[DebugInfo] LiveDebugValues: explicitly terminate overwritten stack locations

[jmorse]

Summary

Currently, if a stack spill location is overwritten to by another spill instruction, nothing notices and any variables located in that slot are not terminated. This can lead to incorrect variable values being reported by debuggers, see PR42772 [0]. The test added replicates this scenario. We should notice this memory-clobber and terminate variable locations when it happens.

Scanning for clobbered spill locations happens before scanning for new spills/restores, terminates open ranges and adds DBG_VALUE $noregs. The early scanning is because the current spill/restore logic appears to quit after moving one location, even if there could be multiple candidates, and I don't want to go near that right now. One function gets refactored into two: the old isSpillInstruction performed a few register liveness tests to see whether an instruction _looked_ like a spill, wheras all we're interested in for terminating existing locations is whether there's a stack write at all.

Note that this creates some slight differences between how registers and spill locations are treated: right now DbgEntityHistoryCalculator will spot register clobbers and terminate any variable locations that have been overwritten, thus manually inserting DBG_VALUE $noreg isn't necessary for registers in LiveDebugValues. IMO it's much harder for DbgEntityHistoryCalculator to spot memory/stack clobbers (it would have to dig into DIExpressions) wheras in LiveDebugValues we already have that information to hand.

I've left quite a lot of machine-code in the MIR test where it could be simpler; IMHO it's good to have a slightly larger test here just in case LiveDebugValues tries to do something clever in the future and gets it wrong.

There's a fractional increase in scope-bytes-covered by 0.05%, it's not completely clear to me why unfortunately D:

[0] https://bugs.llvm.org/show_bug.cgi?id=42772

[aprantl]

Currently, if a stack spill location is overwritten to by another spill instruction, nothing notices and any variables located in that slot are not terminated.

Are all DBG_VALUEs pointing to spill slots created by LiveDebugValues and is this indicative of a bug elsewhere, i.e., should we instead make sure to insert a DBG_VALUE undef, var right before the value is popped from the spill slot (followed by a DBG_VALUE pointing to the restored register)?

lib/CodeGen/LiveDebugValues.cpp

367

out of curiosity: is @MI legal doxygen?

878–904

any previous DBG_VALUEs pointing to the spill slot? or does this mean something else?

[jmorse]

In D66941#1651703, @aprantl wrote:

Are all DBG_VALUEs pointing to spill slots created by LiveDebugValues and is this indicative of a bug elsewhere,

LiveDebugVariables also creates DBG_VALUEs that point at spill slots. However, I think that's a feature, because a variable location may move to a spill slot because of a DBG_VALUE of a vreg that's already been spilt. i.e.,

CALL foo
DBG_VALUE %0
CALL bar

gets regalloc'd to

MOV %stack.0, $rax ; store to spill slot
CALL foo
DBG_VALUE %stack.0
CALL bar

This actually leads to all kinds of trouble (I've got a patch cooking about this), because we bake all information about the spill location into DIExpressions after the prologepilog pass eliminates stack-frame references. LiveDebugValues currently interprets any spill location created by LiveDebugVariables as being a register location with a complex expression, rather than being a spill location. Fixing this leads to some interesting choices, I'm currently writing that up.

There's also dbg.addr intrinsics to consider. These are special because they currently present as normal indirect DBG_VALUE instructions, but don't actually want to be terminated when their stack slot is written to. I haven't thought about how to represent that yet, but AFAIUI dbg.addrs are rarely produced right now, and terminating them early is better than allowing invalid locations to continue.

should we instead make sure to insert a DBG_VALUE undef, var right before the value is popped from the spill slot (followed by a DBG_VALUE pointing to the restored register)?

I believe we're handling that operation accurately with the existing restore mechanism (at least, for spills created by LiveDebugValues). Although, that code only ever restores a single variable location (even if there are multiple variables located there), which needs a bit of investigation.

The problem this patch addresses is when the spill has gone out of liveness without being restored -- there's no record from the register allocator of when that happens, thus we have to look for overwrites. It would be hard to fix at a higher level too, because stack slots get merged after register allocation.

lib/CodeGen/LiveDebugValues.cpp

367

Dunno, I think it might be javadoc that doxygen swallows? Either way it appears from the live docs on llvm.org that doxygen produces no documentation for the LiveDebugValues pass, possibly because there's no class level comment? (I haven't dug into this, only moved the comment block).

878–904

Ah yeah, that should be something like "First, if there are any DBG_VALUEs pointing to a spill slot that is written to..."

[jmorse]

Update a comment, we don't terminate variables, instead we close locations.

[aprantl]

lib/CodeGen/LiveDebugValues.cpp

807

This doesn't appear to be used in the function?

889

One last question: Could/should we fix this in DbgEntityHistoryCalculator instead?

[jmorse]

Delete a vector that isn't used by anything.

[jmorse]

lib/CodeGen/LiveDebugValues.cpp

889

Overall it would make more sense to fix it in DbgEntityHistoryCalculator, that way the two logically similar things (working out when a location no longer contains the desired value) would happen in the same place. However, doing so would require parsing a DIExpression to work out where in the stack frame a DBG_VALUE points, which IMO is generally a bad plan.

My feeling is that creating some undef DBG_VALUEs here where the information is readily available is better than parsing a DIExpression at a later stage.

[aprantl]

I see we would have to do an abstract interpretation of a DIExpression together with the implicit register operand bound by the DBG_VALUE to computes all stack slots that my be touched by the expression, assuming we even know where non-frame-pointer registers point. Could you add an explanation to the comment that lays out why this isn't feasible?

[jmorse]

Expand a comment, explaining why we terminate overwritten spill-locations during LiveDebugValues, instead of leaving it until later.

lib/CodeGen/LiveDebugValues.cpp

public:
 
     /// Return whether the set is empty or not.
     bool empty() const {
       assert(Vars.empty() == VarLocs.empty() && "open ranges are inconsistent");
       return VarLocs.empty();
     }
   };
 
-  bool isSpillInstruction(const MachineInstr &MI, MachineFunction *MF,
-                          unsigned &Reg);
+  /// Tests whether this instruction is a spill to a stack location.
+  bool isSpillInstruction(const MachineInstr &MI, MachineFunction *MF);
+
+  /// Decide if @MI is a spill instruction and return true if it is. We use 2
+  /// criteria to make this decision:
+  /// - Is this instruction a store to a spill slot?
+  /// - Is there a register operand that is both used and killed?
+  /// TODO: Store optimization can fold spills into other stores (including
+  /// other spills). We do not handle this yet (more than one memory operand).
+  bool isLocationSpill(const MachineInstr &MI, MachineFunction *MF,
+
   /// If a given instruction is identified as a spill, return the spill location
   /// and set \p Reg to the spilled register.
   Optional<VarLoc::SpillLoc> isRestoreInstruction(const MachineInstr &MI,
                                                   MachineFunction *MF,
                                                   unsigned &Reg);
   /// Given a spill instruction, extract the register and offset used to
   /// address the spill location in a target independent way.
   VarLoc::SpillLoc extractSpillBaseRegAndOffset(const MachineInstr &MI);
void LiveDebugValues::transferRegisterDef(
   if (auto *TPC = getAnalysisIfAvailable<TargetPassConfig>()) {
     auto &TM = TPC->getTM<TargetMachine>();
     if (TM.Options.EnableDebugEntryValues)
       emitEntryValues(MI, OpenRanges, VarLocIDs, Transfers, DebugEntryVals,
                       KillSet);
   }
 }
 
-/// Decide if @MI is a spill instruction and return true if it is. We use 2
-/// criteria to make this decision:
-/// - Is this instruction a store to a spill slot?
-/// - Is there a register operand that is both used and killed?
-/// TODO: Store optimization can fold spills into other stores (including
-/// other spills). We do not handle this yet (more than one memory operand).
 bool LiveDebugValues::isSpillInstruction(const MachineInstr &MI,
-                                         MachineFunction *MF, unsigned &Reg) {
-  SmallVector<const MachineMemOperand*, 1> Accesses;
-
+                                         MachineFunction *MF) {
   // TODO: Handle multiple stores folded into one.
   if (!MI.hasOneMemOperand())
     return false;
 
   if (!MI.getSpillSize(TII) && !MI.getFoldedSpillSize(TII))
     return false; // This is not a spill instruction, since no valid size was
                   // returned from either function.
 
+  return true;
+}
+
+bool LiveDebugValues::isLocationSpill(const MachineInstr &MI,
+                                      MachineFunction *MF, unsigned &Reg) {
+  if (!isSpillInstruction(MI, MF))
+    return false;
+
   auto isKilledReg = [&](const MachineOperand MO, unsigned &Reg) {
     if (!MO.isReg() || !MO.isUse()) {
       Reg = 0;
       return false;
     }
     Reg = MO.getReg();
     return MO.isKill();
   };
void LiveDebugValues::transferSpillOrRestoreInst(MachineInstr &MI,
                                                  TransferMap &Transfers) {
   MachineFunction *MF = MI.getMF();
   TransferKind TKind;
   unsigned Reg;
   Optional<VarLoc::SpillLoc> Loc;
 
   LLVM_DEBUG(dbgs() << "Examining instruction: "; MI.dump(););
 
-  if (isSpillInstruction(MI, MF, Reg)) {
+  // First, if there are any DBG_VALUEs pointing at a spill slot that is
+  // written to, then close the variable location. The value in memory
+  // will have changed.
+  VarLocSet KillSet;
+  if (isSpillInstruction(MI, MF)) {
+    Loc = extractSpillBaseRegAndOffset(MI);
+    for (unsigned ID : OpenRanges.getVarLocs()) {
+      const VarLoc &VL = VarLocIDs[ID];
+      if (VL.Kind == VarLoc::SpillLocKind && VL.Loc.SpillLocation == *Loc) {
+        // This location is overwritten by the current instruction -- terminate
+        // the open range, and insert an explicit DBG_VALUE $noreg. Reg clobbers
+        // can be detected by DbgEntityHistoryCalculator, but stack writes are
+        // not.
+        KillSet.set(ID);
+        MachineInstr *NewDebugInstr =
+            BuildMI(*MF, VL.MI.getDebugLoc(), VL.MI.getDesc(),
+                    VL.MI.isIndirectDebugValue(), 0, // $noreg
+                    VL.MI.getDebugVariable(), VL.MI.getDebugExpression());
+        Transfers.push_back({&MI, NewDebugInstr});
+      }
+    }
+    OpenRanges.erase(KillSet, VarLocIDs);
+  }
+
+  // Try to recognise spill and restore instructions that may create a new
+  // variable location.
+  if (isLocationSpill(MI, MF, Reg)) {
     TKind = TransferKind::TransferSpill;
     LLVM_DEBUG(dbgs() << "Recognized as spill: "; MI.dump(););
     LLVM_DEBUG(dbgs() << "Register: " << Reg << " " << printReg(Reg, TRI)
                       << "\n");
   } else {
     if (!(Loc = isRestoreInstruction(MI, MF, Reg)))
       return;
     TKind = TransferKind::TransferRestore;

