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[RyuJIT] Unroll single-iteration loops#43947

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Merged

sandreenko merged 6 commits intodotnet:masterfromEgorBo:unroll-empty-loops

Nov 12, 2020

Merged

[RyuJIT] Unroll single-iteration loops#43947

sandreenko merged 6 commits intodotnet:masterfromEgorBo:unroll-empty-loops

Nov 12, 2020

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EgorBo commentedOct 28, 2020•
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JIT is able to unroll small constant-size loops viaoptUnrollLoops, but currently it only works ifVector_.Count is used (magic) as the upper limit. This PR extends that behavior to also handle single iteration loops (thus it won't regress anything)

staticintGetCount()=>1;// e.g. static readonly field inited via an envvarstaticintTest(){intx=0;for(inti=0;i<GetCount();i++)x++;returnx;}

Current codegen for`Test`:

G_M58954_IG01:G_M58954_IG02:xoreax,eaxxoredx,edxG_M58954_IG03:inceaxincedxtestedx,edxjle      SHORT G_M58954_IG03G_M58954_IG04:ret

New codegen for`Test`:

G_M58954_IG02:moveax,1G_M58954_IG03:ret

jit-diff (`-f --cctors --pmi`):

(surprisingly, it's not empty)

Total bytes of delta: -352 (-0.00% of base)    diff is an improvement.Top file improvements (bytes):        -211 : System.Private.CoreLib.dasm (-0.00% of base)        -103 : Microsoft.CodeAnalysis.dasm (-0.01% of base)         -38 : Microsoft.Diagnostics.Tracing.TraceEvent.dasm (-0.00% of base)3 total files with Code Size differences (3 improved, 0 regressed), 265 unchanged.Top method improvements (bytes):        -103 (-12.50% of base) : Microsoft.CodeAnalysis.dasm - SmallDictionary`2:LeftComplex(AvlNode):AvlNode (8 base, 7 diff methods)         -66 (-12.69% of base) : System.Private.CoreLib.dasm - Vector64`1:Equals(Vector64`1):bool:this (6 methods)         -66 (-11.21% of base) : System.Private.CoreLib.dasm - Vector64`1:GetHashCode():int:this (6 methods)         -62 (-4.52% of base) : System.Private.CoreLib.dasm - Utf8Formatter:TryFormat(TimeSpan,Span`1,byref,StandardFormat):bool         -17 (-5.28% of base) : System.Private.CoreLib.dasm - Utf8Parser:TryParseByteX(ReadOnlySpan`1,byref,byref):bool         -14 (-24.56% of base) : Microsoft.Diagnostics.Tracing.TraceEvent.dasm - FileVersionTraceData:get_FileDescription():String:this         -13 (-1.53% of base) : Microsoft.Diagnostics.Tracing.TraceEvent.dasm - FileVersionTraceData:ToXml(StringBuilder):StringBuilder:this         -11 (-1.75% of base) : Microsoft.Diagnostics.Tracing.TraceEvent.dasm - FileVersionTraceData:PayloadValue(int):Object:thisTop method improvements (percentages):         -14 (-24.56% of base) : Microsoft.Diagnostics.Tracing.TraceEvent.dasm - FileVersionTraceData:get_FileDescription():String:this         -66 (-12.69% of base) : System.Private.CoreLib.dasm - Vector64`1:Equals(Vector64`1):bool:this (6 methods)        -103 (-12.50% of base) : Microsoft.CodeAnalysis.dasm - SmallDictionary`2:LeftComplex(AvlNode):AvlNode (8 base, 7 diff methods)         -66 (-11.21% of base) : System.Private.CoreLib.dasm - Vector64`1:GetHashCode():int:this (6 methods)         -17 (-5.28% of base) : System.Private.CoreLib.dasm - Utf8Parser:TryParseByteX(ReadOnlySpan`1,byref,byref):bool         -62 (-4.52% of base) : System.Private.CoreLib.dasm - Utf8Formatter:TryFormat(TimeSpan,Span`1,byref,StandardFormat):bool         -11 (-1.75% of base) : Microsoft.Diagnostics.Tracing.TraceEvent.dasm - FileVersionTraceData:PayloadValue(int):Object:this         -13 (-1.53% of base) : Microsoft.Diagnostics.Tracing.TraceEvent.dasm - FileVersionTraceData:ToXml(StringBuilder):StringBuilder:this8 total methods with Code Size differences (8 improved, 0 regressed), 259114 unchanged.Completed analysis in 23.84

Example:Utf8Parser:TryParseByteX (ByteOverflowLengthHex = 2) - looks suspicious btw: