improve line number merging for reflection tools (#42247)

We were representing some frames as discontiguous that were clearly the
same frame, but the detection logic for it was off-by-one.

Author: vtjnash

base/compiler/ssair/show.jl

@@ -382,7 +382,7 @@ function DILineInfoPrinter(linetable::Vector, showtypes::Bool=false)
                     # if so, drop all existing calls to it from the top of the context
                     # AND check if instead the context was previously printed that way
                     # but now has removed the recursive frames
-                    let method = method_name(context[nctx])
+                    let method = method_name(context[nctx]) # last matching frame
                         if (nctx < nframes && method_name(DI[nframes - nctx]) === method) ||
                            (nctx < length(context) && method_name(context[nctx + 1]) === method)
                             update_line_only = true
@@ -391,8 +391,15 @@ function DILineInfoPrinter(linetable::Vector, showtypes::Bool=false)
                             end
                         end
                     end
-                elseif length(context) > 0
-                    update_line_only = true
+                end
+                # look at the first non-matching element to see if we are only changing the line number
+                if !update_line_only && nctx < length(context) && nctx < nframes
+                    let CtxLine = context[nctx + 1],
+                        FrameLine = DI[nframes - nctx]
+                        if method_name(CtxLine) === method_name(FrameLine)
+                            update_line_only = true
+                        end
+                    end
                 end
             elseif nctx < length(context) && nctx < nframes
                 # look at the first non-matching element to see if we are only changing the line number

src/disasm.cpp

@@ -229,17 +229,21 @@ void DILineInfoPrinter::emit_lineinfo(raw_ostream &Out, std::vector<DILineInfo>
             // if so, drop all existing calls to it from the top of the context
             // AND check if instead the context was previously printed that way
             // but now has removed the recursive frames
-            StringRef method = StringRef(context.at(nctx - 1).FunctionName).rtrim(';');
+            StringRef method = StringRef(context.at(nctx - 1).FunctionName).rtrim(';'); // last matching frame
             if ((nctx < nframes && StringRef(DI.at(nframes - nctx - 1).FunctionName).rtrim(';') == method) ||
                 (nctx < context.size() && StringRef(context.at(nctx).FunctionName).rtrim(';') == method)) {
                 update_line_only = true;
-                while (nctx > 0 && StringRef(context.at(nctx - 1).FunctionName).rtrim(';') == method) {
+                // transform nctx to exclude the combined frames
+                while (nctx > 0 && StringRef(context.at(nctx - 1).FunctionName).rtrim(';') == method)
                     nctx -= 1;
-                }
             }
         }
-        else if (context.size() > 0) {
-            update_line_only = true;
+        if (!update_line_only && nctx < context.size() && nctx < nframes) {
+            // look at the first non-matching element to see if we are only changing the line number
+            const DILineInfo &CtxLine = context.at(nctx);
+            const DILineInfo &FrameLine = DI.at(nframes - 1 - nctx);
+            if (StringRef(CtxLine.FunctionName).rtrim(';') == StringRef(FrameLine.FunctionName).rtrim(';'))
+                update_line_only = true;
         }
     }
     else if (nctx < context.size() && nctx < nframes) {