A little more regex source generator tweaking (#62605)

* Remove now unecessary "Variable declared but never used" warning suppression

* Avoid declaring inputSpan in FindFirstChar if we don't have to

* Add RegexNode.Ref/Bol/Eol to non-backtracking list

* Move some additionalDeclarations back to where they're used

I was overaggressive in moving these to the beginning.  Some are fine where they're needed.

* Delete dead code

* Emit IsEmpty for more slice.Length checks

* Apply suggestions from code review

Author: stephentoub

src/libraries/System.Text.RegularExpressions/gen/RegexGenerator.Emitter.cs

@@ -36,7 +36,6 @@ public partial class RegexGenerator
             "#nullable enable",
             "#pragma warning disable CS0162 // Unreachable code",
             "#pragma warning disable CS0164 // Unreferenced label",
-            "#pragma warning disable CS0168 // Variable declared but never used",
             "#pragma warning disable CS0219 // Variable assigned but never used",
             "",
         };
@@ -274,13 +273,11 @@ private static void EmitFindFirstChar(IndentedTextWriter writer, RegexMethod rm,
             bool hasTextInfo = false;
 
             // In some cases, we need to emit declarations at the beginning of the method, but we only discover we need them later.
-            // To handle that, we emit a placeholder value that's not valid C#, and then at the end of the code generation we either
-            // delete it if no additional declarations are required, or we replace it with the list of additional declarations
-            // built up while generating code.
+            // To handle that, we build up a collection of all the declarations to include, track where they should be inserted,
+            // and then insert them at that position once everything else has been output.
             var additionalDeclarations = new HashSet<string>();
 
             // Emit locals initialization
-            writer.WriteLine("global::System.ReadOnlySpan<char> inputSpan = base.runtext;");
             writer.WriteLine("int pos = base.runtextpos, end = base.runtextend;");
             writer.Flush();
             int additionalDeclarationsPosition = ((StringWriter)writer.InnerWriter).GetStringBuilder().Length;
@@ -315,15 +312,17 @@ private static void EmitFindFirstChar(IndentedTextWriter writer, RegexMethod rm,
                     {
                         case FindNextStartingPositionMode.LeadingPrefix_LeftToRight_CaseSensitive:
                             Debug.Assert(!string.IsNullOrEmpty(code.FindOptimizations.LeadingCaseSensitivePrefix));
-                            EmitIndexOf_LeftToRight(code.FindOptimizations.LeadingCaseSensitivePrefix);
+                            additionalDeclarations.Add("global::System.ReadOnlySpan<char> inputSpan = base.runtext;");
+                            EmitIndexOf(code.FindOptimizations.LeadingCaseSensitivePrefix);
                             break;
 
                         case FindNextStartingPositionMode.FixedSets_LeftToRight_CaseSensitive:
                         case FindNextStartingPositionMode.FixedSets_LeftToRight_CaseInsensitive:
                         case FindNextStartingPositionMode.LeadingSet_LeftToRight_CaseSensitive:
                         case FindNextStartingPositionMode.LeadingSet_LeftToRight_CaseInsensitive:
                             Debug.Assert(code.FindOptimizations.FixedDistanceSets is { Count: > 0 });
-                            EmitFixedSet_LeftToRight();
+                            additionalDeclarations.Add("global::System.ReadOnlySpan<char> inputSpan = base.runtext;");
+                            EmitFixedSet();
                             break;
 
                         default:
@@ -338,7 +337,7 @@ private static void EmitFindFirstChar(IndentedTextWriter writer, RegexMethod rm,
             }
             writer.WriteLine();
 
-            writer.WriteLine("// No match");
+            writer.WriteLine("// No starting position found");
             writer.WriteLine("ReturnFalse:");
             writer.WriteLine("base.runtextpos = end;");
             writer.WriteLine("return false;");
@@ -368,8 +367,7 @@ bool EmitAnchors()
 
                         case RegexPrefixAnalyzer.Start:
                             writer.WriteLine("// Start \\G anchor");
-                            additionalDeclarations.Add("int start = base.runtextstart;");
-                            using (EmitBlock(writer, "if (pos > start)"))
+                            using (EmitBlock(writer, "if (pos > base.runtextstart)"))
                             {
                                 writer.WriteLine("goto ReturnFalse;");
                             }
@@ -400,6 +398,7 @@ bool EmitAnchors()
                             // the other anchors, which all skip all subsequent processing if found, with BOL we just use it
                             // to boost our position to the next line, and then continue normally with any searches.
                             writer.WriteLine("// Beginning-of-line anchor");
+                            additionalDeclarations.Add("global::System.ReadOnlySpan<char> inputSpan = base.runtext;");
                             additionalDeclarations.Add("int beginning = base.runtextbeg;");
                             using (EmitBlock(writer, "if (pos > beginning && inputSpan[pos - 1] != '\\n')"))
                             {
@@ -418,8 +417,8 @@ bool EmitAnchors()
                 return false;
             }
 
