Optimize empty string pattern matching with null-safe length checks (#19189)

Copilot · web-flow · commit 37e18beedede · 2026-01-22T22:21:46.000Z
diff --git a/docs/release-notes/.FSharp.Compiler.Service/10.0.200.md b/docs/release-notes/.FSharp.Compiler.Service/10.0.200.md
@@ -5,6 +5,7 @@
 * Type relations cache: optimize key generation ([Issue #19116](https://github.com/dotnet/fsharp/issues/18767)) ([PR #19120](https://github.com/dotnet/fsharp/pull/19120))
 * Fixed QuickParse to correctly handle optional parameter syntax with `?` prefix, resolving syntax highlighting issues. ([Issue #11008753](https://developercommunity.visualstudio.com/t/F-Highlighting-fails-on-optional-parame/11008753)) ([PR #XXXXX](https://github.com/dotnet/fsharp/pull/XXXXX))
 * Fix `--preferreduilang` switch leaking into `fsi.CommandLineArgs` when positioned after script file ([PR #19151](https://github.com/dotnet/fsharp/pull/19151))
+* Optimize empty string pattern matching to use null-safe .Length check instead of string equality comparison for better performance.
 * Fixed runtime crash when using interfaces with unimplemented static abstract members as constrained type arguments. ([Issue #19184](https://github.com/dotnet/fsharp/issues/19184))
 * Fix delegates with `[<OptionalArgument>]` and caller info attributes failing to compile. ([Issue #18868](https://github.com/dotnet/fsharp/issues/18868), [PR #19069](https://github.com/dotnet/fsharp/pull/19069))
 * Type checker: mark generated event tree nodes as synthetic ([PR #19213](https://github.com/dotnet/fsharp/pull/19213))
diff --git a/src/Compiler/Checking/PatternMatchCompilation.fs b/src/Compiler/Checking/PatternMatchCompilation.fs
@@ -767,8 +767,7 @@ let (|ConstNeedsDefaultCase|_|) c =
 ///   - Compact integer switches become a single switch.  Non-compact integer
 ///     switches, string switches and floating point switches are treated in the
 ///     same way as DecisionTreeTest.IsInst.
-let rec BuildSwitch inpExprOpt g expr edges dflt m =
-    if verbose then dprintf "--> BuildSwitch@%a, #edges = %A, dflt.IsSome = %A\n" outputRange m (List.length edges) (Option.isSome dflt)
+let rec BuildSwitch inpExprOpt g isNullFiltered expr edges dflt m =
     match edges, dflt with
     | [], None      -> failwith "internal error: no edges and no default"
     | [], Some dflt -> dflt
@@ -780,11 +779,13 @@ let rec BuildSwitch inpExprOpt g expr edges dflt m =
     // In this case the 'expr' already holds the result of the 'isinst' test.
 
     | TCase(DecisionTreeTest.IsInst _, success) :: edges, dflt  when Option.isSome inpExprOpt ->
-        TDSwitch(expr, [TCase(DecisionTreeTest.IsNull, BuildSwitch None g expr edges dflt m)], Some success, m)
+        TDSwitch(expr, [TCase(DecisionTreeTest.IsNull, BuildSwitch None g false expr edges dflt m)], Some success, m)
 
     // isnull and isinst tests
     | TCase((DecisionTreeTest.IsNull | DecisionTreeTest.IsInst _), _) as edge :: edges, dflt  ->
-        TDSwitch(expr, [edge], Some (BuildSwitch None g expr edges dflt m), m)
+        // After an IsNull test, in the fallthrough branch (Some), we know the value is not null
+        let nullFiltered = match edge with TCase(DecisionTreeTest.IsNull, _) -> true | _ -> isNullFiltered
+        TDSwitch(expr, [edge], Some (BuildSwitch None g nullFiltered expr edges dflt m), m)
 
