[ADT] Reduce copies and recursion in StringSwitch (#125362)

Optimize the `.Cases` and `.CasesLower` functions to avoid needlessly
recursing on each case and copying the associated values. We can instead
take `Value` by reference and short-circuit by using the `||` operator.

Note that while the implementation uses variadic templates, we cannot
simplify the public functions in the same way. This is because the
current API forces the arguments to be converted to `StringLiterals` and
places the `Value` parameter at the very end. Even if we did some tricks
like split the parameter pack to separate out the `Value`, I do not see
how we could force conversion to `StringLiteral`.

Author: kuhar

llvm/include/llvm/ADT/StringSwitch.h

@@ -14,7 +14,6 @@
 #define LLVM_ADT_STRINGSWITCH_H
 
 #include "llvm/ADT/StringRef.h"
-#include "llvm/Support/Compiler.h"
 #include <cassert>
 #include <cstring>
 #include <optional>
@@ -67,9 +66,7 @@ class StringSwitch {
 
   // Case-sensitive case matchers
   StringSwitch &Case(StringLiteral S, T Value) {
-    if (!Result && Str == S) {
-      Result = std::move(Value);
-    }
+    CaseImpl(Value, S);
     return *this;
   }
 
@@ -88,61 +85,59 @@ class StringSwitch {
   }
 
   StringSwitch &Cases(StringLiteral S0, StringLiteral S1, T Value) {
-    return Case(S0, Value).Case(S1, Value);
+    return CasesImpl(Value, S0, S1);
   }
 
   StringSwitch &Cases(StringLiteral S0, StringLiteral S1, StringLiteral S2,
                       T Value) {
-    return Case(S0, Value).Cases(S1, S2, Value);
+    return CasesImpl(Value, S0, S1, S2);
   }
 
   StringSwitch &Cases(StringLiteral S0, StringLiteral S1, StringLiteral S2,
                       StringLiteral S3, T Value) {
-    return Case(S0, Value).Cases(S1, S2, S3, Value);
+    return CasesImpl(Value, S0, S1, S2, S3);
   }
 
   StringSwitch &Cases(StringLiteral S0, StringLiteral S1, StringLiteral S2,
                       StringLiteral S3, StringLiteral S4, T Value) {
-    return Case(S0, Value).Cases(S1, S2, S3, S4, Value);
+    return CasesImpl(Value, S0, S1, S2, S3, S4);
   }
 
   StringSwitch &Cases(StringLiteral S0, StringLiteral S1, StringLiteral S2,
                       StringLiteral S3, StringLiteral S4, StringLiteral S5,
                       T Value) {
-    return Case(S0, Value).Cases(S1, S2, S3, S4, S5, Value);
+    return CasesImpl(Value, S0, S1, S2, S3, S4, S5);
   }
 
   StringSwitch &Cases(StringLiteral S0, StringLiteral S1, StringLiteral S2,
                       StringLiteral S3, StringLiteral S4, StringLiteral S5,
                       StringLiteral S6, T Value) {
-    return Case(S0, Value).Cases(S1, S2, S3, S4, S5, S6, Value);
+    return CasesImpl(Value, S0, S1, S2, S3, S4, S5, S6);
   }
 
   StringSwitch &Cases(StringLiteral S0, StringLiteral S1, StringLiteral S2,
                       StringLiteral S3, StringLiteral S4, StringLiteral S5,
                       StringLiteral S6, StringLiteral S7, T Value) {
-    return Case(S0, Value).Cases(S1, S2, S3, S4, S5, S6, S7, Value);
+    return CasesImpl(Value, S0, S1, S2, S3, S4, S5, S6, S7);
   }
 
   StringSwitch &Cases(StringLiteral S0, StringLiteral S1, StringLiteral S2,
                       StringLiteral S3, StringLiteral S4, StringLiteral S5,
                       StringLiteral S6, StringLiteral S7, StringLiteral S8,
                       T Value) {
-    return Case(S0, Value).Cases(S1, S2, S3, S4, S5, S6, S7, S8, Value);
+    return CasesImpl(Value, S0, S1, S2, S3, S4, S5, S6, S7, S8);
   }
 
   StringSwitch &Cases(StringLiteral S0, StringLiteral S1, StringLiteral S2,
                       StringLiteral S3, StringLiteral S4, StringLiteral S5,
                       StringLiteral S6, StringLiteral S7, StringLiteral S8,
                       StringLiteral S9, T Value) {
-    return Case(S0, Value).Cases(S1, S2, S3, S4, S5, S6, S7, S8, S9, Value);
+    return CasesImpl(Value, S0, S1, S2, S3, S4, S5, S6, S7, S8, S9);
   }
 