-            // Emits a case-sensitive left-to-right prefix search for a string at the beginning of the pattern.
-            void EmitIndexOf_LeftToRight(string prefix)
+            // Emits a case-sensitive prefix search for a string at the beginning of the pattern.
+            void EmitIndexOf(string prefix)
             {
                 writer.WriteLine($"int i = global::System.MemoryExtensions.IndexOf(inputSpan.Slice(pos, end - pos), {Literal(prefix)});");
                 writer.WriteLine("if (i >= 0)");
@@ -429,9 +428,9 @@ void EmitIndexOf_LeftToRight(string prefix)
                 writer.WriteLine("}");
             }
 
-            // Emits a left-to-right search for a set at a fixed position from the start of the pattern,
+            // Emits a search for a set at a fixed position from the start of the pattern,
             // and potentially other sets at other fixed positions in the pattern.
-            void EmitFixedSet_LeftToRight()
+            void EmitFixedSet()
             {
                 List<(char[]? Chars, string Set, int Distance, bool CaseInsensitive)>? sets = code.FindOptimizations.FixedDistanceSets;
                 (char[]? Chars, string Set, int Distance, bool CaseInsensitive) primarySet = sets![0];
@@ -600,15 +599,14 @@ private static void EmitGo(IndentedTextWriter writer, RegexMethod rm, string id)
 
             RegexOptions options = (RegexOptions)rm.Options;
             RegexCode code = rm.Code;
-            bool hasTimeout = false;
 
             // Helper to define names.  Names start unadorned, but as soon as there's repetition,
             // they begin to have a numbered suffix.
             var usedNames = new Dictionary<string, int>();
 
             // Every RegexTree is rooted in the implicit Capture for the whole expression.
             // Skip the Capture node. We handle the implicit root capture specially.
-            RegexNode node = rm.Code.Tree.Root;
+            RegexNode node = code.Tree.Root;
             Debug.Assert(node.Type == RegexNode.Capture, "Every generated tree should begin with a capture node");
             Debug.Assert(node.ChildCount() == 1, "Capture nodes should have one child");
             node = node.Child(0);
@@ -635,9 +633,8 @@ private static void EmitGo(IndentedTextWriter writer, RegexMethod rm, string id)
             }
 
             // In some cases, we need to emit declarations at the beginning of the method, but we only discover we need them later.
-            // To handle that, we emit a placeholder value that's not valid C#, and then at the end of the code generation we either
-            // delete it if no additional declarations are required, or we replace it with the list of additional declarations
-            // built up while generating code.
+            // To handle that, we build up a collection of all the declarations to include, track where they should be inserted,
+            // and then insert them at that position once everything else has been output.
             var additionalDeclarations = new HashSet<string>();
             var additionalLocalFunctions = new Dictionary<string, string[]>();
 
@@ -646,14 +643,11 @@ private static void EmitGo(IndentedTextWriter writer, RegexMethod rm, string id)
             writer.WriteLine("global::System.ReadOnlySpan<char> inputSpan = base.runtext;");
             writer.WriteLine("int pos = base.runtextpos, end = base.runtextend;");
             writer.WriteLine($"int original_pos = pos;");
-            hasTimeout = EmitLoopTimeoutCounterIfNeeded(writer, rm);
+            bool hasTimeout = EmitLoopTimeoutCounterIfNeeded(writer, rm);
+            bool hasTextInfo = EmitInitializeCultureForGoIfNecessary(writer, rm);
             writer.Flush();
             int additionalDeclarationsPosition = ((StringWriter)writer.InnerWriter).GetStringBuilder().Length;
             int additionalDeclarationsIndent = writer.Indent;
-            writer.WriteLine();
-
-            // TextInfo textInfo = CultureInfo.CurrentCulture.TextInfo; // only if the whole expression or any subportion is ignoring case, and we're not using invariant
-            bool hasTextInfo = EmitInitializeCultureForGoIfNecessary(writer, rm);
 
             // The implementation tries to use const indexes into the span wherever possible, which we can do
             // for all fixed-length constructs.  In such cases (e.g. single chars, repeaters, strings, etc.)
@@ -936,8 +930,7 @@ void EmitAllBranches()
 