     // All these should also always have default cases
     | TCase(DecisionTreeTest.Const ConstNeedsDefaultCase, _) :: _, None ->
@@ -799,7 +800,17 @@ let rec BuildSwitch inpExprOpt g expr edges dflt m =
                     match discrim with
                     | DecisionTreeTest.ArrayLength(n, _)       ->
                         let _v, vExpr, bind = mkCompGenLocalAndInvisibleBind g "testExpr" m testexpr
-                        mkLetBind m bind (mkLazyAnd g m (mkNonNullTest g m vExpr) (mkILAsmCeq g m (mkLdlen g m vExpr) (mkInt g m n)))
+                        // Skip null check if we're in a null-filtered context
+                        let test = mkILAsmCeq g m (mkLdlen g m vExpr) (mkInt g m n)
+                        let finalTest = if isNullFiltered then test else mkLazyAnd g m (mkNonNullTest g m vExpr) test
+                        mkLetBind m bind finalTest
+                    | DecisionTreeTest.Const (Const.String "")  ->
+                        // Optimize empty string check to use null-safe length check
+                        let _v, vExpr, bind = mkCompGenLocalAndInvisibleBind g "testExpr" m testexpr
+                        let test = mkILAsmCeq g m (mkGetStringLength g m vExpr) (mkInt g m 0)
+                        // Skip null check if we're in a null-filtered context
+                        let finalTest = if isNullFiltered then test else mkLazyAnd g m (mkNonNullTest g m vExpr) test
+                        mkLetBind m bind finalTest
                     | DecisionTreeTest.Const (Const.String _ as c)  ->
                         mkCallEqualsOperator g m g.string_ty testexpr (Expr.Const (c, m, g.string_ty))
                     | DecisionTreeTest.Const (Const.Decimal _ as c)  ->
@@ -1152,7 +1163,7 @@ let CompilePatternBasic
                     // OK, build the whole tree and whack on the binding if any
                     let finalDecisionTree =
                         let inpExprToSwitch = (match inpExprOpt with Some vExpr -> vExpr | None -> GetSubExprOfInput subexpr)
-                        let tree = BuildSwitch inpExprOpt g inpExprToSwitch simulSetOfCases defaultTreeOpt mMatch
+                        let tree = BuildSwitch inpExprOpt g false inpExprToSwitch simulSetOfCases defaultTreeOpt mMatch
                         match bindOpt with
                         | None -> tree
                         | Some bind -> TDBind (bind, tree)
diff --git a/src/Compiler/TypedTree/TypedTreeOps.fsi b/src/Compiler/TypedTree/TypedTreeOps.fsi
@@ -2348,6 +2348,8 @@ val mkIncr: TcGlobals -> range -> Expr -> Expr
 
 val mkLdlen: TcGlobals -> range -> Expr -> Expr
 
+val mkGetStringLength: TcGlobals -> range -> Expr -> Expr
+
 val mkLdelem: TcGlobals -> range -> TType -> Expr -> Expr -> Expr
 
