JIT: Fold null checks against initialized static readonly fields

[EgorBo]

Follow up to #49930

This PR folds nullchecks for static readonly fields of ref types (already statically inited, known to be not null), e.g.:

class Program
{
    private static string PathEnvVar { get; } = Environment.GetEnvironmentVariable("PATH");

    public bool IsNotNull() => PathEnvVar != null;
}

Current codegen for IsNotNull (tier1):

; Assembly listing for method IsNotNull():bool:this
G_M52841_IG01:             
G_M52841_IG02:            
       48B8382DF5C507020000 mov      rax, 0x207C5F52D38
       48833800             cmp      gword ptr [rax], 0
       0F95C0               setne    al
       0FB6C0               movzx    rax, al
G_M52841_IG03:              
       C3                   ret      
; Total bytes of code 21

New codegen for IsNotNull:

; Assembly listing for method IsNotNull():bool:this
G_M52841_IG01:
G_M52841_IG02:             
       B801000000           mov      eax, 1
G_M52841_IG03:              
       C3                   ret      
; Total bytes of code 6

jit-diff (-f -cctors)

PMI CodeSize Diffs for System.Private.CoreLib.dll, framework assemblies [invoking .cctors] for  default jit

Summary of Code Size diffs:
(Lower is better)

Total bytes of base: 52242582
Total bytes of diff: 52203720
Total bytes of delta: -38862 (-0.07% of base)
    diff is an improvement.


Top file improvements (bytes):
       -8900 : FSharp.Core.dasm (-0.25% of base)
       -3924 : Microsoft.CodeAnalysis.VisualBasic.dasm (-0.07% of base)
       -3681 : System.Threading.Tasks.Dataflow.dasm (-0.39% of base)
       -3018 : System.Private.Xml.dasm (-0.09% of base)
       -2009 : Microsoft.CodeAnalysis.CSharp.dasm (-0.05% of base)
       -1626 : System.Private.CoreLib.dasm (-0.04% of base)
       -1286 : Microsoft.CodeAnalysis.dasm (-0.07% of base)
       -1166 : System.Data.Common.dasm (-0.08% of base)
       -1060 : System.Collections.Immutable.dasm (-0.08% of base)
        -983 : CommandLine.dasm (-0.20% of base)
        -631 : System.Linq.Parallel.dasm (-0.03% of base)
        -571 : Newtonsoft.Json.dasm (-0.07% of base)
        -567 : System.Net.Http.dasm (-0.09% of base)
        -519 : System.Data.OleDb.dasm (-0.19% of base)
        -508 : System.Configuration.ConfigurationManager.dasm (-0.15% of base)
        -498 : System.Speech.dasm (-0.12% of base)
        -376 : System.Net.Mail.dasm (-0.20% of base)
        -374 : Microsoft.Diagnostics.Tracing.TraceEvent.dasm (-0.01% of base)
        -367 : xunit.execution.dotnet.dasm (-0.16% of base)
        -366 : System.CodeDom.dasm (-0.21% of base)

106 total files with Code Size differences (106 improved, 0 regressed), 165 unchanged.

Top method regressions (bytes):
           6 ( 0.16% of base) : Microsoft.Diagnostics.Tracing.TraceEvent.dasm - TraceEventSession:EnableProvider(Guid,int,long,TraceEventProviderOptions):bool:this

Top method improvements (bytes):
       -2160 (-7.68% of base) : FSharp.Core.dasm - FSharpMailboxProcessor`1:PostAndTryAsyncReply(FSharpFunc`2,FSharpOption`1):FSharpAsync`1:this (64 methods)
       -1143 (-11.13% of base) : FSharp.Core.dasm - Mailbox`1:TryScan(FSharpFunc`2,int):FSharpAsync`1:this (64 methods)
       -1143 (-11.13% of base) : FSharp.Core.dasm - Mailbox`1:Scan(FSharpFunc`2,int):FSharpAsync`1:this (64 methods)
        -863 (-3.21% of base) : FSharp.Core.dasm - FSharpMailboxProcessor`1:PostAndAsyncReply(FSharpFunc`2,FSharpOption`1):FSharpAsync`1:this (64 methods)
        -575 (-6.21% of base) : System.Threading.Tasks.Dataflow.dasm - ReceiveTarget`1:CleanupAndComplete(int):this (8 methods)
        -384 (-3.07% of base) : FSharp.Core.dasm - Array2DModule:CopyTo(ref,int,int,ref,int,int,int,int) (8 methods)
        -277 (-9.84% of base) : FSharp.Core.dasm - FSharpAsync:AwaitAndBindChildResult(CancellationTokenSource,ResultCell`1,FSharpOption`1):FSharpAsync`1 (8 methods)
        -252 (-6.51% of base) : System.Collections.Immutable.dasm - Builder:set_KeyComparer(IEqualityComparer`1):this (16 methods)
        -245 (-5.25% of base) : System.ComponentModel.TypeConverter.dasm - <>c:<get_IntrinsicTypeConverters>b__23_0():Dictionary`2:this
        -236 (-6.94% of base) : CommandLine.dasm - <>c:<EnforceRequired>b__2_0(IEnumerable`1):IEnumerable`1:this
        -233 (-4.71% of base) : System.Threading.Tasks.Dataflow.dasm - TargetCore`1:ProcessAsyncIfNecessary_Slow(bool):this (8 methods)
        -223 (-9.93% of base) : System.Private.Xml.dasm - XmlUntypedStringConverter:StringToListType(String,Type,IXmlNamespaceResolver):Object:this
        -218 (-7.48% of base) : CommandLine.dasm - <>c__DisplayClass4_0:<EnforceSingle>b__0(IEnumerable`1):IEnumerable`1:this
        -217 (-18.72% of base) : Microsoft.CodeAnalysis.dasm - UICultureUtilities:WithCurrentUICulture(Action`1):Action`1 (8 methods)
        -217 (-18.72% of base) : Microsoft.CodeAnalysis.dasm - UICultureUtilities:WithCurrentUICulture(Func`1):Func`1 (8 methods)
        -215 (-12.38% of base) : System.Threading.Tasks.Dataflow.dasm - OutputAvailableAsyncTarget`1:CancelAndUnlink(Object) (8 methods)
        -215 (-11.58% of base) : System.Threading.Tasks.Dataflow.dasm - TargetCore`1:CompleteBlockIfPossible_Slow():this (8 methods)
        -173 (-7.44% of base) : System.Threading.Tasks.Dataflow.dasm - DataflowBlock:TryChooseFromSource(ISourceBlock`1,Action`1,int,TaskScheduler,byref):bool (8 methods)
        -168 (-6.34% of base) : System.Collections.Immutable.dasm - ImmutableSortedDictionary`2:WithComparers(IComparer`1,IEqualityComparer`1):ImmutableSortedDictionary`2:this (8 methods)
        -168 (-18.71% of base) : System.Private.CoreLib.dasm - Dictionary`2:System.Collections.Generic.ICollection<System.Collections.Generic.KeyValuePair<TKey,TValue>>.Contains(KeyValuePair`2):bool:this (8 methods)

