the optimized functions.

Inlining the thunk makes the callsite calling the optimized function for free. i.e.
without any rewriting.

I did not measure any regression with this change.

Author: trentxintong

lib/SILOptimizer/Analysis/FunctionSignatureAnalysis.cpp

@@ -252,8 +252,6 @@ std::string FunctionSignatureInfo::getOptimizedName() const {
   auto P = SpecializationPass::FunctionSignatureOpts;
   FunctionSignatureSpecializationMangler FSSM(P, M, F);
 
-  std::string ArgEnc;
-
   // Handle arguments' changes.
   for (unsigned i : indices(ArgDescList)) {
     const ArgumentDescriptor &Arg = ArgDescList[i];
@@ -271,9 +269,6 @@ std::string FunctionSignatureInfo::getOptimizedName() const {
     // mangling.
     if (Arg.Explode && !Arg.IsEntirelyDead) {
       FSSM.setArgumentSROA(i);
-      // Generate a string of encoding for the argument projection tree.
-      // TODO: we can put this into the mangler itself.
-      ArgEnc += "_arg" + std::to_string(i) + "_" + Arg.ProjTree.getNameEncoding();
     }
   }
 
@@ -285,7 +280,7 @@ std::string FunctionSignatureInfo::getOptimizedName() const {
 
   FSSM.mangle();
 
-  return M.finalize() + ArgEnc;
+  return M.finalize();
 }
 
 

lib/SILOptimizer/PassManager/Passes.cpp

@@ -340,11 +340,6 @@ void swift::runSILOptimizationPasses(SILModule &Module) {
 
   PM.setStageName("LowLevel");
 
-  // Rewrite to get the benefit of release devirtualizer.
-  // Also, Make sure the run the rewriter to create the optimized functions before
-  // the cloner on the current function is run !.
-  PM.addFunctionSignatureOptRewriter();
-
   // Should be after FunctionSignatureOpts and before the last inliner.
   PM.addReleaseDevirtualizer();
 

lib/SILOptimizer/Transforms/FunctionSignatureOptCloner.cpp

@@ -566,7 +566,13 @@ class FunctionSignatureOptCloner : public SILFunctionTransform {
                  createOptimizedFunction(RCIA->get(F), FSA->get(F), AA, F);
 
     if (NewF) { 
-      // Make sure the PM knows about this function.
+      // The thunk now carries the information on how the signature is
+      // optimized. If we inline the thunk, we will get the benefit of calling
+      // the signature optimized function without additional setup on the
+      // caller side.
+      F->setInlineStrategy(AlwaysInline);
+      // Make sure the PM knows about this function. This will also help us
+      // with self-recursion.
       notifyPassManagerOfFunction(NewF);
       invalidateAnalysis(SILAnalysis::InvalidationKind::Everything);
     }

test/SILOptimizer/devirt_default_case.swift

@@ -109,7 +109,7 @@ func foo(a: A3) -> Int {
 // Check that call to A3.f() can be devirtualized.
 //
 // CHECK-NORMAL: sil{{( hidden)?}} [noinline] @_TF19devirt_default_case3fooFCS_2A3Si
-// CHECK-TESTABLE: sil{{( hidden)?}} [thunk] [noinline] @_TF19devirt_default_case3fooFCS_2A3Si
+// CHECK-TESTABLE: sil{{( hidden)?}} [thunk] [always_inline] @_TF19devirt_default_case3fooFCS_2A3Si
 // CHECK-NORMAL: function_ref @{{.*}}TFC19devirt_default_case2B31f
 // CHECK-NORMAL: function_ref @{{.*}}TFC19devirt_default_case2A31f
 // CHECK-NORMAL-NOT: class_method