[mem2reg][debuginfo] Handle op_deref when converting dbg.declare

The conversion of dbg.declare into dbg.values doesn't take into account
the DIExpression attached to the intrinsic. In particular, when
converting:

```
store %val, ptr %alloca
dbg.declare(ptr %alloca, !SomeVar, !DIExpression())
```

Mem2Reg will try to figure out if `%val` has the size of `!SomeVar`. If
it does, then a non-undef dbg.value is inserted:

```
dbg.value(%val, !SomeVar, !DIExpression())
```

This makes sense: the alloca is _the_ address of the variable. So a
store to the alloca is a store to the variable. However, if the
expression in the original intrinsic is a `DW_OP_deref`, this logic is
not applicable:

```
store ptr %val, ptr %alloca
dbg.declare(ptr %alloca, !SomeVar, !DIExpression(DW_OP_deref))
```

Here, the alloca is *not* the address of the variable. A store to the
alloca is *not* a store to the variable. As such, querying whether
`%val` has the same size as `!SomeVar` is meaningless.

This patch addresses the issue by:
1. Allowing the conversion when the expression is _only_ a `DW_OP_deref`
without any other expressions (see code comment).
2. Checking that the expression does not start with a `DW_OP_deref`
before applying the logic that checks whether the value being stored and
the variable have the same length.

Differential Revision: https://reviews.llvm.org/D142160

1 files changed, 24 insertions, 9 deletions

diff --git a/llvm/lib/Transforms/Utils/Local.cpp b/llvm/lib/Transforms/Utils/Local.cpp
index 633f0bb..b6c6c5a 100644
--- a/llvm/lib/Transforms/Utils/Local.cpp
+++ b/llvm/lib/Transforms/Utils/Local.cpp
@@ -1524,19 +1524,34 @@ void llvm::ConvertDebugDeclareToDebugValue(DbgVariableIntrinsic *DII,
 
   DebugLoc NewLoc = getDebugValueLoc(DII);
 
-  if (!valueCoversEntireFragment(DV->getType(), DII)) {
-    // FIXME: If storing to a part of the variable described by the dbg.declare,
-    // then we want to insert a dbg.value for the corresponding fragment.
-    LLVM_DEBUG(dbgs() << "Failed to convert dbg.declare to dbg.value: "
-                      << *DII << '\n');
-    // For now, when there is a store to parts of the variable (but we do not
-    // know which part) we insert an dbg.value intrinsic to indicate that we
-    // know nothing about the variable's content.
-    DV = UndefValue::get(DV->getType());
+  // If the alloca describes the variable itself, i.e. the expression in the
+  // dbg.declare doesn't start with a dereference, we can perform the
+  // conversion if the value covers the entire fragment of DII.
+  // If the alloca describes the *address* of DIVar, i.e. DIExpr is
+  // *just* a DW_OP_deref, we use DV as is for the dbg.value.
+  // We conservatively ignore other dereferences, because the following two are
+  // not equivalent:
+  //     dbg.declare(alloca, ..., !Expr(deref, plus_uconstant, 2))
+  //     dbg.value(DV, ..., !Expr(deref, plus_uconstant, 2))
+  // The former is adding 2 to the address of the variable, whereas the latter
+  // is adding 2 to the value of the variable. As such, we insist on just a
+  // deref expression.
+  bool CanConvert =
+      DIExpr->isDeref() || (!DIExpr->startsWithDeref() &&
+                            valueCoversEntireFragment(DV->getType(), DII));
+  if (CanConvert) {
     Builder.insertDbgValueIntrinsic(DV, DIVar, DIExpr, NewLoc, SI);
     return;
   }
 
+  // FIXME: If storing to a part of the variable described by the dbg.declare,
+  // then we want to insert a dbg.value for the corresponding fragment.
+  LLVM_DEBUG(dbgs() << "Failed to convert dbg.declare to dbg.value: " << *DII
+                    << '\n');
+  // For now, when there is a store to parts of the variable (but we do not
+  // know which part) we insert an dbg.value intrinsic to indicate that we
+  // know nothing about the variable's content.
+  DV = UndefValue::get(DV->getType());
   Builder.insertDbgValueIntrinsic(DV, DIVar, DIExpr, NewLoc, SI);
 }