Avoid exponential behavior in rust_evaluate_subexp

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.

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index c2d3f2d..71aea52 100644 (file)
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -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.
 

diff --git a/gdb/rust-lang.c b/gdb/rust-lang.c
index b4a529d..898edda 100644 (file)
--- a/gdb/rust-lang.c
+++ b/gdb/rust-lang.c
@@ -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;