[DebugInfo][DAG] Distinguish different kinds of location indirection

From SelectionDAGs point of view, debug variable locations specified with
dbg.declare and dbg.addr are indirect -- they specify the address of
something. But calling conventions might mean that a Value is placed on
the stack somewhere, and this too is indirection. Previously this was
mixed up in the "IsIndirect" field of DBG_VALUE insts; this patch
separates them by encoding the indirection in a DIExpression.

If we have a dbg.declare or dbg.addr, then the expression produces an
address that then becomes a DWARF memory location. We can represent
this by putting a DW_OP_deref on the _end_ of the expression. If a Value
has been placed on the stack, then we need to put a DW_OP_deref on the
_start_ of the expression, to load the Value from the stack and have
the rest of the expression operate on it.

Differential Revision: https://reviews.llvm.org/D69028

1 files changed, 19 insertions, 5 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
index da96c9ac435..67d24025abe 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
@@ -5493,7 +5493,6 @@ bool SelectionDAGBuilder::EmitFuncArgumentDbgValue(
   MachineFunction &MF = DAG.getMachineFunction();
   const TargetInstrInfo *TII = DAG.getSubtarget().getInstrInfo();
 
-  bool IsIndirect = false;
   Optional<MachineOperand> Op;
   // Some arguments' frame index is recorded during argument lowering.
   int FI = FuncInfo.getArgumentFrameIndex(Arg);
@@ -5515,7 +5514,6 @@ bool SelectionDAGBuilder::EmitFuncArgumentDbgValue(
     }
     if (Reg) {
       Op = MachineOperand::CreateReg(Reg, false);
-      IsIndirect = IsDbgDeclare;
     }
   }
 
@@ -5559,7 +5557,6 @@ bool SelectionDAGBuilder::EmitFuncArgumentDbgValue(
       }
 
       Op = MachineOperand::CreateReg(VMI->second, false);
-      IsIndirect = IsDbgDeclare;
     } else if (ArgRegsAndSizes.size() > 1) {
       // This was split due to the calling convention, and no virtual register
       // mapping exists for the value.
@@ -5573,9 +5570,26 @@ bool SelectionDAGBuilder::EmitFuncArgumentDbgValue(
 
   assert(Variable->isValidLocationForIntrinsic(DL) &&
          "Expected inlined-at fields to agree");
-  IsIndirect = (Op->isReg()) ? IsIndirect : true;
-  if (IsIndirect)
+
+  // If the argument arrives in a stack slot, then what the IR thought was a
+  // normal Value is actually in memory, and we must add a deref to load it.
+  if (Op->isFI()) {
+    int FI = Op->getIndex();
+    unsigned Size = DAG.getMachineFunction().getFrameInfo().getObjectSize(FI);
+    if (Expr->isImplicit()) {
+      SmallVector<uint64_t, 2> Ops = {dwarf::DW_OP_deref_size, Size};
+      Expr = DIExpression::prependOpcodes(Expr, Ops);
+    } else {
+      Expr = DIExpression::prepend(Expr, DIExpression::DerefBefore);
+    }
+  }
+
+  // If this location was specified with a dbg.declare, then it and its
+  // expression calculate the address of the variable. Append a deref to
+  // force it to be a memory location.
+  if (IsDbgDeclare)
     Expr = DIExpression::append(Expr, {dwarf::DW_OP_deref});
+
   FuncInfo.ArgDbgValues.push_back(
       BuildMI(MF, DL, TII->get(TargetOpcode::DBG_VALUE), false,
               *Op, Variable, Expr));