[PATCH v1 2/6] perf dwarf-aux: More accurate variable type match for breg

[Zecheng Li <zecheng@google.com>]

Introduces the function is_breg_access_indirect to determine whether a
memory access involving a DW_OP_breg* operation refers to the variable's
value directly or requires dereferencing the variable's type as a
pointer based on the DWARF expression. Previously, all breg based
accesses were assumed to directly access the variable's value
(is_pointer = false).

The is_breg_access_indirect function handles three main cases:

1. Base register + offset only: (e.g., DW_OP_breg7 RSP+88) The
   calculated address is the location of the variable. The access is
   direct, so no type dereference is needed. Returns false.

2. Base register + offset, followed by other operations ending in
   DW_OP_stack_value, including DW_OP_deref: (e.g., DW_OP_breg7 RSP+96,
   DW_OP_deref, DW_OP_plus_uconst 0x64, DW_OP_stack_value) The DWARF
   expression computes the variable's value, but that value requires a
   dereference. The memory access is fetching that value, so no type
   dereference is needed. Returns false.

3. Base register + offset, followed only by DW_OP_stack_value: (e.g.,
   DW_OP_breg7 RSP+176, DW_OP_stack_value) This indicates the value at
   the base + offset is the variable's value. Since this value is being
   used as an address in the memory access, the variable's type is
   treated as a pointer and requires a type dereference. Returns true.

The is_pointer argument passed to match_var_offset is now set by
is_breg_access_indirect for breg accesses.

Signed-off-by: Zecheng Li <zecheng@google.com>
---
 tools/perf/util/dwarf-aux.c | 42 ++++++++++++++++++++++++++++++++-----
 1 file changed, 37 insertions(+), 5 deletions(-)

diff --git a/tools/perf/util/dwarf-aux.c b/tools/perf/util/dwarf-aux.c
index bf906dff9ef0..814c96ea509f 100644
--- a/tools/perf/util/dwarf-aux.c
+++ b/tools/perf/util/dwarf-aux.c
@@ -1424,6 +1424,38 @@ static bool match_var_offset(Dwarf_Die *die_mem, struct find_var_data *data,
 	return true;
 }
 
+/**
+ * is_breg_access_indirect - Check if breg based access implies type dereference
+ * @ops: DWARF operations array
+ * @nops: Number of operations in @ops
+ *
+ * Returns true if the DWARF expression indicates the variable's value is
+ * a pointer that the memory access dereferences.
+ * Returns false if the expression evaluates to the variable's value directly.
+ * This is called after check_allowed_ops.
+ */
+static bool is_breg_access_indirect(Dwarf_Op *ops, size_t nops)
+{
+	ops++;
+	nops--;
+
+	/* only the base register */
+	if (nops == 0)
+		return false;
+
+	switch (ops->atom) {
+	case DW_OP_stack_value:
+		return true;
+	case DW_OP_deref_size:
+	case DW_OP_deref:
+	case DW_OP_piece:
+		return false;
+	default:
+		/* unreachable, OP not supported */
+		return false;
+	}
+}
+
 /* Only checks direct child DIEs in the given scope. */
 static int __die_find_var_reg_cb(Dwarf_Die *die_mem, void *arg)
 {
@@ -1452,7 +1484,7 @@ static int __die_find_var_reg_cb(Dwarf_Die *die_mem, void *arg)
 		if (data->is_fbreg && ops->atom == DW_OP_fbreg &&
 		    check_allowed_ops(ops, nops) &&
 		    match_var_offset(die_mem, data, data->offset, ops->number,
-				     /*is_pointer=*/false))
+				     /*is_pointer=*/is_breg_access_indirect(ops, nops)))
 			return DIE_FIND_CB_END;
 
 		/* Only match with a simple case */
@@ -1464,11 +1496,11 @@ static int __die_find_var_reg_cb(Dwarf_Die *die_mem, void *arg)
 					     /*is_pointer=*/true))
 				return DIE_FIND_CB_END;
 
-			/* Local variables accessed by a register + offset */
+			/* variables accessed by a register + offset */
 			if (ops->atom == (DW_OP_breg0 + data->reg) &&
 			    check_allowed_ops(ops, nops) &&
 			    match_var_offset(die_mem, data, data->offset, ops->number,
-					     /*is_pointer=*/false))
+					     /*is_pointer=*/is_breg_access_indirect(ops, nops)))
 				return DIE_FIND_CB_END;
 		} else {
 			/* pointer variables saved in a register 32 or above */
@@ -1478,11 +1510,11 @@ static int __die_find_var_reg_cb(Dwarf_Die *die_mem, void *arg)
 					     /*is_pointer=*/true))
 				return DIE_FIND_CB_END;
 
-			/* Local variables accessed by a register + offset */
+			/* variables accessed by a register + offset */
 			if (ops->atom == DW_OP_bregx && data->reg == ops->number &&
 			    check_allowed_ops(ops, nops) &&
 			    match_var_offset(die_mem, data, data->offset, ops->number2,
-					     /*is_poitner=*/false))
+					     /*is_pointer=*/is_breg_access_indirect(ops, nops)))
 				return DIE_FIND_CB_END;
 		}
 	}
-- 
2.50.1.470.g6ba607880d-goog