Support SkipLocalsinit

src/fsharp/IlxGen.fs

@@ -967,6 +967,9 @@ and IlxGenEnv =
 
       /// Are we inside of a recursive let binding, while loop, or a for loop?
       isInLoop: bool
+
+      /// Indicates that the .locals init flag should be set on a method and all its nested methods and lambdas
+      initLocals: bool
     }
 
     override _.ToString() = "<IlxGenEnv>"
@@ -2105,7 +2108,7 @@ let CodeGenMethod cenv mgbuf (entryPointInfo, methodName, eenv, alreadyUsedArgs,
     let maxStack = maxStack + 2
 
     // Build an Abstract IL method
-    instrs, mkILMethodBody (true, locals, maxStack, code, sourceRange)
+    instrs, mkILMethodBody (eenv.initLocals, locals, maxStack, code, sourceRange)
 
 let StartDelayedLocalScope nm cgbuf =
     let startScope = CG.GenerateDelayMark cgbuf ("start_" + nm)
@@ -5609,7 +5612,8 @@ and GenBindingAfterDebugPoint cenv cgbuf eenv sp (TBind(vspec, rhsExpr, _)) star
     | Some _, Some e -> cgbuf.mgbuf.AddReflectedDefinition(vspec, e)
     | _ -> ()
 
-    let eenv = {eenv with letBoundVars= (mkLocalValRef vspec) :: eenv.letBoundVars}
+    let eenv = { eenv with letBoundVars = (mkLocalValRef vspec) :: eenv.letBoundVars;
+                           initLocals = eenv.initLocals && (match vspec.ApparentEnclosingEntity with Parent ref -> not (HasFSharpAttribute g g.attrib_SkipLocalsInitAttribute ref.Attribs) | _ -> true) }
 
     let access = ComputeMethodAccessRestrictedBySig eenv vspec
 
@@ -6181,6 +6185,7 @@ and GenMethodForBinding
         let eenvForMeth = eenvForMeth |> AddStorageForLocalWitnesses (methLambdaWitnessInfos |> List.mapi (fun i w -> (w, Arg (numArgsUsed+i))))
         let numArgsUsed = numArgsUsed + methLambdaWitnessInfos.Length
         let eenvForMeth = eenvForMeth |> AddStorageForLocalVals cenv.g (List.mapi (fun i v -> (v, Arg (numArgsUsed+i))) nonUnitNonSelfMethodVars)
+        let eenvForMeth = if eenvForMeth.initLocals && HasFSharpAttribute g g.attrib_SkipLocalsInitAttribute v.Attribs then { eenvForMeth with initLocals = false } else eenvForMeth
         eenvForMeth
 
     let tailCallInfo =
@@ -6953,7 +6958,7 @@ and GenModuleBinding cenv (cgbuf: CodeGenBuffer) (qname: QualifiedNameOfFile) la
 
     let eenvinner =
         if mspec.IsNamespace then eenv else
-        {eenv with cloc = CompLocForFixedModule cenv.opts.fragName qname.Text mspec }
+        { eenv with cloc = CompLocForFixedModule cenv.opts.fragName qname.Text mspec; initLocals = eenv.initLocals && not (HasFSharpAttribute cenv.g cenv.g.attrib_SkipLocalsInitAttribute mspec.Attribs) }
 
     // Create the class to hold the contents of this module. No class needed if
     // we're compiling it as a namespace.
@@ -8044,7 +8049,8 @@ let GetEmptyIlxGenEnv (g: TcGlobals) ccu =
       innerVals = []
       sigToImplRemapInfo = [] (* "module remap info" *)
       withinSEH = false
-      isInLoop = false }
+      isInLoop = false
+      initLocals = true }
 
 type IlxGenResults =
     { ilTypeDefs: ILTypeDef list

src/fsharp/TcGlobals.fs

@@ -1193,6 +1193,7 @@ type public TcGlobals(compilingFslib: bool, ilg:ILGlobals, fslibCcu: CcuThunk, d
   member val attrib_CallerLineNumberAttribute = findSysAttrib "System.Runtime.CompilerServices.CallerLineNumberAttribute"
   member val attrib_CallerFilePathAttribute = findSysAttrib "System.Runtime.CompilerServices.CallerFilePathAttribute"
   member val attrib_CallerMemberNameAttribute = findSysAttrib "System.Runtime.CompilerServices.CallerMemberNameAttribute"
+  member val attrib_SkipLocalsInitAttribute  = findSysAttrib "System.Runtime.CompilerServices.SkipLocalsInitAttribute"
 
   member val attrib_ProjectionParameterAttribute           = mk_MFCore_attrib "ProjectionParameterAttribute"
   member val attrib_CustomOperationAttribute               = mk_MFCore_attrib "CustomOperationAttribute"

src/fsharp/ilx/EraseClosures.fs

@@ -314,7 +314,7 @@ let convILMethodBody (thisClo, boxReturnTy) (il: ILMethodBody) =
         match boxReturnTy with
         | None    -> code
         | Some ty -> morphILInstrsInILCode (convReturnInstr ty) code
-    {il with MaxStack=newMax; IsZeroInit=true; Code= code }
+    { il with MaxStack = newMax; Code = code }
 
 let convMethodBody thisClo = function
     | MethodBody.IL il -> 

tests/FSharp.Compiler.ComponentTests/EmittedIL/SkipLocalsInit.fs

@@ -0,0 +1,133 @@
+// Copyright (c) Microsoft Corporation.  All Rights Reserved.  See License.txt in the project root for license information.
+
+namespace FSharp.Compiler.ComponentTests.EmittedIL
+
+open Xunit
+open FSharp.Test.Utilities.Compiler
+
+#if NETCOREAPP
+module ``SkipLocalsInit`` =
+    [<Fact>]
+    let ``Init in function and closure not emitted when applied on function``() =
+        FSharp """
+module SkipLocalsInit
+
+[<System.Runtime.CompilerServices.SkipLocalsInitAttribute>]
+let x () =
+    [||] |> Array.filter (fun x -> let y = "".Length in y + y = x) |> ignore
+"""
+         |> compile
+         |> shouldSucceed
+         |> verifyIL ["""
+.method public static void  x() cil managed
+{
+  .custom instance void [runtime]System.Runtime.CompilerServices.SkipLocalsInitAttribute::.ctor() = ( 01 00 00 00 )
+
+  .maxstack  4
+  .locals (int32[] V_0)
+"""
+
+                      """
+.method public strict virtual instance bool 
+        Invoke(int32 x) cil managed
+{
+          
+  .maxstack  6
+  .locals (int32 V_0)"""]
+
+    [<Fact>]
+    let ``Init in static method not emitted when applied on class``() =
+        FSharp """
+module SkipLocalsInit
+
+[<System.Runtime.CompilerServices.SkipLocalsInitAttribute>]
+type X () =
+    static member Y () =
+        let x = "ssa".Length
+        x + x
+         """
+         |> compile
+         |> shouldSucceed
+         |> verifyIL ["""
+.custom instance void [runtime]System.Runtime.CompilerServices.SkipLocalsInitAttribute::.ctor() = ( 01 00 00 00 ) 
+.custom instance void [FSharp.Core]Microsoft.FSharp.Core.CompilationMappingAttribute::.ctor(valuetype [FSharp.Core]Microsoft.FSharp.Core.SourceConstructFlags) = ( 01 00 03 00 00 00 00 00 ) 
+"""
+
+                      """
+.method public static int32  Y() cil managed
+{
+  
+  .maxstack  4
+  .locals (int32 V_0)"""]
+
+    [<Fact>]
+    let ``Init in static method and function not emitted when applied on module``() =
+        FSharp """
+[<System.Runtime.CompilerServices.SkipLocalsInitAttribute>]
+module SkipLocalsInit
+
+let x () =
+    let x = "ssa".Length
+    x + x
+
+type X () =
+    static member Y () =
+        let x = "ssa".Length
+        x + x
+         """
+         |> compile
+         |> shouldSucceed
+         |> verifyIL ["""
+.custom instance void [runtime]System.Runtime.CompilerServices.SkipLocalsInitAttribute::.ctor() = ( 01 00 00 00 ) 
+.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 int32  x() cil managed
+{
+
+  .maxstack  4
+  .locals (int32 V_0)
+"""
+
+                      """
+.method public static int32  Y() cil managed
+{
+  
+  .maxstack  4
+  .locals (int32 V_0)"""]
+
+    [<Fact>]
+    let ``Init in method and closure not emitted when applied on method``() =
+        FSharp """
+module SkipLocalsInit
+
+type X () =
+    [<System.Runtime.CompilerServices.SkipLocalsInitAttribute>]
+    member _.Y () =
+        [||] |> Array.filter (fun x -> let y = "".Length in y + y = x) |> ignore
+        let x = "ssa".Length
+        x + x
+         """
+         |> compile
+         |> shouldSucceed
+         |> verifyIL ["""
+.method public hidebysig instance int32 
+        Y() cil managed
+{
+  .custom instance void [runtime]System.Runtime.CompilerServices.SkipLocalsInitAttribute::.ctor() = ( 01 00 00 00 ) 
+  
+  .maxstack  4
+  .locals (int32[] V_0,
+           int32 V_1)
+"""
+
+                      """
+.method public strict virtual instance bool 
+        Invoke(int32 x) cil managed
+{
+  
+  .maxstack  6
+  .locals (int32 V_0)
+"""]
+#endif

tests/FSharp.Compiler.ComponentTests/FSharp.Compiler.ComponentTests.fsproj

@@ -22,6 +22,7 @@
     <Compile Include="EmittedIL\Operators.fs" />
     <Compile Include="EmittedIL\Literals.fs" />
     <Compile Include="EmittedIL\Misc.fs" />
+    <Compile Include="EmittedIL\SkipLocalsInit.fs" />
     <Compile Include="EmittedIL\TailCalls.fs" />
     <Compile Include="EmittedIL\TupleElimination.fs" />
     <Compile Include="ErrorMessages\TypeEqualsMissingTests.fs" />