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Allow lazy 'zero' value

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This changes value_zero to create a lazy value.  In many cases,
value_zero is called in expression evaluation to wrap a type in a
non-eval context.  It seems senseless to allocate a buffer in these
cases.

A new 'is_zero' flag is added so we can preserve the existing
assertions in value_fetch_lazy.

A subsequent patch will add a test where creating a zero value would
fail, due to the variable size check.  However, the contents of this
value are never needed, and so creating a lazy value avoids the error
case.
This commit is contained in:
Tom Tromey
2021-09-13 12:31:20 -06:00
parent a519e8ffe2
commit 3e44c3049f
2 changed files with 23 additions and 12 deletions
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11
gdb/valops.c
View File

@ -935,17 +935,6 @@ value_dynamic_cast (struct type *type, struct value *arg)
error (_("dynamic_cast failed"));
}
/* Create a value of type TYPE that is zero, and return it. */
struct value *
value_zero (struct type *type, enum lval_type lv)
{
struct value *val = allocate_value (type);
VALUE_LVAL (val) = (lv == lval_computed ? not_lval : lv);
return val;
}
/* Create a not_lval value of numeric type TYPE that is one, and return it. */
struct value *

24
gdb/value.c
View File

@ -180,6 +180,7 @@ struct value
lazy (1),
initialized (1),
stack (0),
is_zero (false),
type (type_),
enclosing_type (type_)
{
@ -230,6 +231,10 @@ struct value
used instead of read_memory to enable extra caching. */
unsigned int stack : 1;
/* True if this is a zero value, created by 'value_zero'; false
otherwise. */
bool is_zero : 1;
/* Location of value (if lval). */
union
{
@ -1704,6 +1709,7 @@ value_copy (struct value *arg)
val->pointed_to_offset = arg->pointed_to_offset;
val->modifiable = arg->modifiable;
val->stack = arg->stack;
val->is_zero = arg->is_zero;
val->initialized = arg->initialized;
if (!value_lazy (val))
{
@ -3507,6 +3513,18 @@ pack_unsigned_long (gdb_byte *buf, struct type *type, ULONGEST num)
}
/* Create a value of type TYPE that is zero, and return it. */
struct value *
value_zero (struct type *type, enum lval_type lv)
{
struct value *val = allocate_value_lazy (type);
VALUE_LVAL (val) = (lv == lval_computed ? not_lval : lv);
val->is_zero = true;
return val;
}
/* Convert C numbers into newly allocated values. */
struct value *
@ -4026,7 +4044,11 @@ value_fetch_lazy (struct value *val)
value. */
gdb_assert (val->optimized_out.empty ());
gdb_assert (val->unavailable.empty ());
if (value_bitsize (val))
if (val->is_zero)
{
/* Nothing. */
}
else if (value_bitsize (val))
value_fetch_lazy_bitfield (val);
else if (VALUE_LVAL (val) == lval_memory)
value_fetch_lazy_memory (val);