Sema: Add -solver-disable-splitter flag for debugging

We're not planning on removing the splitter because it is a big win
in some cases, but we want to run it less often since it can also
be a source of overhead. This flag allows us to compare performance
to understand the tradeoffs better.

Author: slavapestov

include/swift/Basic/LangOptions.h

@@ -913,13 +913,6 @@ namespace swift {
     /// is for testing purposes.
     std::vector<std::string> DebugForbidTypecheckPrefixes;
 
-    /// The upper bound to number of sub-expressions unsolved
-    /// before termination of the shrink phrase of the constraint solver.
-    unsigned SolverShrinkUnsolvedThreshold = 10;
-
-    /// Disable the shrink phase of the expression type checker.
-    bool SolverDisableShrink = false;
-
     /// Enable experimental operator designated types feature.
     bool EnableOperatorDesignatedTypes = false;
 
@@ -935,6 +928,9 @@ namespace swift {
     /// Allow request evalutation to perform type checking lazily, instead of
     /// eagerly typechecking source files after parsing.
     bool EnableLazyTypecheck = false;
+
+    /// Disable the component splitter phase of the expression type checker.
+    bool SolverDisableSplitter = false;
   };
 
   /// Options for controlling the behavior of the Clang importer.

include/swift/Option/FrontendOptions.td

@@ -825,15 +825,17 @@ def downgrade_typecheck_interface_error : Flag<["-"], "downgrade-typecheck-inter
 def enable_volatile_modules : Flag<["-"], "enable-volatile-modules">,
   HelpText<"Load Swift modules in memory">;
 
-def solver_expression_time_threshold_EQ : Joined<["-"], "solver-expression-time-threshold=">;
+def solver_expression_time_threshold_EQ : Joined<["-"], "solver-expression-time-threshold=">,
+  HelpText<"Expression type checking timeout, in seconds">;
 
-def solver_scope_threshold_EQ : Joined<["-"], "solver-scope-threshold=">;
+def solver_scope_threshold_EQ : Joined<["-"], "solver-scope-threshold=">,
+  HelpText<"Expression type checking scope limit">;
 
-def solver_trail_threshold_EQ : Joined<["-"], "solver-trail-threshold=">;
+def solver_trail_threshold_EQ : Joined<["-"], "solver-trail-threshold=">,
+  HelpText<"Expression type checking trail change limit">;
 
-def solver_disable_shrink :
-  Flag<["-"], "solver-disable-shrink">,
-  HelpText<"Disable the shrink phase of expression type checking">;
+def solver_disable_splitter : Flag<["-"], "solver-disable-splitter">,
+  HelpText<"Disable the component splitter phase of expression type checking">;
 
 def disable_constraint_solver_performance_hacks : Flag<["-"], "disable-constraint-solver-performance-hacks">,
   HelpText<"Disable all the hacks in the constraint solver">;

lib/Frontend/CompilerInvocation.cpp

@@ -1775,8 +1775,6 @@ static bool ParseTypeCheckerArgs(TypeCheckerOptions &Opts, ArgList &Args,
                              Opts.SolverScopeThreshold);
   setUnsignedIntegerArgument(OPT_solver_trail_threshold_EQ,
                              Opts.SolverTrailThreshold);
-  setUnsignedIntegerArgument(OPT_solver_shrink_unsolved_threshold,
-                             Opts.SolverShrinkUnsolvedThreshold);
 
   Opts.DebugTimeFunctionBodies |= Args.hasArg(OPT_debug_time_function_bodies);
   Opts.DebugTimeExpressions |=
@@ -1865,8 +1863,8 @@ static bool ParseTypeCheckerArgs(TypeCheckerOptions &Opts, ArgList &Args,
     Opts.DebugForbidTypecheckPrefixes.push_back(A);
   }
 
-  if (Args.getLastArg(OPT_solver_disable_shrink))
-    Opts.SolverDisableShrink = true;
+  if (Args.getLastArg(OPT_solver_disable_splitter))
+    Opts.SolverDisableSplitter = true;
 
   if (FrontendOpts.RequestedAction == FrontendOptions::ActionType::Immediate)
     Opts.DeferToRuntime = true;

lib/Sema/CSStep.cpp

@@ -95,6 +95,12 @@ void SplitterStep::computeFollowupSteps(
   // Contract the edges of the constraint graph.
   CG.optimize();
 
+  if (CS.getASTContext().TypeCheckerOpts.SolverDisableSplitter) {
+    steps.push_back(std::make_unique<ComponentStep>(
+          CS, 0, &CS.InactiveConstraints, Solutions));
+    return;
+  }
+
   // Compute the connected components of the constraint graph.
   auto components = CG.computeConnectedComponents(CS.getTypeVariables());
   unsigned numComponents = components.size();

validation-test/Sema/type_checker_perf/fast/array_concatenation.swift

@@ -1,4 +1,4 @@
-// RUN: %target-typecheck-verify-swift -solver-disable-shrink
+// RUN: %target-typecheck-verify-swift
 
 // Self-contained test case
 protocol P1 {}; func f<T: P1>(_: T, _: T) -> T { fatalError() }

validation-test/Sema/type_checker_perf/fast/property_vs_unapplied_func.swift

@@ -1,4 +1,4 @@
-// RUN: %target-typecheck-verify-swift -solver-expression-time-threshold=1 -solver-disable-shrink
+// RUN: %target-typecheck-verify-swift -solver-expression-time-threshold=1
 // REQUIRES: tools-release,no_asan
 
 struct Date {

validation-test/Sema/type_checker_perf/slow/rdar26564101.swift

@@ -1,4 +1,4 @@
-// RUN: %target-typecheck-verify-swift -solver-expression-time-threshold=1 -solver-disable-shrink
+// RUN: %target-typecheck-verify-swift -solver-expression-time-threshold=1
 // REQUIRES: tools-release,no_asan
 // UNSUPPORTED: swift_test_mode_optimize_none && OS=linux-gnu