[DebugInfo][InstrRef] Permit and track variable assignments in out-of-scope blocks

[jmorse]

Summary

Allow and track variable assignments that happen in blocks that are out-of-scope. This happens when a DBG_INSTR_REF or DBG_VALUE gets moved to a block where no instructions are in its lexical scope, or the instructions in-scope are moved / merged so that the block is no longer in scope.

What to do in the general case is arguable -- LiveDebugValues already loses some variable locations due to its scope trimming, see PR48091 -- but the scope trimming is necessary for performance reasons. There's already a test though, llvm/test/DebugInfo/X86/stack-value-dwarf2.ll , that depends on assignments in out-of-scope blocks being preserved, so lets preserved VarLoc LiveDebugValues' behaviour for now. I ran it through the compile-time-tracker [0] and its performance effects are fractional.

[0] http://llvm-compile-time-tracker.com/compare.php?from=83be083d8567316ff2dd6e9651009cc2d615a69a&to=fbcd0356ed20c959cba82b1d710d97ef40b7e547&stat=instructions

[djtodoro]

lgtm

llvm/lib/CodeGen/LiveDebugValues/InstrRefBasedImpl.cpp

void InstrRefBasedLDV::buildVLocValueMap(const DILocation *DILoc,
   };
 
   LS.getMachineBasicBlocks(DILoc, BlocksToExplore);
 
   // A separate container to distinguish "blocks we're exploring" versus
   // "blocks that are potentially in scope. See comment at start of vlocJoin.
   SmallPtrSet<const MachineBasicBlock *, 8> InScopeBlocks = BlocksToExplore;
 
-  // Old LiveDebugValues tracks variable locations that come out of blocks
-  // not in scope, where DBG_VALUEs occur. This is something we could
-  // legitimately ignore, but lets allow it for now.
-  if (EmulateOldLDV)
-    BlocksToExplore.insert(AssignBlocks.begin(), AssignBlocks.end());
+  // VarLoc LiveDebugValues tracks variable locations that are defined in
+  // blocks not in scope. This is something we could legitimately ignore, but
+  // lets allow it for now for the sake of coverage.
 
   // We also need to propagate variable values through any artificial blocks
   // that immediately follow blocks in scope.
   DenseSet<const MachineBasicBlock *> ToAdd;
 
   // Helper lambda: For a given block in scope, perform a depth first search
   // of all the artificial successors, adding them to the ToAdd collection.
   auto AccumulateArtificialBlocks =

llvm/test/DebugInfo/X86/stack-value-dwarf2.ll

-; RUN: llc -filetype=obj -o - %s -experimental-debug-variable-locations=false | llvm-dwarfdump -debug-loc - | FileCheck %s
+; RUN: llc -filetype=obj -o - %s -experimental-debug-variable-locations=true | llvm-dwarfdump -debug-loc - | FileCheck %s
 
 ; Note that it would be even better to avoid emitting the empty piece.
 ; CHECK:  {{.*}}: DW_OP_piece 0x8{{$}}
 
 source_filename = "bugpoint-reduced-simplified.ll"
 target triple = "i386-apple-ios7.0.0"
 
 %class.K = type { %class.N, %struct.F, i32 }