 //-------------------------------------------------------------------------
diff --git a/tests/FSharp.Compiler.ComponentTests/EmittedIL/Inlining/EmptyStringPattern.fs b/tests/FSharp.Compiler.ComponentTests/EmittedIL/Inlining/EmptyStringPattern.fs
@@ -0,0 +1,28 @@
+module TestLibrary
+
+let inline classifyString (s: string) =
+    match s with
+    | "" -> "empty"
+    | null -> "null"
+    | _ -> "other"
+
+let inline testEmptyStringOnly (s: string) =
+    match s with
+    | "" -> 1
+    | _ -> 0
+
+let inline testBundledNullAndEmpty (s: string) =
+    match s with
+    | null | "" -> 0
+    | _ -> 1
+
+let inline testBundledEmptyAndNull (s: string) =
+    match s with
+    | "" | null -> 0
+    | _ -> 1
+
+// Usage functions to show inlining in action
+let useClassifyString s = classifyString s
+let useTestEmptyStringOnly s = testEmptyStringOnly s  
+let useBundledNullAndEmpty s = testBundledNullAndEmpty s
+let useBundledEmptyAndNull s = testBundledEmptyAndNull s
diff --git a/tests/FSharp.Compiler.ComponentTests/EmittedIL/Inlining/EmptyStringPattern.fs.il.bsl b/tests/FSharp.Compiler.ComponentTests/EmittedIL/Inlining/EmptyStringPattern.fs.il.bsl
@@ -0,0 +1,276 @@
+
+
+
+
+
+.assembly extern runtime { }
+.assembly extern FSharp.Core { }
+.assembly assembly
+{
+  .custom instance void [FSharp.Core]Microsoft.FSharp.Core.FSharpInterfaceDataVersionAttribute::.ctor(int32,
+                                                                                                      int32,
+                                                                                                      int32) = ( 01 00 02 00 00 00 00 00 00 00 00 00 00 00 00 00 ) 
+
+  
+  
+
+  .hash algorithm 0x00008004
+  .ver 0:0:0:0
+}
+.module assembly.exe
+
+.imagebase {value}
+.file alignment 0x00000200
+.stackreserve 0x00100000
+.subsystem 0x0003       
+.corflags 0x00000001    
+
+
+
+
+
+.class public abstract auto ansi sealed TestLibrary
+       extends [runtime]System.Object
+{
+  .custom instance void [FSharp.Core]Microsoft.FSharp.Core.CompilationMappingAttribute::.ctor(valuetype [FSharp.Core]Microsoft.FSharp.Core.SourceConstructFlags) = ( 01 00 07 00 00 00 00 00 ) 
+  .method public static string  classifyString(string s) cil managed
+  {
+    
+    .maxstack  8
+    IL_0000:  nop
+    IL_0001:  ldarg.0
+    IL_0002:  brfalse.s  IL_0010
+
+    IL_0004:  ldarg.0
+    IL_0005:  callvirt   instance int32 [runtime]System.String::get_Length()
+    IL_000a:  ldc.i4.0
+    IL_000b:  ceq
+    IL_000d:  nop
+    IL_000e:  br.s       IL_0012
+
+    IL_0010:  ldc.i4.0
+    IL_0011:  nop
+    IL_0012:  brtrue.s   IL_0019
+
+    IL_0014:  ldarg.0
+    IL_0015:  brfalse.s  IL_001f
+
+    IL_0017:  br.s       IL_0025
+
+    IL_0019:  ldstr      "empty"
+    IL_001e:  ret
+
+    IL_001f:  ldstr      "null"
+    IL_0024:  ret
+
+    IL_0025:  ldstr      "other"
+    IL_002a:  ret
+  } 
+
+  .method public static int32  testEmptyStringOnly(string s) cil managed
+  {
+    
+    .maxstack  8
+    IL_0000:  nop
+    IL_0001:  ldarg.0
+    IL_0002:  brfalse.s  IL_000e
+
+    IL_0004:  ldarg.0
+    IL_0005:  callvirt   instance int32 [runtime]System.String::get_Length()
+    IL_000a:  brtrue.s   IL_000e
+
+    IL_000c:  ldc.i4.1
+    IL_000d:  ret
+
+    IL_000e:  ldc.i4.0
+    IL_000f:  ret
+  } 
+
+  .method public static int32  testBundledNullAndEmpty(string s) cil managed
+  {
+    
+    .maxstack  8
+    IL_0000:  nop
+    IL_0001:  ldarg.0
+    IL_0002:  brfalse.s  IL_0017
+
+    IL_0004:  ldarg.0
+    IL_0005:  brfalse.s  IL_0013
+
+    IL_0007:  ldarg.0
+    IL_0008:  callvirt   instance int32 [runtime]System.String::get_Length()
+    IL_000d:  ldc.i4.0
+    IL_000e:  ceq
+    IL_0010:  nop
+    IL_0011:  br.s       IL_0015
+
+    IL_0013:  ldc.i4.0
+    IL_0014:  nop
+    IL_0015:  brfalse.s  IL_0019
+
+    IL_0017:  ldc.i4.0
+    IL_0018:  ret
+
+    IL_0019:  ldc.i4.1
+    IL_001a:  ret
+  } 
+
