Codegen for some bitflag checks could be optimized

[GrabYourPitchforks]

More info: dotnet/coreclr#27603 (comment)

Sample code:

public static bool HasHighBitSet(uint value) {
    return (value & 0x8000_0000) != 0;
}

Actual codegen:

    L0000: test ecx, 0x80000000
    L0006: setnz al
    L0009: movzx eax, al
    L000c: ret

Replacing the first two instructions with test ecx, ecx and setl al would save 4 bytes of codegen. It's a minor savings but might help if this pattern occurs within a hot loop.

category:cq
theme:basic-cq
skill-level:intermediate
cost:small

[mikedn]

Replacing the first two instructions with test ecx, ecx and setl al would save 4 bytes of codegen.

Actually this specific case should generate:

mov eax, ecx
shr eax, 31
ret

The test approach is only useful if such code is used as branch condition, then it would generate test and js/jns.

[omariom]

if such code is used as branch condition

The codegen of the inlined method looks good

[mikedn]

The codegen of the inlined method looks good

No, the point is that this could generate test reg, reg instead of test reg, imm to avoid the 32 bit immediate.

[GrabYourPitchforks]

The test approach is only useful if such code is used as branch condition

Yeah, this suggestion was for branch conditions specifically. I thought there was already a tracking issue for more general branchless bit manipulation of Boolean conditions?

[mikedn]

There definitely are other related issues - #6815 and #7088 come to mind and there may be others. I also have some code written that does some of this, gotta check what state it's in.

[EgorBo]

Possible fix: EgorBo@6aaf47a (with help of #35627)

public static bool HasHighBitSet(uint value)
{
    return (value & 0x8000_0000) != 0;
}

new asm:

; Method CC:HasHighBitSet(int):bool
       8BC1                 mov      eax, ecx
       C1E81F               shr      eax, 31
       C3                   ret      

pmi jit-diff:

Analyzing CodeSize diffs...
Found 33 files with textual diffs.
PMI CodeSize Diffs for System.Private.CoreLib.dll, framework assemblies for  default jit
Summary of Code Size diffs:
(Lower is better)
Total bytes of diff: -5184 (-0.01% of base)
    diff is an improvement.
Top file improvements (bytes):
       -1101 : System.Private.CoreLib.dasm (-0.02% of base)
        -976 : System.Numerics.Tensors.dasm (-0.40% of base)
        -846 : System.Security.Cryptography.Pkcs.dasm (-0.22% of base)
        -489 : System.Text.Json.dasm (-0.08% of base)
        -400 : System.Memory.dasm (-0.16% of base)
        -270 : System.Security.Cryptography.Algorithms.dasm (-0.08% of base)
        -137 : System.Reflection.Metadata.dasm (-0.03% of base)
        -129 : System.Security.Cryptography.Cng.dasm (-0.07% of base)
        -110 : System.Net.Http.dasm (-0.02% of base)
        -108 : System.Formats.Asn1.dasm (-0.11% of base)
         -99 : System.Formats.Cbor.dasm (-0.21% of base)
         -99 : System.Security.Cryptography.X509Certificates.dasm (-0.06% of base)
         -49 : System.Reflection.MetadataLoadContext.dasm (-0.03% of base)
         -47 : Microsoft.CodeAnalysis.dasm (-0.00% of base)
         -36 : System.Net.WebSockets.dasm (-0.09% of base)
         -33 : System.IO.Pipelines.dasm (-0.05% of base)
         -30 : System.Net.WebSockets.WebSocketProtocol.dasm (-0.09% of base)
         -28 : System.Private.Xml.dasm (-0.00% of base)
         -24 : ILCompiler.Reflection.ReadyToRun.dasm (-0.02% of base)
         -21 : Microsoft.Extensions.Logging.Console.dasm (-0.05% of base)
33 total files with Code Size differences (33 improved, 0 regressed), 235 unchanged.
Top method regressions (bytes):
           9 ( 0.44% of base) : System.Security.Cryptography.Pkcs.dasm - ManagedPkcsPal:Decode(ReadOnlySpan`1,byref,byref,byref,byref,byref):DecryptorPal:this
Top method improvements (bytes):
        -234 (-1.62% of base) : System.Numerics.Tensors.dasm - CompressedSparseTensor`1:EnsureCapacity(int,int):this (7 methods)
         -99 (-1.75% of base) : System.Numerics.Tensors.dasm - CompressedSparseTensor`1:RemoveAt(int,int):this (7 methods)
         -98 (-2.97% of base) : System.Private.CoreLib.dasm - Bucket:Rent():ref:this (7 methods)
         -98 (-3.45% of base) : System.Private.CoreLib.dasm - Bucket:Return(ref):this (7 methods)
         -64 (-0.81% of base) : System.Numerics.Tensors.dasm - CompressedSparseTensor`1:Reshape(ReadOnlySpan`1):Tensor`1:this (7 methods)
         -63 (-1.35% of base) : System.Numerics.Tensors.dasm - CompressedSparseTensor`1:ToDenseTensor():DenseTensor`1:this (7 methods)
         -63 (-0.66% of base) : System.Security.Cryptography.Algorithms.dasm - KeyFormatHelper:ReadEncryptedPkcs8(ref,ReadOnlyMemory`1,ReadOnlySpan`1,ReadOnlySpan`1,KeyReader`1,byref,byref) (7 methods)
         -63 (-0.66% of base) : System.Security.Cryptography.Pkcs.dasm - KeyFormatHelper:ReadEncryptedPkcs8(ref,ReadOnlyMemory`1,ReadOnlySpan`1,ReadOnlySpan`1,KeyReader`1,byref,byref) (7 methods)
         -60 (-0.90% of base) : System.Numerics.Tensors.dasm - CompressedSparseTensor`1:Clone():Tensor`1:this (7 methods)
         -60 (-0.88% of base) : System.Numerics.Tensors.dasm - CompressedSparseTensor`1:ToCompressedSparseTensor():CompressedSparseTensor`1:this (7 methods)
         -60 (-1.22% of base) : System.Numerics.Tensors.dasm - CompressedSparseTensor`1:ToSparseTensor():SparseTensor`1:this (7 methods)
         -46 (-2.35% of base) : System.Private.CoreLib.dasm - Memory`1:CopyTo(Memory`1):this (7 methods)
         -45 (-0.70% of base) : System.Text.Json.dasm - JsonDocument:TryGetValue(int,byref):bool:this (15 methods)
         -43 (-2.24% of base) : System.Private.CoreLib.dasm - ReadOnlyMemory`1:CopyTo(Memory`1):this (7 methods)
         -42 (-1.58% of base) : System.Numerics.Tensors.dasm - CompressedSparseTensor`1:TryFindIndex(int,int,byref):bool:this (7 methods)
         -42 (-2.03% of base) : System.Private.CoreLib.dasm - Memory`1:TryCopyTo(Memory`1):bool:this (7 methods)
         -39 (-1.92% of base) : System.Private.CoreLib.dasm - ReadOnlyMemory`1:TryCopyTo(Memory`1):bool:this (7 methods)
         -23 (-1.24% of base) : System.Reflection.MetadataLoadContext.dasm - EcmaAssembly:ComputeAssemblyReferences():ref:this
         -21 (-0.19% of base) : Microsoft.Extensions.Logging.Console.dasm - JsonConsoleFormatter:Write(byref,IExternalScopeProvider,TextWriter):this (7 methods)
         -21 (-1.75% of base) : System.Numerics.Tensors.dasm - CompressedSparseTensor`1:InsertAt(int,ubyte,int,int):this
Top method regressions (percentages):
           9 ( 0.44% of base) : System.Security.Cryptography.Pkcs.dasm - ManagedPkcsPal:Decode(ReadOnlySpan`1,byref,byref,byref,byref,byref):DecryptorPal:this
Top method improvements (percentages):
          -8 (-53.33% of base) : System.Private.CoreLib.dasm - ManualResetEventSlim:get_IsSet():bool:this
          -4 (-28.57% of base) : System.Reflection.Metadata.dasm - MetadataTokens:GetToken(EntityHandle):int
          -4 (-23.53% of base) : System.Reflection.Metadata.dasm - MetadataTokens:GetHeapOffset(BlobHandle):int
          -3 (-23.08% of base) : System.Private.CoreLib.dasm - MethodTable:get_HasComponentSize():bool:this
          -3 (-23.08% of base) : System.Reflection.Metadata.dasm - EntityHandle:get_IsVirtual():bool:this
          -3 (-23.08% of base) : System.Reflection.Metadata.dasm - AssemblyReferenceHandle:get_IsVirtual():bool:this
          -3 (-23.08% of base) : System.Reflection.Metadata.dasm - StringHandle:get_IsVirtual():bool:this
          -3 (-23.08% of base) : System.Reflection.Metadata.dasm - NamespaceDefinitionHandle:get_IsVirtual():bool:this
          -3 (-23.08% of base) : System.Reflection.Metadata.dasm - NamespaceDefinitionHandle:get_HasFullName():bool:this
          -3 (-23.08% of base) : System.Reflection.Metadata.dasm - BlobHandle:get_IsVirtual():bool:this
          -3 (-21.43% of base) : System.Private.Xml.dasm - XsdDuration:get_IsNegative():bool:this
          -3 (-21.43% of base) : System.Reflection.Metadata.dasm - AssemblyReference:get_IsVirtual():bool:this
          -3 (-18.75% of base) : System.Private.CoreLib.dasm - RuntimeHelpers:ObjectHasComponentSize(Object):bool
          -4 (-18.18% of base) : System.Reflection.Metadata.dasm - MetadataTokens:GetRowNumber(EntityHandle):int
          -4 (-18.18% of base) : System.Reflection.Metadata.dasm - MetadataTokens:GetHeapOffset(StringHandle):int
          -4 (-14.29% of base) : System.Reflection.Metadata.dasm - DocumentNameBlobHandle:op_Explicit(BlobHandle):DocumentNameBlobHandle
          -6 (-13.64% of base) : System.Private.CoreLib.dasm - SpinLock:Exit(bool):this
          -4 (-10.81% of base) : System.Reflection.Metadata.dasm - StringHeap:GetString(StringHandle,MetadataStringDecoder):String:this
          -4 (-9.09% of base) : System.Reflection.Metadata.dasm - AssemblyReference:get_PublicKeyOrToken():BlobHandle:this
          -3 (-8.33% of base) : System.Private.CoreLib.dasm - RuntimeHelpers:GetRawObjectDataSize(Object):long
828 total methods with Code Size differences (827 improved, 1 regressed), 258028 unchanged.
Completed analysis in 26.51s
PS C:\prj>