LiveDebugValues: Fix another crash related to unreachable blocks

[aprantl]

Summary

This is a follow-up patch to D130999. In the test, the MIR contains an unreachable MBB but the code attempts to look it up in MLocs. This patch fixes this issue by checking for the default-constructed value.

[jryans]

Thanks for the fix, looks good to me! 😄

[jmorse]

Fix and test LGTM, much appreciated.

An alternative might be to identify unreachable blocks in depthFirstVLocAndEmit's EjectBlock lambda, and not do any work for them. It might be non-trivial to identify unreachable blocks, so this fix is fine.

llvm/lib/CodeGen/LiveDebugValues/InstrRefBasedImpl.cpp

loadInlocs(MachineBasicBlock &MBB, ValueTable &MLocs,
     ActiveVLocs.reserve(VLocs.size());
 
     // Produce a map of value numbers to the current machine locs they live
     // in. When emulating VarLocBasedImpl, there should only be one
     // location; when not, we get to pick.
     for (auto Location : MTracker->locations()) {
       LocIdx Idx = Location.Idx;
       ValueIDNum &VNum = MLocs[Idx.asU64()];
+      if (VNum == ValueIDNum::EmptyValue)
+        continue;
       VarLocs.push_back(VNum);
 
       // Is there a variable that wants a location for this value? If not, skip.
       auto VIt = ValueToLoc.find(VNum);
       if (VIt == ValueToLoc.end())
         continue;
 
       LocIdx CurLoc = VIt->second;

llvm/test/DebugInfo/MIR/X86/ldv_unreachable_blocks2.mir

+# RUN: llc -mtriple=x86_64-apple-macos %s -start-before=livedebugvalues -filetype=obj -o - | llvm-dwarfdump - | FileCheck %s
+# CHECK: DW_TAG_subprogram
+# Test that LiveDebugValues can handle MBBs that are not reachable in a RPOT.
+--- |
+  define hidden zeroext i1 @__foo_block_invoke(i8* nocapture noundef readonly %.block_descriptor, i64 noundef %type) !dbg !7 {
+  entry:
+    unreachable
+  do.body.i129.i:                                   ; preds = %if.else6.i128.i
+    unreachable
+  }
+  !llvm.dbg.cu = !{!3}
+  !llvm.module.flags = !{!5, !6}
+  !2 = !{}
+  !3 = distinct !DICompileUnit(language: DW_LANG_C99, file: !4, producer: "clang", runtimeVersion: 0, emissionKind: FullDebug)
+  !4 = !DIFile(filename: "t.c", directory: "/")
+  !5 = !{i32 2, !"Debug Info Version", i32 3}
+  !6 = !{i32 1, !"LTOPostLink", i32 1}
+  !7 = distinct !DISubprogram(name: "__foo_block_invoke", scope: !4, file: !4, line: 573, type: !9, scopeLine: 573, flags: DIFlagPrototyped, spFlags: DISPFlagLocalToUnit | DISPFlagDefinition | DISPFlagOptimized, unit: !3, retainedNodes: !2)
+  !9 = !DISubroutineType(types: !2)
+  !11 = !DILocalVariable(name: ".block_descriptor", arg: 1, scope: !7, file: !4, line: 557, type: !12, flags: DIFlagArtificial)
+  !12 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !13, size: 64)
+  !13 = !DICompositeType(tag: DW_TAG_structure_type, name: "__block_literal_1", scope: !4, file: !4, line: 557, size: 392, elements: !2)
+  !15 = !DILocation(line: 558, column: 7, scope: !7)
+name:            __foo_block_invoke
+body:             |
+  bb.0.entry:
+    DBG_VALUE $rdi, $noreg, !11, !DIExpression(), debug-location !15
+    $ecx = MOV32rr undef $edi, implicit $dil, debug-location !15
+  bb.2:
+    successors: 
+    TRAP
+  bb.21.do.body.i129.i (address-taken):