+  .method public static int32  testBundledEmptyAndNull(string s) cil managed
+  {
+    
+    .maxstack  8
+    IL_0000:  nop
+    IL_0001:  ldarg.0
+    IL_0002:  brfalse.s  IL_0010
+
+    IL_0004:  ldarg.0
+    IL_0005:  callvirt   instance int32 [runtime]System.String::get_Length()
+    IL_000a:  ldc.i4.0
+    IL_000b:  ceq
+    IL_000d:  nop
+    IL_000e:  br.s       IL_0012
+
+    IL_0010:  ldc.i4.0
+    IL_0011:  nop
+    IL_0012:  brtrue.s   IL_0017
+
+    IL_0014:  ldarg.0
+    IL_0015:  brtrue.s   IL_0019
+
+    IL_0017:  ldc.i4.0
+    IL_0018:  ret
+
+    IL_0019:  ldc.i4.1
+    IL_001a:  ret
+  } 
+
+  .method public static string  useClassifyString(string s) cil managed
+  {
+    
+    .maxstack  8
+    IL_0000:  nop
+    IL_0001:  ldarg.0
+    IL_0002:  brfalse.s  IL_0010
+
+    IL_0004:  ldarg.0
+    IL_0005:  callvirt   instance int32 [runtime]System.String::get_Length()
+    IL_000a:  ldc.i4.0
+    IL_000b:  ceq
+    IL_000d:  nop
+    IL_000e:  br.s       IL_0012
+
+    IL_0010:  ldc.i4.0
+    IL_0011:  nop
+    IL_0012:  brtrue.s   IL_0019
+
+    IL_0014:  ldarg.0
+    IL_0015:  brfalse.s  IL_001f
+
+    IL_0017:  br.s       IL_0025
+
+    IL_0019:  ldstr      "empty"
+    IL_001e:  ret
+
+    IL_001f:  ldstr      "null"
+    IL_0024:  ret
+
+    IL_0025:  ldstr      "other"
+    IL_002a:  ret
+  } 
+
+  .method public static int32  useTestEmptyStringOnly(string s) cil managed
+  {
+    
+    .maxstack  8
+    IL_0000:  nop
+    IL_0001:  ldarg.0
+    IL_0002:  brfalse.s  IL_000e
+
+    IL_0004:  ldarg.0
+    IL_0005:  callvirt   instance int32 [runtime]System.String::get_Length()
+    IL_000a:  brtrue.s   IL_000e
+
+    IL_000c:  ldc.i4.1
+    IL_000d:  ret
+
+    IL_000e:  ldc.i4.0
+    IL_000f:  ret
+  } 
+
+  .method public static int32  useBundledNullAndEmpty(string s) cil managed
+  {
+    
+    .maxstack  8
+    IL_0000:  nop
+    IL_0001:  ldarg.0
+    IL_0002:  brfalse.s  IL_0017
+
+    IL_0004:  ldarg.0
+    IL_0005:  brfalse.s  IL_0013
+
+    IL_0007:  ldarg.0
+    IL_0008:  callvirt   instance int32 [runtime]System.String::get_Length()
+    IL_000d:  ldc.i4.0
+    IL_000e:  ceq
+    IL_0010:  nop
+    IL_0011:  br.s       IL_0015
+
+    IL_0013:  ldc.i4.0
+    IL_0014:  nop
+    IL_0015:  brfalse.s  IL_0019
+
+    IL_0017:  ldc.i4.0
+    IL_0018:  ret
+
+    IL_0019:  ldc.i4.1
+    IL_001a:  ret
+  } 
+
+  .method public static int32  useBundledEmptyAndNull(string s) cil managed
+  {
+    
+    .maxstack  8
+    IL_0000:  nop
+    IL_0001:  ldarg.0
+    IL_0002:  brfalse.s  IL_0010
+
+    IL_0004:  ldarg.0
+    IL_0005:  callvirt   instance int32 [runtime]System.String::get_Length()
+    IL_000a:  ldc.i4.0
+    IL_000b:  ceq
+    IL_000d:  nop
+    IL_000e:  br.s       IL_0012
+
+    IL_0010:  ldc.i4.0
+    IL_0011:  nop
+    IL_0012:  brtrue.s   IL_0017
+
+    IL_0014:  ldarg.0
+    IL_0015:  brtrue.s   IL_0019
+
+    IL_0017:  ldc.i4.0
+    IL_0018:  ret
+
+    IL_0019:  ldc.i4.1
+    IL_001a:  ret
+  } 
+
+} 
+
+.class private abstract auto ansi sealed '<StartupCode$assembly>'.$TestLibrary
+       extends [runtime]System.Object
+{
+  .method public static void  main@() cil managed
+  {
+    .entrypoint
+    
+    .maxstack  8
+    IL_0000:  ret
+  } 
+
+} 
+
+
+
+
+
diff --git a/tests/FSharp.Compiler.ComponentTests/EmittedIL/Inlining/Inlining.fs b/tests/FSharp.Compiler.ComponentTests/EmittedIL/Inlining/Inlining.fs
@@ -229,3 +229,10 @@ printfn $"{(SecondType ()).SecondMethod()}"
         |> compileAndRun
         |> shouldSucceed
 
+
+    // Test empty string pattern optimization in inlining
+    [<Theory; FileInlineData("EmptyStringPattern.fs")>]
+    let ``EmptyStringPattern_fs`` compilation =
+        compilation
+        |> getCompilation
+        |> verifyCompilation
diff --git a/tests/FSharp.Compiler.ComponentTests/EmittedIL/TypeTestsInPatternMatching.fs b/tests/FSharp.Compiler.ComponentTests/EmittedIL/TypeTestsInPatternMatching.fs
