[FYI,3/3] Avoid exponential behavior in rust_evaluate_subexp
Commit Message
The STRUCTOP_STRUCT case in rust_evaluate_subexp would evaluate its
LHS, and then, if it did not need Rust-specific treatment, it would
back up and re-evaluate the entire STRUCTOP_STRUCT part of the
expression using evaluate_subexp_standard. This yields exponential
behavior and causes some expressions to evaluate extremely slowly.
The fix is to simply do the needed work inline.
This is PR rust/21483.
ChangeLog
2017-05-12 Tom Tromey <tom@tromey.com>
PR rust/21483:
* rust-lang.c (rust_evaluate_subexp) <STRUCTOP_STRUCT>: Don't
recurse, just call value_struct_elt directly.
---
gdb/ChangeLog | 6 ++++++
gdb/rust-lang.c | 15 +++++++--------
2 files changed, 13 insertions(+), 8 deletions(-)
@@ -1,5 +1,11 @@
2017-05-12 Tom Tromey <tom@tromey.com>
+ PR rust/21483:
+ * rust-lang.c (rust_evaluate_subexp) <STRUCTOP_STRUCT>: Don't
+ recurse, just call value_struct_elt directly.
+
+2017-05-12 Tom Tromey <tom@tromey.com>
+
* rust-lang.c (rust_dump_subexp_body) <STRUCTOP_ANONYMOUS,
OP_RUST_ARRAY>: Fix.
@@ -1762,7 +1762,7 @@ tuple structs, and tuple-like enum variants"));
case STRUCTOP_STRUCT:
{
- struct value* lhs;
+ struct value *lhs;
struct type *type;
int tem, pc;
@@ -1771,16 +1771,14 @@ tuple structs, and tuple-like enum variants"));
(*pos) += 3 + BYTES_TO_EXP_ELEM (tem + 1);
lhs = evaluate_subexp (NULL_TYPE, exp, pos, noside);
+ const char *field_name = &exp->elts[pc + 2].string;
type = value_type (lhs);
if (TYPE_CODE (type) == TYPE_CODE_UNION
&& !rust_union_is_untagged (type))
{
int i, start;
struct disr_info disr;
- struct type* variant_type;
- char* field_name;
-
- field_name = &exp->elts[pc + 2].string;
+ struct type *variant_type;
disr = rust_get_disr_info (type, value_contents (lhs),
value_embedded_offset (lhs),
@@ -1817,9 +1815,10 @@ which has only anonymous fields"),
}
else
{
- /* Field access in structs and untagged unions works like C. */
- *pos = pc;
- result = evaluate_subexp_standard (expect_type, exp, pos, noside);
+ result = value_struct_elt (&lhs, NULL, field_name, NULL,
+ "structure");
+ if (noside == EVAL_AVOID_SIDE_EFFECTS)
+ result = value_zero (value_type (result), VALUE_LVAL (result));
}
}
break;