test/DebugInfo/MIR/X86/live-debug-values-stack-clobber.mir

+# RUN: llc -mtriple=x86_64-unknown-unknown %s -o - -run-pass=livedebugvalues | FileCheck %s
+#
+# Fix some of PR42772. Consider the code below: the arguments are forced onto
+# the stack by the FORCE_SPILL macro, and go out of liveness if the
+# "bees == 2" conditional is not taken. A spill slot is then re-used to
+# preserve quux over the second FORCE_SPILL, over-writing the value of either
+# "a" or "b". LiveDebugValues should detect when this happens, and terminate
+# stack-spill locations when they get overwritten by a new value.
+#
+# --------8<--------
+# #define FORCE_SPILL() \
+#   __asm volatile("" : : : \
+#                    "rax", "rbx", "rcx", "rdx", "rsi", "rdi", "rbp", "r8", \
+#                    "r9", "r10", "r11", "r12", "r13", "r14", "r15")
+# 
+# volatile int bees = 0;
+# 
+# long int f(long int a, long int b) {
+#   if (bees == 12)
+#    return 3;
+# 
+#   FORCE_SPILL();
+#   if (bees == 2)
+#     return a - b;
+# 
+#   int quux = sum(1, 2);
+#   FORCE_SPILL();
+#   bees++;
+#   return quux;
+# }
+# -------->8--------
+#
+# CHECK:       ![[ANUM:[0-9]+]] = !DILocalVariable(name: "a"
+# CHECK:       ![[BNUM:[0-9]+]] = !DILocalVariable(name: "b"
+#
+# These variables should be spilt,
+# CHECK-LABEL: bb.1.if.end:
+# CHECK:       MOV64mr $rsp, 1, $noreg, 16, $noreg, killed renamable $rsi
+# CHECK-NEXT:  DBG_VALUE $rsp, 0, ![[BNUM]], !DIExpression(
+# CHECK-NEXT:  MOV64mr $rsp, 1, $noreg, 8, $noreg, killed renamable $rdi
+# CHECK-NEXT:  DBG_VALUE $rsp, 0, ![[ANUM]], !DIExpression(
+# CHECK-NEXT:  INLINEASM
+#
+# Then the location of "a" should be terminated when overwritten
+# CHECK-LABEL: bb.3.if.end3:
+# CHECK:       CALL64pcrel32 @sum
+# CHECK-NEXT:  MOV64mr $rsp, 1, $noreg, 8, $noreg, $rax
+# CHECK-NEXT:  DBG_VALUE $noreg, $noreg, ![[ANUM]], !DIExpression()
+# CHECK-NEXT:  INLINEASM
+
+--- |
+  target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+  target triple = "x86_64-unknown-linux-gnu"
+  
+  @bees = external global i32, !dbg !0
+  
+  ; Function Attrs: noinline norecurse nounwind readnone uwtable
+  declare i64 @sum(i64, i64)
+  
+  ; Function Attrs: noinline nounwind uwtable
+  define i64 @f(i64 %a, i64 %b) !dbg !12 {
+  entry:
+    br label %if.end
+  if.end:
+    br label %if.then2
+  if.then2:
+    br label %if.end3
+  if.end3:
+    br label %return
+  return:
+    ret i64 0
+  }
+  
+  !llvm.dbg.cu = !{!2}
+  !llvm.module.flags = !{!8, !9, !10}
+  
+  !0 = !DIGlobalVariableExpression(var: !1, expr: !DIExpression())
+  !1 = distinct !DIGlobalVariable(name: "bees", scope: !2, file: !3, line: 6, type: !6, isLocal: false, isDefinition: true)
+  !2 = distinct !DICompileUnit(language: DW_LANG_C99, file: !3, producer: "clang", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !4, globals: !5, nameTableKind: None)
+  !3 = !DIFile(filename: "pr42772.c", directory: ".")
+  !4 = !{}
+  !5 = !{!0}
+  !6 = !DIDerivedType(tag: DW_TAG_volatile_type, baseType: !7)
+  !7 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+  !8 = !{i32 2, !"Dwarf Version", i32 4}
+  !9 = !{i32 2, !"Debug Info Version", i32 3}
+  !10 = !{i32 1, !"wchar_size", i32 4}
+  !12 = distinct !DISubprogram(name: "f", scope: !3, file: !3, line: 15, type: !13, scopeLine: 15, flags: DIFlagPrototyped, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !2, retainedNodes: !16)
+  !13 = !DISubroutineType(types: !14)
+  !14 = !{!15, !15, !15}
+  !15 = !DIBasicType(name: "long int", size: 64, encoding: DW_ATE_signed)
+  !16 = !{!17, !18, !19}
+  !17 = !DILocalVariable(name: "a", arg: 1, scope: !12, file: !3, line: 15, type: !15)
+  !18 = !DILocalVariable(name: "b", arg: 2, scope: !12, file: !3, line: 15, type: !15)
