Index: llvm/lib/Analysis/IRSimilarityIdentifier.cpp
===================================================================
--- llvm/lib/Analysis/IRSimilarityIdentifier.cpp
+++ llvm/lib/Analysis/IRSimilarityIdentifier.cpp
@@ -1039,8 +1039,9 @@
     // If the basic block is the starting block, then the shared instruction may
     // not be the first instruction in the block, it will be the first
     // instruction in the similarity region.
-    Value *FirstOutlineInst =
-        BB == getStartBB() ? frontInstruction() : &BB->front();
+    Value *FirstOutlineInst = BB == getStartBB()
+                                  ? frontInstruction()
+                                  : &*BB->instructionsWithoutDebug().begin();
 
     unsigned FirstInstGVN = *getGVN(FirstOutlineInst);
     unsigned FirstInstCanonNum = *getCanonicalNum(FirstInstGVN);
Index: llvm/test/Analysis/IRSimilarityIdentifier/debug-inst-starts-block.ll
===================================================================
--- /dev/null
+++ llvm/test/Analysis/IRSimilarityIdentifier/debug-inst-starts-block.ll
@@ -0,0 +1,74 @@
+; RUN: opt -disable-output -S -passes=print-ir-similarity < %s 2>&1 | FileCheck %s
+
+; When a debug instruction is the first instruction in a block, when that block
+; has not been given a canonical numbering, since debug instructions are not
+; counted in similarity matching they must be ignored when creating canonical
+; relations from one region to another.  This checks that this is enforced.
+
+; CHECK: 2 candidates of length 3.  Found in: 
+; CHECK-NEXT:   Function: main, Basic Block: entry
+; CHECK-NEXT:     Start Instruction:   br label %for.body169
+; CHECK-NEXT:       End Instruction:   %1 = sub i32 1, 4
+; CHECK-NEXT:   Function: main, Basic Block: for.body169
+; CHECK-NEXT:     Start Instruction:   br label %for.end122
+; CHECK-NEXT:       End Instruction:   %3 = sub i32 1, 4
+; CHECK-NEXT: 2 candidates of length 2.  Found in: 
+; CHECK-NEXT:   Function: main, Basic Block: for.end122
+; CHECK-NEXT:     Start Instruction:   store i32 30, ptr undef, align 1
+; CHECK-NEXT:       End Instruction:   %1 = sub i32 1, 4
+; CHECK-NEXT:   Function: main, Basic Block: for.end246
+; CHECK-NEXT:     Start Instruction:   store i32 0, ptr undef, align 1
+; CHECK-NEXT:       End Instruction:   %3 = sub i32 1, 4
+; CHECK-NEXT: 2 candidates of length 4.  Found in: 
+; CHECK-NEXT:   Function: main, Basic Block: entry
+; CHECK-NEXT:     Start Instruction:   %0 = add i32 1, 4
+; CHECK-NEXT:       End Instruction:   %1 = sub i32 1, 4
+; CHECK-NEXT:   Function: main, Basic Block: for.body169
+; CHECK-NEXT:     Start Instruction:   %2 = add i32 1, 4
+; CHECK-NEXT:       End Instruction:   %3 = sub i32 1, 4
+
+source_filename = "irsimilarity_crash.ll"
+
+@v_13 = external dso_local global ptr, align 1
+
+; Function Attrs: nocallback nofree nosync nounwind readnone speculatable willreturn
+declare void @llvm.dbg.declare(metadata, metadata, metadata) #0
+
+define dso_local i16 @main() {
+entry:
+  %0 = add i32 1, 4
+  br label %for.body169
+
+for.end122:                                       ; preds = %for.cond108
+  store i32 30, ptr undef, align 1
+  %1 = sub i32 1, 4
+  ret i16 1
+
+for.body169:                                      ; preds = %for.cond167
+  %2 = add i32 1, 4
+  br label %for.end122
+
+for.end246:                                     ; preds = %for.cond167
+  call void @llvm.dbg.declare(metadata ptr undef, metadata !1, metadata !DIExpression()), !dbg !11
+  store i32 0, ptr undef, align 1
+  %3 = sub i32 1, 4
+  unreachable
+}
+
+attributes #0 = { nocallback nofree nosync nounwind readnone speculatable willreturn }
+
+!llvm.dbg.cu = !{}
+!llvm.module.flags = !{!0}
+
+!0 = !{i32 2, !"Debug Info Version", i32 3}
+!1 = !DILocalVariable(name: "v_68", scope: !2, file: !3, line: 522, type: !10)
+!2 = distinct !DILexicalBlock(scope: !4, file: !3, line: 522, column: 9)
+!3 = !DIFile(filename: "41097217.c", directory: "rt.outdir")
+!4 = distinct !DISubprogram(name: "main", scope: !3, file: !3, line: 480, type: !5, scopeLine: 481, spFlags: DISPFlagDefinition, unit: !8, retainedNodes: !9)
+!5 = !DISubroutineType(types: !6)
+!6 = !{!7}
+!7 = !DIBasicType(name: "int", size: 16, encoding: DW_ATE_signed)
+!8 = distinct !DICompileUnit(language: DW_LANG_C99, file: !3, producer: "clang version 15.0.0.prerel", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug, retainedTypes: !9, globals: !9, splitDebugInlining: false, nameTableKind: None)
+!9 = !{}
+!10 = !DIBasicType(name: "long", size: 32, encoding: DW_ATE_signed)
+!11 = !DILocation(line: 522, column: 23, scope: !2)