SIL optimizer: Add a new string optimization.

Optimizes String operations with constant operands.

Specifically:
  * Replaces x.append(y) with x = y if x is empty.
  * Removes x.append("")
  * Replaces x.append(y) with x = x + y if x and y are constant strings.
  * Replaces _typeName(T.self) with a constant string if T is statically known.

With this optimization it's possible to constant fold string interpolations, like "the \(Int.self) type" -> "the Int type"

This new pass runs on high-level SIL, where semantic calls are still in place.

rdar://problem/65642843

Author: eeckstein

include/swift/AST/ASTContext.h

@@ -553,6 +553,9 @@ class ASTContext final {
   /// Array.reserveCapacityForAppend(newElementsCount: Int)
   FuncDecl *getArrayReserveCapacityDecl() const;
 
+  /// Retrieve the declaration of String.init(_builtinStringLiteral ...)
+  ConstructorDecl *getMakeUTF8StringDecl() const;
+
   // Retrieve the declaration of Swift._stdlib_isOSVersionAtLeast.
   FuncDecl *getIsOSVersionAtLeastDecl() const;
 

include/swift/AST/SemanticAttrs.def

@@ -30,6 +30,7 @@ SEMANTICS_ATTR(STRING_ESCAPE_PERCENT_GET, "string.escapePercent.get")
 SEMANTICS_ATTR(STRING_CONCAT, "string.concat")
 SEMANTICS_ATTR(STRING_APPEND, "string.append")
 SEMANTICS_ATTR(STRING_INIT_EMPTY, "string.init_empty")
+SEMANTICS_ATTR(STRING_INIT_EMPTY_WITH_CAPACITY, "string.init_empty_with_capacity")
 SEMANTICS_ATTR(STRING_PLUS_EQUALS, "string.plusequals")
 SEMANTICS_ATTR(FIND_STRING_SWITCH_CASE, "findStringSwitchCase")
 SEMANTICS_ATTR(FIND_STRING_SWITCH_CASE_WITH_CACHE, "findStringSwitchCaseWithCache")
@@ -63,6 +64,7 @@ SEMANTICS_ATTR(ARRAY_UNINITIALIZED_INTRINSIC, "array.uninitialized_intrinsic")
 SEMANTICS_ATTR(ARRAY_FINALIZE_INTRINSIC, "array.finalize_intrinsic")
 
 SEMANTICS_ATTR(SEQUENCE_FOR_EACH, "sequence.forEach")
+SEMANTICS_ATTR(TYPENAME, "typeName")
 
 SEMANTICS_ATTR(OPTIMIZE_SIL_SPECIALIZE_GENERIC_NEVER, "optimize.sil.specialize.generic.never")
 SEMANTICS_ATTR(OPTIMIZE_SIL_SPECIALIZE_GENERIC_PARTIAL_NEVER,

include/swift/SILOptimizer/PassManager/Passes.def

@@ -317,6 +317,8 @@ PASS(StackPromotion, "stack-promotion",
      "Stack Promotion of Class Objects")
 PASS(StripDebugInfo, "strip-debug-info",
      "Strip Debug Information")
+PASS(StringOptimization, "string-optimization",
+     "Optimization for String operations")
 PASS(SwiftArrayPropertyOpt, "array-property-opt",
      "Loop Specialization for Array Properties")
 PASS(UnsafeGuaranteedPeephole, "unsafe-guaranteed-peephole",

lib/AST/ASTContext.cpp

@@ -40,6 +40,7 @@
 #include "swift/AST/PropertyWrappers.h"
 #include "swift/AST/ProtocolConformance.h"
 #include "swift/AST/RawComment.h"
+#include "swift/AST/SemanticAttrs.h"
 #include "swift/AST/SourceFile.h"
 #include "swift/AST/SubstitutionMap.h"
 #include "swift/AST/SILLayout.h"
@@ -240,6 +241,9 @@ struct ASTContext::Implementation {
   /// func append(Element) -> void
   FuncDecl *ArrayAppendElementDecl = nullptr;
 
+  /// init(Builtin.RawPointer, Builtin.Word, Builtin.Int1)
+  ConstructorDecl *MakeUTF8StringDecl = nullptr;
+
   /// func reserveCapacityForAppend(newElementsCount: Int)
   FuncDecl *ArrayReserveCapacityDecl = nullptr;
 
@@ -1247,6 +1251,33 @@ FuncDecl *ASTContext::getArrayReserveCapacityDecl() const {
   return nullptr;
 }
 
+ConstructorDecl *ASTContext::getMakeUTF8StringDecl() const {
+  if (getImpl().MakeUTF8StringDecl)
+    return getImpl().MakeUTF8StringDecl;
+
+  auto initializers =
+    getStringDecl()->lookupDirect(DeclBaseName::createConstructor());
+
+  for (Decl *initializer : initializers) {
+    auto *constructor = cast<ConstructorDecl>(initializer);
+    auto Attrs = constructor->getAttrs();
+    for (auto *A : Attrs.getAttributes<SemanticsAttr, false>()) {
+      if (A->Value != semantics::STRING_MAKE_UTF8)
+        continue;
+      auto ParamList = constructor->getParameters();
+      if (ParamList->size() != 3)
+        continue;
+      ParamDecl *param = constructor->getParameters()->get(0);
+      if (param->getArgumentName().str() != "_builtinStringLiteral")
+        continue;
+
+      getImpl().MakeUTF8StringDecl = constructor;
+      return constructor;
+    }
+  }
+  return nullptr;
+}
+
 FuncDecl *ASTContext::getIsOSVersionAtLeastDecl() const {
   if (getImpl().IsOSVersionAtLeastDecl)
     return getImpl().IsOSVersionAtLeastDecl;

lib/SILOptimizer/PassManager/PassPipeline.cpp

@@ -524,6 +524,9 @@ static void addSerializePipeline(SILPassPipelinePlan &P) {
 
 static void addMidLevelFunctionPipeline(SILPassPipelinePlan &P) {
   P.startPipeline("MidLevel,Function", true /*isFunctionPassPipeline*/);
+
+  P.addStringOptimization();
+
   addFunctionPasses(P, OptimizationLevelKind::MidLevel);
 
   // Specialize partially applied functions with dead arguments as a preparation

lib/SILOptimizer/Transforms/CMakeLists.txt

@@ -39,6 +39,7 @@ target_sources(swiftSILOptimizer PRIVATE
   Sink.cpp
   SpeculativeDevirtualizer.cpp
   StackPromotion.cpp
+  StringOptimization.cpp
   TempLValueOpt.cpp
   TempRValueElimination.cpp
   UnsafeGuaranteedPeephole.cpp)