+  !19 = !DILocalVariable(name: "quux", scope: !12, file: !3, line: 23, type: !7)
+  !28 = !DILocation(line: 1, column: 1, scope: !12)
+
+...
+---
+name:            f
+tracksRegLiveness: true
+liveins:
+  - { reg: '$rdi', virtual-reg: '' }
+  - { reg: '$rsi', virtual-reg: '' }
+frameInfo:
+  stackSize:       72
+  offsetAdjustment: -72
+  maxAlignment:    8
+  adjustsStack:    true
+  hasCalls:        true
+  cvBytesOfCalleeSavedRegisters: 48
+fixedStack:
+  - { id: 0, type: spill-slot, offset: -56, size: 8, alignment: 8, stack-id: default, 
+      callee-saved-register: '$rbx', callee-saved-restored: true, debug-info-variable: '', 
+      debug-info-expression: '', debug-info-location: '' }
+  - { id: 1, type: spill-slot, offset: -48, size: 8, alignment: 16, stack-id: default, 
+      callee-saved-register: '$r12', callee-saved-restored: true, debug-info-variable: '', 
+      debug-info-expression: '', debug-info-location: '' }
+  - { id: 2, type: spill-slot, offset: -40, size: 8, alignment: 8, stack-id: default, 
+      callee-saved-register: '$r13', callee-saved-restored: true, debug-info-variable: '', 
+      debug-info-expression: '', debug-info-location: '' }
+  - { id: 3, type: spill-slot, offset: -32, size: 8, alignment: 16, stack-id: default, 
+      callee-saved-register: '$r14', callee-saved-restored: true, debug-info-variable: '', 
+      debug-info-expression: '', debug-info-location: '' }
+  - { id: 4, type: spill-slot, offset: -24, size: 8, alignment: 8, stack-id: default, 
+      callee-saved-register: '$r15', callee-saved-restored: true, debug-info-variable: '', 
+      debug-info-expression: '', debug-info-location: '' }
+  - { id: 5, type: spill-slot, offset: -16, size: 8, alignment: 16, stack-id: default, 
+      callee-saved-register: '$rbp', callee-saved-restored: true, debug-info-variable: '', 
+      debug-info-expression: '', debug-info-location: '' }
+stack:
+  - { id: 0, name: '', type: spill-slot, offset: -72, size: 8, alignment: 8, 
+      stack-id: default, callee-saved-register: '', callee-saved-restored: true, 
+      debug-info-variable: '', debug-info-expression: '', debug-info-location: '' }
+  - { id: 1, name: '', type: spill-slot, offset: -64, size: 8, alignment: 8, 
+      stack-id: default, callee-saved-register: '', callee-saved-restored: true, 
+      debug-info-variable: '', debug-info-expression: '', debug-info-location: '' }
+body:             |
+  bb.0.entry:
+    successors: %bb.4, %bb.1
+    liveins: $rdi, $rsi, $rbp, $r15, $r14, $r13, $r12, $rbx
+  
+    DBG_VALUE $rdi, $noreg, !17, !DIExpression(), debug-location !28
+    DBG_VALUE $rdi, $noreg, !17, !DIExpression(), debug-location !28
+    DBG_VALUE $rsi, $noreg, !18, !DIExpression(), debug-location !28
+    DBG_VALUE $rsi, $noreg, !18, !DIExpression(), debug-location !28
+    frame-setup PUSH64r killed $rbp, implicit-def $rsp, implicit $rsp, debug-location !28
+    frame-setup PUSH64r killed $r15, implicit-def $rsp, implicit $rsp, debug-location !28
+    frame-setup PUSH64r killed $r14, implicit-def $rsp, implicit $rsp, debug-location !28
+    frame-setup PUSH64r killed $r13, implicit-def $rsp, implicit $rsp, debug-location !28
+    frame-setup PUSH64r killed $r12, implicit-def $rsp, implicit $rsp, debug-location !28
+    frame-setup PUSH64r killed $rbx, implicit-def $rsp, implicit $rsp, debug-location !28
+    $rsp = frame-setup SUB64ri8 $rsp, 24, implicit-def dead $eflags
+    renamable $ecx = MOV32rm $rip, 1, $noreg, @bees, $noreg, debug-location !28 :: (volatile dereferenceable load 4 from @bees)
+    $eax = MOV32ri 3, implicit-def $rax
+    CMP32ri8 killed renamable $ecx, 12, implicit-def $eflags, debug-location !28
+    JCC_1 %bb.4, 4, implicit $eflags, debug-location !28
+  
+  bb.1.if.end:
+    successors: %bb.2, %bb.3