   // Case-insensitive case matchers.
   StringSwitch &CaseLower(StringLiteral S, T Value) {
-    if (!Result && Str.equals_insensitive(S))
-      Result = std::move(Value);
-
+    CaseLowerImpl(Value, S);
     return *this;
   }
 
@@ -161,22 +156,22 @@ class StringSwitch {
   }
 
   StringSwitch &CasesLower(StringLiteral S0, StringLiteral S1, T Value) {
-    return CaseLower(S0, Value).CaseLower(S1, Value);
+    return CasesLowerImpl(Value, S0, S1);
   }
 
   StringSwitch &CasesLower(StringLiteral S0, StringLiteral S1, StringLiteral S2,
                            T Value) {
-    return CaseLower(S0, Value).CasesLower(S1, S2, Value);
+    return CasesLowerImpl(Value, S0, S1, S2);
   }
 
   StringSwitch &CasesLower(StringLiteral S0, StringLiteral S1, StringLiteral S2,
                            StringLiteral S3, T Value) {
-    return CaseLower(S0, Value).CasesLower(S1, S2, S3, Value);
+    return CasesLowerImpl(Value, S0, S1, S2, S3);
   }
 
   StringSwitch &CasesLower(StringLiteral S0, StringLiteral S1, StringLiteral S2,
                            StringLiteral S3, StringLiteral S4, T Value) {
-    return CaseLower(S0, Value).CasesLower(S1, S2, S3, S4, Value);
+    return CasesLowerImpl(Value, S0, S1, S2, S3, S4);
   }
 
   [[nodiscard]] R Default(T Value) {
@@ -189,6 +184,39 @@ class StringSwitch {
     assert(Result && "Fell off the end of a string-switch");
     return std::move(*Result);
   }
+
+private:
+  // Returns true when `Str` matches the `S` argument, and stores the result.
+  bool CaseImpl(T &Value, StringLiteral S) {
+    if (!Result && Str == S) {
+      Result = std::move(Value);
+      return true;
+    }
+    return false;
+  }
+
+  // Returns true when `Str` matches the `S` argument (case-insensitive), and
+  // stores the result.
+  bool CaseLowerImpl(T &Value, StringLiteral S) {
+    if (!Result && Str.equals_insensitive(S)) {
+      Result = std::move(Value);
+      return true;
+    }
+    return false;
+  }
+
+  template <typename... Args> StringSwitch &CasesImpl(T &Value, Args... Cases) {
+    // Stop matching after the string is found.
+    (... || CaseImpl(Value, Cases));
+    return *this;
+  }
+
+  template <typename... Args>
+  StringSwitch &CasesLowerImpl(T &Value, Args... Cases) {
+    // Stop matching after the string is found.
+    (... || CaseLowerImpl(Value, Cases));
+    return *this;
+  }
 };
 
 } // end namespace llvm

llvm/unittests/ADT/StringSwitchTest.cpp

@@ -205,3 +205,28 @@ TEST(StringSwitchTest, CasesLower) {
   EXPECT_EQ(OSType::Unknown, Translate("wind"));
   EXPECT_EQ(OSType::Unknown, Translate(""));
 }
+
+TEST(StringSwitchTest, CasesCopies) {
+  struct Copyable {
+    unsigned &NumCopies;
+    Copyable(unsigned &Value) : NumCopies(Value) {}
+    Copyable(const Copyable &Other) : NumCopies(Other.NumCopies) {
+      ++NumCopies;
+    }
+    Copyable &operator=(const Copyable &Other) {
+      ++NumCopies;
+      return *this;
+    }
+  };
+
+  // Check that evaluating multiple cases does not cause unnecessary copies.
+  unsigned NumCopies = 0;
+  llvm::StringSwitch<Copyable, void>("baz").Cases("foo", "bar", "baz", "qux",
+                                                  Copyable{NumCopies});
+  EXPECT_EQ(NumCopies, 1u);
+
+  NumCopies = 0;
+  llvm::StringSwitch<Copyable, void>("baz").CasesLower(
+      "Foo", "Bar", "Baz", "Qux", Copyable{NumCopies});
+  EXPECT_EQ(NumCopies, 1u);
+}