                     // Save off pos.  We'll need to reset this each time a branch fails.
                     string startingPos = ReserveName("alternation_starting_pos");
-                    additionalDeclarations.Add($"int {startingPos} = 0;");
-                    writer.WriteLine($"{startingPos} = pos;");
+                    writer.WriteLine($"int {startingPos} = pos;");
                     int startingSliceStaticPos = sliceStaticPos;
 
                     // We need to be able to undo captures in two situations:
@@ -964,8 +957,7 @@ void EmitAllBranches()
                     if (expressionHasCaptures && ((node.Options & RegexNode.HasCapturesFlag) != 0 || !isAtomic))
                     {
                         startingCapturePos = ReserveName("alternation_starting_capturepos");
-                        additionalDeclarations.Add($"int {startingCapturePos} = 0;");
-                        writer.WriteLine($"{startingCapturePos} = base.Crawlpos();");
+                        writer.WriteLine($"int {startingCapturePos} = base.Crawlpos();");
                     }
                     writer.WriteLine();
 
@@ -1211,7 +1203,7 @@ void EmitBackreferenceConditional(RegexNode node)
                 // to backtrack to.  So, we expose a single Backtrack label and track which branch was
                 // followed in this resumeAt local.
                 string resumeAt = ReserveName("conditionalbackreference_branch");
-                additionalDeclarations.Add($"int {resumeAt} = 0;");
+                writer.WriteLine($"int {resumeAt} = 0;");
 
                 // While it would be nicely readable to use an if/else block, if the branches contain
                 // anything that triggers backtracking, labels will end up being defined, and if they're
@@ -1340,7 +1332,12 @@ void EmitExpressionConditional(RegexNode node)
                 {
                     startingCapturePos = ReserveName("conditionalexpression_starting_capturepos");
                     writer.WriteLine($"int {startingCapturePos} = base.Crawlpos();");
-                    writer.WriteLine();
+                }
+
+                string resumeAt = ReserveName("conditionalexpression_resumeAt");
+                if (!isAtomic)
+                {
+                    writer.WriteLine($"int {resumeAt} = 0;");
                 }
 
                 // Emit the conditional expression.  We need to reroute any match failures to either the "no" branch
@@ -1353,13 +1350,7 @@ void EmitExpressionConditional(RegexNode node)
                 {
                     doneLabel = originalDoneLabel;
                 }
-
                 string postConditionalDoneLabel = doneLabel;
-                string resumeAt = ReserveName("conditionalexpression_resumeAt");
-                if (!isAtomic)
-                {
-                    additionalDeclarations.Add($"int {resumeAt} = 0;");
-                }
 
                 // If we get to this point of the code, the conditional successfully matched, so run the "yes" branch.
                 // Since the "yes" branch may have a different execution path than the "no" branch or the lack of
@@ -1370,10 +1361,6 @@ void EmitExpressionConditional(RegexNode node)
                 writer.WriteLine();
                 TransferSliceStaticPosToPos(); // ensure all subsequent code sees the same sliceStaticPos value by setting it to 0
                 string postYesDoneLabel = doneLabel;
-                if (!isAtomic && postYesDoneLabel != originalDoneLabel)
-                {
-                    writer.WriteLine($"{resumeAt} = 0;");
-                }
                 if (postYesDoneLabel != originalDoneLabel || noBranch is not null)
                 {
                     writer.WriteLine($"goto {end};");
@@ -1467,8 +1454,7 @@ void EmitCapture(RegexNode node, RegexNode? subsequent = null)
 
                 TransferSliceStaticPosToPos();
                 string startingPos = ReserveName("capture_starting_pos");
-                additionalDeclarations.Add($"int {startingPos} = 0;");
-                writer.WriteLine($"{startingPos} = pos;");
+                writer.WriteLine($"int {startingPos} = pos;");
                 writer.WriteLine();
 
                 RegexNode child = node.Child(0);
@@ -1604,7 +1590,8 @@ node.Type is RegexNode.Atomic or // atomic nodes by definition don't give up any
                              RegexNode.Oneloopatomic or RegexNode.Notoneloopatomic or RegexNode.Setloopatomic or // same for atomic loops
                              RegexNode.One or RegexNode.Notone or RegexNode.Set or // individual characters don't backtrack
                              RegexNode.Multi or // multiple characters don't backtrack
-                             RegexNode.Beginning or RegexNode.Start or RegexNode.End or RegexNode.EndZ or RegexNode.Boundary or RegexNode.NonBoundary or RegexNode.ECMABoundary or RegexNode.NonECMABoundary or // anchors don't backtrack
+                             RegexNode.Ref or // backreferences don't backtrack
+                             RegexNode.Beginning or RegexNode.Bol or RegexNode.Start or RegexNode.End or RegexNode.EndZ or RegexNode.Eol or RegexNode.Boundary or RegexNode.NonBoundary or RegexNode.ECMABoundary or RegexNode.NonECMABoundary or // anchors don't backtrack
                              RegexNode.Nothing or RegexNode.Empty or RegexNode.UpdateBumpalong // empty/nothing don't do anything
                 // Fixed-size repeaters of single characters or atomic don't backtrack
                 || node.Type is RegexNode.Oneloop or RegexNode.Notoneloop or RegexNode.Setloop or RegexNode.Onelazy or RegexNode.Notonelazy or RegexNode.Setlazy && node.M == node.N
@@ -1965,26 +1952,30 @@ void EmitAnchors(RegexNode node)
                         break;
 