Top method regressions (percentages):
           6 ( 0.16% of base) : Microsoft.Diagnostics.Tracing.TraceEvent.dasm - TraceEventSession:EnableProvider(Guid,int,long,TraceEventProviderOptions):bool:this

Top method improvements (percentages):
         -19 (-95.00% of base) : System.Net.Http.WinHttpHandler.dasm - NetEventSource:DebugValidateArg(Object)
         -50 (-89.29% of base) : Microsoft.CodeAnalysis.dasm - HashAlgorithm:get_SupportsTransform():bool:this
         -15 (-65.22% of base) : Microsoft.Extensions.DependencyModel.dasm - EnvironmentWrapper:GetAppContextData(String):Object:this
         -48 (-57.83% of base) : System.Net.Mail.dasm - HeaderCollection:.ctor():this
         -48 (-57.83% of base) : System.Web.HttpUtility.dasm - HttpQSCollection:.ctor():this
         -48 (-57.83% of base) : System.Configuration.ConfigurationManager.dasm - ConfigurationValues:.ctor():this
        -114 (-39.18% of base) : System.Private.Xml.dasm - XmlILOptimizerVisitor:IsPrimitiveNumeric(XmlQueryType):bool:this
        -114 (-39.18% of base) : System.Private.Xml.dasm - XmlILOptimizerVisitor:MatchesContentTest(XmlQueryType):bool:this
         -28 (-38.36% of base) : System.Security.Cryptography.Cng.dasm - Rc2CbcParameters:GetEffectiveKeyBits():int:this
         -28 (-38.36% of base) : System.Security.Cryptography.Pkcs.dasm - Rc2CbcParameters:GetEffectiveKeyBits():int:this
         -28 (-38.36% of base) : System.Security.Cryptography.Algorithms.dasm - Rc2CbcParameters:GetEffectiveKeyBits():int:this
         -20 (-36.36% of base) : System.Collections.Specialized.dasm - NameObjectCollectionBase:.ctor():this
         -20 (-36.36% of base) : System.Collections.Specialized.dasm - NameValueCollection:.ctor():this
         -15 (-29.41% of base) : System.Configuration.ConfigurationManager.dasm - SettingsPropertyValue:IsHostedInAspnet():bool:this
         -40 (-27.59% of base) : System.Configuration.ConfigurationManager.dasm - ConfigurationElement:.ctor():this
         -16 (-26.23% of base) : System.IO.Packaging.dasm - ContentType:IsAllowedCharacter(ushort):bool
         -16 (-26.23% of base) : System.Reflection.MetadataLoadContext.dasm - Helpers:NeedsEscapingInTypeName(ushort):bool
         -18 (-25.35% of base) : Microsoft.CodeAnalysis.VisualBasic.dasm - _Closure$__:_Lambda$__0-3(Symbol):String:this
         -44 (-24.72% of base) : System.Net.Mail.dasm - Message:get_Headers():HeaderCollection:this
         -44 (-24.72% of base) : System.Net.Mail.dasm - Message:get_EnvelopeHeaders():HeaderCollection:this

1939 total methods with Code Size differences (1938 improved, 1 regressed), 266648 unchanged.

--------------------------------------------------------------------------------
Completed analysis in 36.81s

[EgorBo]

Id: 50000 🎉

[antonfirsov]

I'm here only because mine was #49999 unluckily

[EgorBo]

@dotnet/jit-contrib PTAL

[sandreenko]

LGTM, the diffs look awesome!