[DebugInfo][InstrRef] Avoid an asan-crash caused by an untracked DBG_PHI operand

[jmorse]

Summary

An important part of the instruction referencing solution is that we identify all the registers that values move between before we then compute an SSA-like function from the machine code, and from the variable intrinsics. DBG_PHIs weren't causing all the subregisters of their operands to be tracked; this patch forces that to happen.

The practical implications were that not enough space is allocated for storing values when analysing the function -- asan will crash on the attached test case with an unpatched compiler. Non-asan llc's will produce a DBG_VALUE $noreg, where it should be $dil.

[Orlando]

Makes sense, LGTM

llvm/lib/CodeGen/LiveDebugValues/InstrRefBasedImpl.cpp

bool InstrRefBasedLDV::transferDebugPHI(MachineInstr &MI) {
   if (MO.isReg()) {
     // The value is whatever's currently in the register. Read and record it,
     // to be analysed later.
     Register Reg = MO.getReg();
     ValueIDNum Num = MTracker->readReg(Reg);
     auto PHIRec = DebugPHIRecord(
         {InstrNum, MI.getParent(), Num, MTracker->lookupOrTrackRegister(Reg)});
     DebugPHINumToValue.push_back(PHIRec);
+
+    // Subsequent register operations, or variable locations, might occur for
+    // any of the subregisters of this DBG_PHIs operand. Ensure that all
+    // registers aliasing this register are tracked.
+    for (MCRegAliasIterator RAI(MO.getReg(), TRI, true); RAI.isValid(); ++RAI)
+      MTracker->lookupOrTrackRegister(*RAI);
   } else {
     // The value is whatever's in this stack slot.
     assert(MO.isFI());
     unsigned FI = MO.getIndex();
 
     // If the stack slot is dead, then this was optimized away.
     // FIXME: stack slot colouring should account for slots that get merged.
     if (MFI->isDeadObjectIndex(FI))

llvm/test/DebugInfo/MIR/InstrRef/dbg-phi-subregister-location.mir

+# RUN: llc %s -run-pass=livedebugvalues -experimental-debug-variable-locations\
+# RUN:   -o - | FileCheck %s
+#
+# In the MIR below, there's an argument in the lowest byte of $edi. The debug
+# intrinsics correctly identify the value and where it becomes the variables
+# value, however a bug in InstrRefBasedLDV meant that not all subregisters of
+# DBG_PHI operands are tracked. That leads to the wrong DBG_VALUE location
+# being produced, and a crash under asan.
+#
+# CHECK-LABEL: name: foo
+# CHECK:       DBG_PHI $edi
+# CHECK-NEXT:  DBG_INSTR_REF 2, 0
+# CHECK-NEXT:  DBG_VALUE $dil
+--- |
+  ; ModuleID = 'out.ll'
+  source_filename = "out.ll"
+  target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+  target triple = "x86_64-scei-ps4"
+
+  @someglobal = external local_unnamed_addr global i8, align 1
+
+  define hidden void @foo(i1 zeroext %bar) !dbg !7 {
+  entry:
+    ret void, !dbg !13
+  }
+
+  declare void @llvm.dbg.value(metadata, metadata, metadata)
+
+  !llvm.dbg.cu = !{!0}
+  !llvm.module.flags = !{!3, !4, !5}
+  !llvm.ident = !{!6}
+
+  !0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug, enums: !2, splitDebugInlining: false, nameTableKind: None)
+  !1 = !DIFile(filename: "test.c", directory: "/tmp/out.c")
+  !2 = !{}
+  !3 = !{i32 7, !"Dwarf Version", i32 4}
+  !4 = !{i32 2, !"Debug Info Version", i32 3}
+  !5 = !{i32 1, !"wchar_size", i32 4}
+  !6 = !{!""}
+  !7 = distinct !DISubprogram(name: "foo", scope: !1, file: !1, line: 3, type: !8, scopeLine: 3, flags: DIFlagPrototyped, spFlags: DISPFlagDefinition, unit: !0, retainedNodes: !2)
+  !8 = !DISubroutineType(types: !9)
+  !9 = !{!10, !11, !11}
+  !10 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+  !11 = !DIBasicType(name: "long int", size: 64, encoding: DW_ATE_signed)
+  !12 = !DILocalVariable(name: "baz", arg: 2, scope: !7, file: !1, line: 3, type: !11)
+  !13 = !DILocation(line: 0, scope: !7)
+
+...
+---
+name:            foo
+alignment:       16
+tracksRegLiveness: true
+liveins:
+  - { reg: '$edi' }
+frameInfo:
+  maxAlignment:    1
+  maxCallFrameSize: 0
+debugValueSubstitutions:
+  - { srcinst: 2, srcop: 0, dstinst: 1, dstop: 0, subreg: 1 }
+machineFunctionInfo: {}
+body:             |
+  bb.0.entry:
+    liveins: $edi
+
+    DBG_PHI $edi, 1
+    DBG_INSTR_REF 2, 0, !12, !DIExpression(), debug-location !13
+    renamable $rax = MOV64rm $rip, 1, $noreg, target-flags(x86-gotpcrel) @someglobal, $noreg, debug-location !13 :: (load (s64) from got)
+    MOV8mr killed renamable $rax, 1, $noreg, 0, $noreg, renamable $dil, debug-location !13 :: (store (s8) into @someglobal)
+    RETQ debug-location !13
+
+...