                     case RegexNode.End:
-                        using (EmitBlock(writer, $"if ({sliceSpan}.Length > {sliceStaticPos})"))
+                        using (EmitBlock(writer, $"if ({IsSliceLengthGreaterThanSliceStaticPos()})"))
                         {
                             writer.WriteLine($"goto {doneLabel};");
                         }
                         break;
 
                     case RegexNode.EndZ:
-                        writer.WriteLine($"if ({sliceSpan}.Length - 1 > {sliceStaticPos} || ({sliceSpan}.Length > {sliceStaticPos} && {sliceSpan}[{sliceStaticPos}] != '\\n'))");
+                        writer.WriteLine($"if ({sliceSpan}.Length - 1 > {sliceStaticPos} || ({IsSliceLengthGreaterThanSliceStaticPos()} && {sliceSpan}[{sliceStaticPos}] != '\\n'))");
                         using (EmitBlock(writer, null))
                         {
                             writer.WriteLine($"goto {doneLabel};");
                         }
                         break;
 
                     case RegexNode.Eol:
-                        using (EmitBlock(writer, $"if ({sliceSpan}.Length > {sliceStaticPos} && {sliceSpan}[{sliceStaticPos}] != '\\n')"))
+                        using (EmitBlock(writer, $"if ({IsSliceLengthGreaterThanSliceStaticPos()} && {sliceSpan}[{sliceStaticPos}] != '\\n')"))
                         {
                             writer.WriteLine($"goto {doneLabel};");
                         }
                         break;
+
+                        string IsSliceLengthGreaterThanSliceStaticPos() =>
+                            sliceStaticPos == 0 ? $"!{sliceSpan}.IsEmpty" :
+                            $"{sliceSpan}.Length > {sliceStaticPos}";
                 }
             }
 
@@ -2222,8 +2213,7 @@ void EmitSingleCharLazy(RegexNode node, bool emitLengthChecksIfRequired = true)
                     maxIterations = $"{node.N - node.M}";
 
                     iterationCount = ReserveName("lazyloop_iteration");
-                    additionalDeclarations.Add($"int {iterationCount} = 0;");
-                    writer.WriteLine($"{iterationCount} = 0;");
+                    writer.WriteLine($"int {iterationCount} = 0;");
                 }
 
                 // Track the current crawl position.  Upon backtracking, we'll unwind any captures beyond this point.
@@ -2366,18 +2356,15 @@ void EmitLazy(RegexNode node)
                 string body = ReserveName("LazyLoopBody");
                 string endLoop = ReserveName("LazyLoopEnd");
 
-                additionalDeclarations.Add($"int {iterationCount} = 0, {startingPos} = 0, {sawEmpty} = 0;");
-                writer.WriteLine($"{iterationCount} = 0;");
-                writer.WriteLine($"{startingPos} = pos;");
-                writer.WriteLine($"{sawEmpty} = 0;");
-                writer.WriteLine();
+                writer.WriteLine($"int {iterationCount} = 0, {startingPos} = pos, {sawEmpty} = 0;");
 
                 // If the min count is 0, start out by jumping right to what's after the loop.  Backtracking
                 // will then bring us back in to do further iterations.
                 if (minIterations == 0)
                 {
                     writer.WriteLine($"goto {endLoop};");
                 }
+                writer.WriteLine();
 
                 // Iteration body
                 MarkLabel(body, emitSemicolon: false);
@@ -3279,7 +3266,7 @@ private static void ReplaceAdditionalDeclarations(IndentedTextWriter writer, Has
         {
             if (declarations.Count != 0)
             {
-                StringBuilder tmp = new StringBuilder().AppendLine();
+                var tmp = new StringBuilder();
                 foreach (string decl in declarations.OrderBy(s => s))
                 {
                     for (int i = 0; i < indent; i++)