+    liveins: $rdi, $rsi
+  
+    MOV64mr $rsp, 1, $noreg, 16, $noreg, killed renamable $rsi :: (store 8 into %stack.1)
+    MOV64mr $rsp, 1, $noreg, 8, $noreg, killed renamable $rdi :: (store 8 into %stack.0)
+    INLINEASM &"", 1, 12, implicit-def dead early-clobber $rax, 12, implicit-def dead early-clobber $rbx, 12, implicit-def dead early-clobber $rcx, 12, implicit-def dead early-clobber $rdx, 12, implicit-def dead early-clobber $rsi, 12, implicit-def dead early-clobber $rdi, 12, implicit-def dead early-clobber $rbp, 12, implicit-def dead early-clobber $r8, 12, implicit-def dead early-clobber $r9, 12, implicit-def dead early-clobber $r10, 12, implicit-def dead early-clobber $r11, 12, implicit-def dead early-clobber $r12, 12, implicit-def dead early-clobber $r13, 12, implicit-def dead early-clobber $r14, 12, implicit-def dead early-clobber $r15, 12, implicit-def dead early-clobber $df, 12, implicit-def dead early-clobber $fpsw, 12, implicit-def dead early-clobber $eflags, debug-location !28
+    renamable $eax = MOV32rm $rip, 1, $noreg, @bees, $noreg, debug-location !28 :: (volatile dereferenceable load 4 from @bees)
+    CMP32ri8 killed renamable $eax, 2, implicit-def $eflags, debug-location !28
+    JCC_1 %bb.3, 5, implicit killed $eflags, debug-location !28
+  
+  bb.2.if.then2:
+    successors: %bb.4
+  
+    renamable $rax = MOV64rm $rsp, 1, $noreg, 8, $noreg :: (load 8 from %stack.0)
+    renamable $rax = SUB64rm killed renamable $rax, $rsp, 1, $noreg, 16, $noreg, implicit-def dead $eflags, debug-location !28 :: (load 8 from %stack.1)
+    JMP_1 %bb.4
+  
+  bb.3.if.end3:
+    successors: %bb.4
+  
+    $edi = MOV32ri 1, implicit-def $rdi, debug-location !28
+    $esi = MOV32ri 2, implicit-def $rsi, debug-location !28
+    CALL64pcrel32 @sum, csr_64, implicit $rsp, implicit $ssp, implicit $rdi, implicit $rsi, implicit-def $rsp, implicit-def $ssp, implicit-def $rax, debug-location !28
+    MOV64mr $rsp, 1, $noreg, 8, $noreg, $rax :: (store 8 into %stack.0)
+    INLINEASM &"", 1, 12, implicit-def dead early-clobber $rax, 12, implicit-def dead early-clobber $rbx, 12, implicit-def dead early-clobber $rcx, 12, implicit-def dead early-clobber $rdx, 12, implicit-def dead early-clobber $rsi, 12, implicit-def dead early-clobber $rdi, 12, implicit-def dead early-clobber $rbp, 12, implicit-def dead early-clobber $r8, 12, implicit-def dead early-clobber $r9, 12, implicit-def dead early-clobber $r10, 12, implicit-def dead early-clobber $r11, 12, implicit-def dead early-clobber $r12, 12, implicit-def dead early-clobber $r13, 12, implicit-def dead early-clobber $r14, 12, implicit-def dead early-clobber $r15, 12, implicit-def dead early-clobber $df, 12, implicit-def dead early-clobber $fpsw, 12, implicit-def dead early-clobber $eflags, debug-location !28
+    ADD32mi8 $rip, 1, $noreg, @bees, $noreg, 1, implicit-def dead $eflags, debug-location !28 :: (volatile store 4 into @bees), (volatile dereferenceable load 4 from @bees)
+    renamable $rax = MOVSX64rm32 $rsp, 1, $noreg, 8, $noreg, debug-location !28 :: (load 4 from %stack.0, align 8)
+  
+  bb.4.return:
+    liveins: $rax
+  
+    $rsp = frame-destroy ADD64ri8 $rsp, 24, implicit-def dead $eflags, debug-location !28
+    $rbx = frame-destroy POP64r implicit-def $rsp, implicit $rsp, debug-location !28
+    $r12 = frame-destroy POP64r implicit-def $rsp, implicit $rsp, debug-location !28
+    $r13 = frame-destroy POP64r implicit-def $rsp, implicit $rsp, debug-location !28
+    $r14 = frame-destroy POP64r implicit-def $rsp, implicit $rsp, debug-location !28
+    $r15 = frame-destroy POP64r implicit-def $rsp, implicit $rsp, debug-location !28
+    $rbp = frame-destroy POP64r implicit-def $rsp, implicit $rsp, debug-location !28
+    RETQ $rax, debug-location !28
+
+...